Brakes are all well and good, but you need some method of applying them in order for them to work. The method by which the force from your hand or foot reaches the brake itself is all to do with the brake actuator system.
This is about as basic as you get. A cable is connected to a lever at each end. You press on one lever with your foot or squeeze it with your hand, and it pulls the lever at the other end. On the back of the brake-end lever there’s an elliptical cam which rotates inside a circular cup in the brake shoe. As the long axis of the ellipse rotates, it forces the brake shoes to move apart. In the case of a bicycle brake, the brake-end of the cable just pulls the two calipers together.
One step up, and found on the rear brake of most Indian motorbikes, the solid bar connection. This allows the use of mechanical advantage (see below) to amplify your force on the pedal or lever before it gets to the brakes themselves. Typically these systems are used on drum brakes with the elliptical actuator described above. The disadvantage of this system is that it needs hinge and pivot points that match the position of the suspension components. If they’re not present, going over a bump could put the brakes on as the suspension moves relative to the lever.
Another step up and we get to the type of brake system used on most cars and motorbikes today. Gone are the cables and bars, replaced instead with a system of plungers, reservoirs and hydraulic fluid. Single-circuit hydraulic systems have three basic components – the master cylinder, the slave cylinder and the reservoir. They’re joined together with hydraulic hose and filled with a non-compressible hydraulic fluid (see brake fluid below). When you press your foot on the brake, or squeeze the brake lever, you compress a small piston assembly in the master cylinder. Because the brake fluid does not compress, that pressure is instantaneously transferred through the hydraulic brake line to the slave cylinder where it acts on another piston assembly, pushing it out. That slave assembly is either connected to a lever to activate the brakes, or more commonly, is the brake caliper itself, with the slave cylinder being the piston that acts directly on the brake pads. Because of the arrangement of the slave cylinder, heat from the brakes can be transferred back into the brake fluid.
Dual-circuit hydraulic systems are available on high-end luxury vehicles and newer motorbikes, in particular BMW bikes. These have two separate circuits. One is the command circuit – that’s the one you act on with your hand or foot. The second is a separate circuit controlled by an onboard computer, and that’s the one which is actually connected to the brakes. As you apply the brakes, you’re sending a pressure signal via the command circuit to the brake computer. It measures the amount of force you’re applying, and using a servo / pump system, applies the same force to the secondary circuit to activate the brakes. If you do something stupid like trying to slam on the brakes at 100mph, the computer will realise that this would result in a skid or spin, and will not send the full pressure down the secondary circuit, instead deciding to use it’s speed and ABS sensors to determine the optimal brake pressure to maintain control of the vehicle. The advantage of a dual-circuit system is that the command circuit never gets heat transferred into it because it is totally separated from the brakes themselves. The disadvantage of course is that you now have two hydraulic circuits to maintain.
The most advanced system of brakes to date are brake-by-wire. These are a direct copy of some styles of racing brakes and are very similar to the dual-circuit hydraulic system described above, but instead of the command circuit being hydraulic, its replaced with electronics. The brake pedal or lever is connected to a hypersensitive rheostat (measures electrical resistance). The more you push it, the greater the electrical signal sent to the brake computer. From there on, it performs just like the secondary circuit described above. The advantage to this system is that the brake pedal or lever can be placed just about anywhere you like as it no longer is encumbered by the plumbing that goes with a hydraulic circuit. To combat driver complaints of “lack of feel” in the brakes, most brake-by-wire systems have a reverse feedback loop built in. This measures the pressure being applied to the brakes on the secondary circuit, and actuates an electrical resistor in the pedal or lever assembly to provide resistance. This is needed because there is no physical connection to any part of the brake system at all.
Power brakes (also known as power assisted brakes) are designed to use the power of the engine and/or battery to enhance your braking power. Whilst you can generate a fair amount of force using your foot, using systems from elsewhere in the car to help you apply even more force means that you get more powerful brakes as a result. The four most common types of power brakes are: vacuum suspended; air suspended; hydraulic booster, and electrohydraulic booster. Most cars use vacuum suspended units (vacuum boosters). In this type of system, when you press the brake pedal, the push rod to the master cylinder opens a vacuum control valve. This allows vacuum pressure (normally from the intake manifold) to “suck” on a diaphragm inside the vacuum assist unit. This extra vacuum suction helps you to produce more force at the pedal end of the brake system.
Hydraulic booster systems usually utilise pressure from the power steering system to augment pressure on the master brake cylinder.
Electrohydraulic booster systems use an electric motor to pressurize the hydraulic system downwind of the brake pedal which has the effect of amplifying the internal pressure in the whole system.The advantage to this system is that as long as you have battery power, you have power brakes even if the engine fails. With vacuum-assist brakes, no engine means no assistance.
If you’re curious about how power brakes work, go out to your car and with the engine off, step on the brakes. They’ll have a slightly solid, almost wooden feel to them. Turn the engine on and do it again and you’ll notice a lot less back-pressure on the pedal. This is the power assist which is making it easier for you to depress the pedal.
Brake master cylinders are complicated affairs involving finely manufactured parts, minute tolerances, springs, o-rings and rubber seals. The diagram below is a simplified representation of a dual-circuit master brake cylinder. When you step on the brake, its connected to the main plunger (on the right side of this image). As this is pushed into the master cylinder it acts on the components inside. The rear plunger (in blue) is the first one to start moving. As it moves forward, brake fluid from the reservoir is sucked in through the fluid intake and return port. At the same time, fluid is sucked in through the equalisation port. As the second circuit rear seal passes the intake and return port (about 1.5mm after the plunger starts moving), it creates a fixed volume of fluid between the rear and front plungers. The more you step on the brake pedal, the more this fluid is now forced out into the second brake circuit to apply those brakes. At the same time, the pressure building up in this area overcomes the strength of the first circuit return spring and the front plunger (red) begins to move too. As with the rear plunger, it too sucks fluid from the reservoir until the first circuit rear seal passes the fluid intake and return port (again about 1.5mm), trapping fluid between it and the front of the master cylinder. This fluid is then forced out into the first brake circuit, applying those brakes. When you take your foot off the brakes, the return springs push the plungers back into their neutral position. Fluid returns to the brake fluid reservoir and the system goes back to an unpressurised state.
One last thing about brake master cylinders : they cost an absolute bomb to replace. If you find yours is leaking, patching it up is not an option. Brand new master cylinders can go for around $1500 without labour costs. Remanufactured ones come in slightly cheaper at around $900. Bear that in mind when your 20 year old beater develops a leak – it’s probably cheaper to buy another used car than to replace the master cylinder.
In the rendering of the master brake cylinder above, you’ll see there are two plungers and two brake circuits. This is the most common design for cars today. It’s a form of redundancy in the brake system. The idea is that only two brakes, one front and one rear, are on either of the brake circuits. For four brakes, you therefore need two circuits. But why? Well imagine one of your brake lines springs a leak – for the sake of argument, the front-left brake. If all four brakes were on a single circuit, when the master cylinder began to pressurise the brake system, fluid would spurt out of the broken line and pressure would never build up. In turn, that means none of the brakes would ever come on and you’ll sail merrily into the back of the vehicle in front of you. Imagine the same scenario with two circuits. As the first circuit pressurises the front-left and rear-right brakes, fluid spurts out of the broken line and those brakes are never applied. However because the master cylinder is also pressurising a separate second circuit connected to the front-right and rear-left wheels, those brakes do apply and you’ve still got braking force. Sure, it’s reduced, but it’s a hell of a lot better than no brakes at all. Because of the front-left to rear-right and front-right to rear-left linking of the brake circuits, this type of system is known as cross-linked brakes. The rendering below shows an example arrangement of cross-linked brakes.
It’s worth spending a moment here to talk about handbrakes. Or parking brakes, e-brakes or emergency brakes depending on where you come from. Whilst they’re good for doing handbrake turns, they’re not especially effective at actually slowing you down. They will – don’t get me wrong – but you won’t be seeing any stellar performance out of them so the term ‘emergency brake’ is a bit of a misnomer. So why is this? Well, handbrakes are cable-actuated for a start so the amount of power they have is wholly dependent on the amount of tug you have in your arm. There’s no hydraulic system to help you out. Apart from that, they only work on the rear wheels, so you’re not getting four-wheel braking. On drum-brakes, the handbrake is connected to a small lever that pivots against the end of one of the brake actuating pistons. When you pull the handbrake, the lever gets pulled and the brake shoes are pressed out against the inside of the drum. On disc brakes, the handbrake normally works a second set of brake pads in the rear caliper. They’re little spots, about the size of a grown man’s thumbprint and they’re clamped mechanically against the brake rotor. These pads never need changing because they’re normally only used at standstill so generally don’t wear much. Their small size is the other reason you shouldn’t expect stellar stopping performance if you yank on the handbrake. That being said, there are derivatives of disc-based handbrakes that use a mechanical arm to press the main brake pads against the rotor although these are less common as far as I know.
Typically you ought to use your handbrake whenever you’re stopped somewhere, be it parked, on a hill or waiting at traffic lights. The reason is simple : if you’re parked or stopped, you generally don’t want the car to run off without you. At traffic lights, it’s an accident minimisation function as much as anything. If you’re sitting there with your foot on the brake and someone drives into the back of you, the impact will cause you to take your foot off the brake and you’ll go sailing into the car in front, causing more accidents. If you have the handbrake on in the same scenario, your car will largely stay put (apart from the initial shove across the ground as the energy from the impact is dissapated through your tyres). Of course there are personal habits and mechanical complications to contend with here. For example in a car with an automatic gearbox, it’s force of habit to just use the footbrake. Even so, you should still use the handbrake when you’re parked, especially on an incline. The ‘park’ setting on automatic gearboxes isn’t sufficient to hold a car on a hill, and apart from that, it puts incredible strain on the transmission and clutch system if you let the whole weight of the car transfer into the transmission to try to keep it from moving. In some American cars, the handbrake isn’t a handbrake at all, it’s a second footbrake on the far left side of the footwell, which is basically totally useless because it’s a pain to put on and even more of a pain to get off because it’s a one-way ratchet system (you have to force the pedal all the way down to get it to release). Then there’s the ignorance factor. When I went to my new owners orientation evening after buying a Subaru in America, one lady asked what the parking brake was for. (Apparently the name wasn’t obvious enough). The dealer representative told her it was a relic of days gone by, not to be used, and he didn’t understand why manufacturers even put them in cars any more!
The first and most obvious answer to this is : when you’re going at any speed. If you yank on the handbrake at any speed much over 30km/h, the back end of your car will start to slide. Great for stunts and tricks, not so great if you’re trying to stop in 5 lanes of crowded highway traffic.
The other time you should not use your handbrake is in post-snow, freezing conditions. With the salt and grit that gets put down on the roads, you’ll be driving through a salty, snowy slush and it will be spraying all over the underside of your car. If you park and put the handbrake on, you risk it binding on by freezing. Why? Well handbrake cables are almost always exposed to the elements at some point under your car. If you put the handbrake on and the cable is covered in slush, as it freezes again it will lock the handbrake on. There’s no solution to this other than waiting for the weather to warm up. Well, not unless you fancy a crack at the Darwin Awards, because some people have tried using blowtorches to thaw the ice, not understanding that they were working right underneath the petrol tank. So here’s a tip : don’t. If you need to park in those types of conditions, try to find level ground and leave your automatic gearbox in “p” or your manual gearbox either in first or reverse gears.
By 2006, brake-assist and accident warning systems were starting to find their way into consumer cars. Volvo’s collision warning system (CWS), for example, constantly monitors your speed and uses a radar with a 15° forward field of view to determine the distance to any object in front of you. If the distance begins to shrink but you don’t slow down, the system sounds a buzzer and flashes a bright red light in a heads-up display to alert you. The brake pads are automatically placed against the discs and when the driver finally does use the brakes, the system monitors the pedal pressure. If the pressure is determined to be too light, the braking power is amplified by the system. Brake-assist and auto-brakes go one step further. In some high end vehicle now (top end BMWs and Mercedes’ for example), the collision-detection system is linked into the brakes like it is with the Volvo system, but it’s also been given the flexibility to do all the braking for you. Adaptive cruise control, for example, will control the throttle just like a normal cruise control system, but will also apply the brakes if it determines that you’re getting too close to the vehicle in front. Full auto-brakes will actually stop the car for you if you fail to respond. All these systems work in essentially the same way – they monitor the brake use and distance to the vehicle in front. If the computer thinks you’re not braking hard enough, it will assist you. These systems are all very clever but they tread the thin ethical line. Just because engineers can make their vehicles do this doesn’t mean they should. Consider this: with in-vehicle monitoring and tracking systems like OnStar, and the impending satellite-tracking systems for road tolling, it’s not too hard to imagine all those systems chained together in such a way that the vehicle will literally prevent you from speeding by limiting the throttle availability and controlling the brakes. If you really want to be driven like that in a vehicle over which you have no control at all, take the bus.
There are other brake technologies that are becoming available in vehicles now, and a lot of them are gathered together in the 2006 / 2007 BMW models. They’re the rolling embodiment of clever brake engineers just showing off. Three of the more notable features are:
All the above devices fall into that ethical grey area again, but unlike the brake-assist and collision-detection systems outlined earlier, these three brake technologies don’t actually attempt to compensate for any wrongdoing on the driver’s behalf. They simply help prepare the car for when the driver chooses to use the brakes. From that point of view, I would regard these as better technologies than those which go the whole hog and interfere with your driving.
Reference:
www.carbibles.com – It is a truly wonderful site for getting all of the information on basic automotive domains. Do give it a try.
The simple answer : they slow you down.
The complex answer : brakes are designed to slow down your vehicle but probably not by the means that you think. The common misconception is that brakes squeeze against a drum or disc, and the pressure of the squeezing action is what slows you down. This in fact is only part of the reason you slow down. Brakes are essentially a mechanism to change energy types. When you’re travelling at speed, your vehicle has kinetic energy. When you apply the brakes, the pads or shoes that press against the brake drum or rotor convert that energy into thermal energy via friction. The cooling of the brakes dissipates the heat and the vehicle slows down. This is all to do with The First Law of Thermodynamics, sometimes known as the law of conservation of energy. This states that energy cannot be created nor destroyed, it can only be converted from one form to another. In the case of brakes, it is converted from kinetic energy to thermal energy. Angular force. Because of the configuration of the brake pads and rotor in a disc brake, the location of the point of contact where the friction is generated also provides a mechanical moment to resist the turning motion of the rotor.
If you remember any sort of physics classes from school, you might recall something called mechanical advantage. In its most basic form, mechanical advantage is the ratio of force-in to force-out in a mechanical system. Mechanical Advantage = Effort Torque/Load Torque. For example a 20kg weight 1 metre from a pivot can lift a 40kg weight 0.5m from the pivot on the other side. The effort torque and load torque calculations are to do with force in Newtons and distance from pivot point. Hence torque is measured in Newton-metres, or Nm. A Newton is the amount of force required to accelerate a mass of one kilogram by one metre per second². On Earth, where acceleration due to gravity is 9.8m/s², the force exerted upon a mass of 1kg is 9.8N (usually rounded up to 10N). Another popular notation is lbf.ft – pound-force-feet, commonly referred to as foot-pounds. 1 Newton-metre is equivalent to 0.737 foot-pounds. The diagram below shows a simple lever system on a pivot. The load torque is 200Nm, and the effort torque is also 200Nm. Mechanical advantage = effort / load, which in this case is 200 / 200, which is 1. ie. the system is balanced.
Now imagine increasing the weight on the effort side to 30kg instead of 20kg, but leaving everything else the same. The load torque is still 200Nm, but the effort torque is now 300Nm. Mechanical advantage = effort / load, which is 300 / 200, which is 1.5. Any mechanical advantage value larger than 1.0 means that the effort has the advantage. In this case, a 30kg weight which is lighter than the 40kg load, is able to lift it off the ground.
If you now take your new-found / remembered knowledge about physics and look at the simple lever brake system, you’ll realise how it’s possible to generate enough force using your foot to stop a car or motorbike. Look at this diagram of the lever-operated cam brake.
This system has 4 levers in it. The middle two have no mechanical advantage as the levers are connected the same distance from the pivot in each case. However, look at the pedal. The values I’ve put in are arbitrary but they serve the purpose. On the pedal we have some amount of force 20cm from the pivot, but the other end of the lever is only 5cm from the pivot. This gives us a mechanical advantage of 4 on the brake lever (20cm / 5cm). At the other end, the lever attached to the cam is still a lever system – it’s just bent. The input lever is 10cm long but the cam is only 4cm across – or 2cm to the tip from the pivot. So at the brake cam we have a mechanical advantage of 5. (10cm / 2cm). So across this entire system, we have a total mechanical advantage of 20 – 4 from the brake pedal and 5 from the lever and cam. Apply force to this little system and be amazed. The units of force used are irrelevant – they’re multiplied just the same. To use easier-to-comprehend values, let’s imagine that when you’re braking, your foot is pushing on the brake pedal with about 60pounds of force – 27Kg. Through the brake pedal, that is amplified 4 times to 240pounds, and through the lever and cam its amplified a further 5 times from 240pounds to 1200pounds. You pushed the pedal with 60pounds of force, but the cam inside the drum brake is being forced out against the brake drum with 1200pounds of force – about 544Kg. Sweet.
Most braking systems now use hydraulics. This is a slight change in the equation but the concept of mechanical advantage still exists, this time by the use of pressure equations. Pressure = force / area. If you apply 20 Newtons of pressure to 1m², it’s the same as applying 200 Newtons to 10m². Why? Because 20 Newtons of force divided by 1m² of area generates 20 Pascals of pressure. Similarly, 200N / 10m² is also 20Pa.
If you now think of that in terms of a hydraulic braking system, it becomes clear how mechanical advantage works for you. Brake fluid is incompressible – it has to be. This is good because it makes calculation for hydraulic brake systems quite easy – you can eliminate the internal pressure from the equation. Split the system into two parts – input and output – the brake pedal and the brake caliper piston. For each part, Pressure = Force / Area. The Pressure is the same at all points in the system, so some basic algebra gives a simple formula:
Using our previous example, we apply 60pounds (27Kg) of input force to the brake pedal. This is attached to a master piston which (for example) is 1.25cm across – ie. it has a surface area of 0.000491m² (remember your maths? area = PI x r²). At the other end of the system is the caliper piston, which for example is 2cm across – ie. it has a surface area of 0.001257m². Using our sparkly new formula, the output force from the caliper piston is 60 x (0.001257m² / 0.000491m²) Get your calculator out and that comes out to 154pounds (69.8Kg) – more than double the force at the brake pedal. The ratio of output area to input area is sometimes referred to as the area differential.
So that, my friend, is why you can stop a speeding vehicle with a single foot.
If you ride a motorbike or drive a race car, you’re probably familiar with the term brake fade which is used to describe what happens to brakes when they get too hot. A good example is coming down a mountain pass using your brakes rather than your engine to slow you down. By the First Law of Thermodynamics, as you start to come down the pass, the brakes on your vehicle heat up, slowing you down. But if you keep using the brakes, the drums or discs and brake pads will stay hot and get no chance to cool off. The next time you try to brake, because the brake components are already so hot, they cannot absorb much more heat. Once they get to this stage, you have to look at the brake pads themselves. In every brake pad there is the friction material which is held together with some sort of resin. Once this lot starts to get too hot, the resin holding the pad material together starts to vapourise, forming a gas. That gas has to have somewhere to go, because it can’t stay between the pad and the rotor, so if forms a thin layer between the two trying to escape. The result is very similar to hydroplaning while going too fast in the rain; the pads lose contact with the rotor, thus reducing the amount of friction. Voila. Brake fade. The typical symptom of this would be to get the vehicle to a stop and wait for a few minutes. As the brake components cool down, their ability to absorb heat returns, the pads cool off which means they have more chance to heat up again before the resin vapourises, hence the next time you use the brakes, they seem to work just fine. This type of brake fade was more common in older vehicles. Newer vehicles tend to have less outgassing from the brake pad compounds but they still suffer brake fade. So why? Well it is again to do with the pads getting too hot. With newer brake pad compounds where outgassing isn’t so much of a problem, the pads transfer heat into the calipers because the rotors are already too hot and the brake fluid starts to boil as a result. As this happens, bubbles form in the brake fluid. Air is compressible, brake fluid isn’t, so you can put your foot on the brake pedal and get full travel but have no braking effect at the other end. This is because you’re now compressing the gas bubbles and not actually forcing the pads against the rotors. Voila. Brake fade again. So how do the engineers design brakes to reduce or eliminate brake fade? For older vehicles, you give that vapourised gas somewhere to go. For newer vehicles, you find some way to cool the rotors off more effectively. Either way you end up with cross-drilled or grooved brake rotors. While grooving the surface may reduce the specific heat capacity of the rotor, its effect is negligible in the grand scheme of things. The rotors will heat up to cool down no faster or slower. However, under heavy braking once everything is hot and the resin is vapourising, the grooves give the gas somewhere to go, so the pad can continue to contact the rotor, allowing you to stop.
The whole understanding of the conversion of energy is critical in understanding how and why brakes do what they do, and why they are designed like they are. If you’ve ever watched Formula-1 racing, you’ll see the front wheels have huge scoops inside the wheel pointing to the front (see the picture on the right). This is to duct air to the brake rotors to help them cool off because in Formula-1 racing, the brakes are used viciously every few seconds and spend a lot of their time trying to stay hot. Without some form of cooling assistance, the brakes would be fine for the first few corners but then would fade and become near useless by half way around the track.
Rotor technology. If a brake rotor was a single cast chunk of steel, it would have terrible heat dissipation properties and leave nowhere for the vapourised gas to go. Because of this, brake rotors are typically modified with all manner of extra design features to help them cool down as quickly as possible as well as dissapate any gas from between the pads and rotors. The following diagram shows some examples of rotor types with the various modification that can be done to them to help them create more friction, disperse more heat more quickly, and ventilate gas. From left to right. 1. Basic brake rotor. 2. Grooved rotor. The grooves give more bite and thus more friction as they pass between the brake pads They also allow gas to vent from between the pads and the rotor. 3. Grooved, drilled rotor. The drilled holes again give more bite, but also allow air currents (eddies) to blow through the brake disc to assist cooling and ventilating gas. 4. Dual ventilated rotors. Same as before but now with two rotors instead of one, and with vanes in between them to generate a vortex which will cool the rotors even further whilst trying to actually ’suck’ any gas away from the pads. An important note about drilled rotors: Drilled rotors are typically only found (and to be used on) race cars. The drilling weakens the rotors and typically results in microfractures to the rotor. On race cars this isn’t a problem – the brakes are changed after each race or weekend. But on a road car, this can eventually lead to brake rotor failure – not what you want.
Big rotors You know I’ve been drumming into you the whole mechanism that causes you to stop? How does it apply to bigger brake rotors; a common sports car upgrade? Well sports cars and race bikes typically have much bigger discs or rotors than your average family saloon car. The reason again is to do with heat and friction. A bigger rotor has more material in it so it can absorb more heat. More material also means a larger surface area, which as well as meaning more area for the pads to generate friction with, also translates to better heat dissipation. On top of that, the larger rotors mean that the brake pads make contact further away from the axle of rotation. This provides a larger mechanical advantage to resist the turning of the rotor itself. To best illustrate how this works, imagine a spinning steel disc on a pivot in front of you. If you clamped your thumbs either side of the disc close to the middle, your thumbs would heat up very quickly and you’d need to push pretty hard to generate the friction required to slow the disc down. Now imagine doing the same thing but clamping your thumbs together close to the outer rim of the disc. The disc will stop spinning much more quickly and your thumbs won’t get as hot. That, in a nutshell explains the whole principle behind why bigger rotors = better stopping power. Taking it one step further, composite brake rotors, as found on high-end Ferraris, the McLaren F1, and most Formula-1 race cars, are even better again at heat transfer.
All brakes work by friction. Friction causes heat which is part of the kinetic energy conversion process. How they create friction is down to the various designs.
I thought I’d cover these because they’re about the most basic type of functioning brake that you can see, watch working, and understand. The construction is very simple and out-in-the-open. A pair of rubber blocks are attached to a pair of calipers which are pivoted on the frame. When you pull the brake cable, the pads are pressed against the side or inner edge of the bicycle wheel rim. The rubber creates friction, which creates heat, which is the transfer of kinetic energy that slows you down. There’s only really two types of bicycle brake – those on which each brake shoe shares the same pivot point, and those with two pivot points.
The next, more complicated type of brake is a drum brake. The concept here is simple. Two semicircular brake shoes sit inside a spinning drum which is attached to the wheel. When you apply the brakes, the shoes are expanded outwards to press against the inside of the drum. This creates friction, which creates heat, which transfers kinetic energy, which slows you down. The example below shows a simple model. The actuator in this case is the blue elliptical object. As that is twisted, it forces against the brake shoes and in turn forces them to expand outwards. The return spring is what pulls the shoes back away from the surface of the brake drum when the brakes are released. See the later section for more information on actuator types.
The “single leading edge” refers to the number of parts of the brake shoe which actually contact the spinning drum. Because the brake shoe pivots at one end, simple geometry means that the entire brake pad cannot contact the brake drum. The leading edge is the term given to the part of the brake pad which does contact the drum, and in the case of a single leading edge system, it’s the part of the pad closest to the actuator. The diagram below shows what happens as the brakes are applied. The shoes are pressed outwards and the part of the brake pad which first contacts the drum is the leading edge. The action of the drum spinning actually helps to draw the brake pad outwards because of friction, which causes the brakes to “bite”. The trailing edge of the brake shoe makes virtually no contact with the drum at all. This simple geometry explains why it’s really difficult to stop a vehicle rolling backwards if it’s equipped only with single leading edge drum brakes. As the drum spins backwards, the leading edge of the shoe becomes the trailing edge and thus doesn’t bite.
The drawbacks of the single leading edge style of drum brake can be eliminated by adding a second return spring and turning the pivot point into a second actuator. Now when the brakes are applied, the shoes are pressed outwards at two points. So each brake pad now has one leading and one trailing edge. Because there are two brake shoes, there are two brake pads, which means there are two leading edges. Hence the name double leading edge.
Some background: Disc brakes were invented in 1902 and patented by Birmingham car maker Frederick William Lanchester. His original design had two discs which pressed against each other to generate friction and slow his car down. It wasn’t until 1949 that disc brakes appeared on a production car though. The obscure American car builder Crosley made a vehicle called the Hotshot which used the more familiar brake rotor and calipers that we all know and love today. His original design was a bit crap though – the brakes lasted less than a year each. Finally in 1954 Citroën launched the way-ahead-of-its-time DS which had the first modern incarnation of disc brakes along with other nifty stuff like self-levelling suspension, semi-automatic gearbox, active headlights and composite body panels. (all things which were re-introduced as “new” by car makers in the 90’s).
Disc brakes are an order of magnitude better at stopping vehicles than drum brakes, which is why you’ll find disc brakes on the front of almost every car and motorbike built today. Sportier vehicles with higher speeds need better brakes to slow them down, so you’ll likely see disc brakes on the rear of those too. Disc brakes are again a two-part system. Instead of the drum, you have a disc or rotor, and instead of the brake shoes, you now have brake caliper assemblies. The caliper assemblies contain one or more hydraulic pistons which push against the back of the brake pads, clamping them together around the spinning rotor. The harder they clamp together, the more friction is generated, which means more heat, which means more kinetic energy transfer, which slows you down. You get the idea by now.
Standard disc brakes have one or two cylinders in them – also know as one or two-pot calipers. Where more force is required, three, or more cylinders can be used. Sports bikes have 4- or 6-pot calipers arranged in pairs. The disadvantage of disc brakes is that they are extremely intolerant of faulty workmanship or bad machining. If you have a regular car disc rotor which is off by so much as 0.07mm (3/1000 inch) it will be Hell when you step on the brakes. That ever-so-slight warp or misalignment is going to spin through the clamped calipers at some ungodly speed and the resulting vibration will make you wonder if you’re driving down stairs. So you can imagine the kind of tolerances required for these components.
There is a quiet but major revolution happening in the world of brakes, and its being brought about by a Canadian company called NewTech. Rather than the piecemeal improvements we’ve seen over the last few years, with slight design changes, and materials improvements, the new system is a radical redesign from the ground up. NewTech have designed a disc brake system called “full contact disc brakes”. They looked at traditional pad and rotor design and figured that the pads only contact about 15% of the rotor surface at any one time. With a change of design, NewTech have been able to add 5 more pads to the system so that 75% of the brake rotor is in contact with the pads at any one time. With traditional pads and rotors, the brake rotor is clamped between the pad. With the NewTech design, the brake rotor itself becomes a floating rotor, similar to those found on motorbikes. It is covered with a ’spider’ (the red structure in my renderings below) and the spider has 6 brake pads on the inside of it. The hydraulic system acts on fully circular elastomer composite diaphragm behind the brake disc, mounted in the black structure in the renderings. This had 6 pads on it which push the entire disc out against the 6 pads inside the spider. This provides and even force across the entire disc to push it out, and the disc gets an even contact with all 12 pads. To ensure the brakes remain cool, the system is covered in cooling fins connected to the outer pads to dissipate heat. The inner pads are fitted with a moulded thermal barrier made of a composite material. Special inserts made of a variety of frictional materials are distributed evenly on the entire surface of the pad. The range of materials is used to ensure performance under diverse conditions. NewTech believe that the system has considerable advantages over conventional brakes with better cooling, higher strength and reduced noise and vibration. NewTech have sold truck and bus versions of these brakes into the haulage and public transport industry, but now Renault is considering introducing this system on its cars in conjunction with a new brake-by-wire system. Newtech’s first OEM customer was to be Saleen who were going to put the system on their S7 supercar, but in the end went with conventional six-piston monoblock calipers instead. NewTech’s website can be found here.
It’s worth nothing that this isn’t actually the first time this has been tried in cars. Bugattiexperimented with a system like this in the late 80’s for inclusion on their 1991 EB110 supercar; it was going to be available as an option for the car. People who had experienced the brakes said they were just otherworldy, that the braking power was way beyond capabilities of the average driver. They came from Aerospatiale, the French aerospace company, who also designed the chassis for the EB110 (this type of brake was being used in aircraft at the time). Bugatti dropped the idea because the brakes would have cost more than the rest of the EB110, which at $350,000 was by no means a cheap car.
Siemens VDO in Germany are trying to bring a prototype electric wedge brake (EWB) to the market. The EWB is an innovative idea based on technology developed by a company called eStop. Siemens acquired eStop early in 2005 and have been continuing their work on the wedge system ever since. The principle is both simple and clever. The brake pad is pressed against the brake rotor by means of a wedge-shaped thrust plate. The more the brake rotor turns, the harder the slope of the wedge forces the pads against it. Because of the shape of the wedge bearings and thrust plate and the rotation of the brake rotor, the pad is actually forced against the rotor harder the faster the rotor is spinning. In effect, a lot of braking force for very little input. The system runs off a normal 12v vehicle electrical system which means no more hydraulics. It also allows the system to eliminate all the plumbing associated with ABS as the EWB is entirely electronically controlled. The final advantage, if you could call it that, is that it allows the first true all-electronic brake-by-wire system. Current brake-by-wire systems use electronics behind the brake pedal to send signals to actuators in the hydraulic system. With the EWB there is no hydraulic system so the only link from the brake pedal to the brake caliper is a 12v electrical feed and signal actuation wire. The operation of the wedge system is based on several roller bearings and a wedge-shaped thrust plate connected to a pair of 12v electric motors. As the brake pedal is depressed, the signal is sent to the motors to start moving the thrust plate. Because of its shape and the design of the roller bearings, as the thrust plate moves, it forces the brake pad to press against the brake rotor. The reaction time of the electric motors can be measured in milliseconds – far quicker than any hydraulic system could react, so in theory, when connected to a full computer-monitored brake-by-wire system, the EWB ought to be able to shave milliseconds off brake reaction time. Doesn’t sound like much but if it means a few less metres in stopping distance, that can only be a good thing. The brake caliper unit itself has an intelligent wheel-braking module built into it. As well as the motors, bearings and wedges, the module also has a sensor system for monitoring movement and force – basically this is what replaces the traditional ABS items so each brake caliper becomes a self-governing ABS unit. Because there’s no physical link back to the brake pedal any more, the ABS doesn’t force the brake pedal to judder when it activates which will make it far more acceptable for a lot more drivers. Finally, because the system is totally electronic, the traditional cable-pulled handbrake can also be eliminated and replaced with a parking switch that simply activates all four EWB modules. Of course there are pros and cons to any new system like this. Obviously reducing the weight and complexity of the braking system is a good thing, and because of the design of the EWB, there’s a lot less space taken up in the engine bay, freeing up more room for the car designers to work with. But by removing the hydraulic lines, ABS actuators and sensors, and master and slave brake cylinders, the EWB concept becomes entirely reliant on the 12v electrical system and the vagaries of a computer. Knowing how often a single dodgy earth connections in a car can totally screw up the electrics, I’ve got to wonder what would happen if a grounding strap came loose and the electronic brake system started playing up. Will these brakes have a fail-safe or backup system like the double hydraulic circuits we use now, or will you sail off into some solid object because you’ve got no brakes left? Siemens aren’t clear on this matter. If you want to see a video demonstrating the EWB, Siemens VDO have one available here(27.8Mb mpeg).
Just a quick word on brake pad compounds. Most pads used to use asbestos but we all know what that stuff is like. Today they use all manner of combinations of materials. The pads themselves are made up of a friction material bonded to the backing plate. The brake caliper piston pushes against the backing plate and the friction material is pushed against the brake rotor. The material combinations typically fall into the following broad categories now.
Organic
These pads are well-suited for street driving because they wear well, are easy on the ears, don’t chew up the rotors and don’t spew dust everywhere. They’re favoured for your average family saloon because they work well when they’re cold. Of course the drawback is that they don’t work so well when they get hot.
Semi-metallic / sintered
This is a good compromise between street and track. These seem to be the pad of choice for sportier vehicles such as the Subaru Impreza WRX. They won’t work as well as organic pads when they are cold, so you need to be a bit wary of the first couple of stops. Conversely they do work well when hot. Occasionally the weak link in semi-metallic pads is the bonding material that holds the friction pad to the backing plate. There have been occasions where the friction material has come away completely. That’s infrequent though.
Metallic
These pads are typically reserved for racing or the extremely rich. They squeal and dust like crazy, are hard on rotors and don’t work well when cold.
Ceramic
Ceramic pads still have metal fibers (about 15% vs. about 40% for semi-metallic) but they are copper instead of steel and therefore cause less wear and transfer heat better. They don’t fade as easily as other pads, cool faster, last longer, and are effectively silent, as the sound they genereate is outside of the human range of hearing. Dogs will go crazy thought. The dust created by ceramic pads is also very light in color so your wheels look cleaner.
The Bugatti Veyron EB 16.4 is the most recent version of a mid-engined full-sized grand tourer developed by the German car-manufacturer Volkswagen and produced by the Volkswagen-brand Bugatti Automobiles SAS at their headquarters in Château St. Jean in Molsheim (Alsace, France), and whose production and development is often credited to Ferdinand Karl Piech. It is named after French racing driver Pierre Veyron, who won the 24 hours of Le Mans in 1939 while racing for the original Bugatti company. It was named “Car Of The Decade” by the BBC television programme Top Gear.
Two hundred and twenty Veyrons are known to have been built and delivered since production began in 2005 and ended in late 2008. Special variants of the Veyron include the Pur Sang, the Fbg Par Hermes, the Sang Noir, the Targa, the Vincero, and the Bleu Centenaire. It will be replaced with the Grand Sport, which is essentially a Veyron convertible.
The Veyron features an 8.0 litre W16 engine — sixteen cylinders in two banks of eight cylinders, or the equivalent of two narrow angle V8 engines mated in a “W” configuration. Each cylinder has four valves for a total of sixty four, but the narrow staggered eight configuration allows two overhead camshafts to drive two banks of cylinders so only four camshafts are needed. The engine is fed by four turbochargers and displaces 7,993 cubic centimetres (487.8 cu in), with a square 86 mm by 86 mm (3.4 in × 3.4 in) bore and stroke.
The transmission is a dual clutch Direct-Shift Gearbox computer-controlled automatic with seven gear ratios, with magnesium paddles behind the steering wheel and a shift time of less than 150 milliseconds. This is designed and manufactured by Ricardo of England (and not Borg-Warner who designed the six speed DSG used in the mainstream marques of the Volkswagen Group). The Veyron can be driven in either semi automatic or fully automatic mode. A replacement transmission for the Veyron costs just over $120,000. It also features fulltime permanent four wheel drive, using the Haldex Traction system. It uses special Michelin PAX run flat tyres, designed specifically for the Veyron to accommodate its top speed, which reportedly cost $25,000 US per set. The tyres can only be removed from the rims in France, a service which reportedly costs $70,000. Kerb weight is 2,034.8 kilograms (4,486 lb). This gives the car a power to weight ratio, according to Volkswagen Group’s 736 kilowatts (1,001 PS; 987 bhp) figures, of 446.3 bhp per ton.
The car’s wheelbase is 2,710 mm (106.7 in). Overall length is 4,462 mm (175.7 in), width 1,998 mm (78.7 in) and height 1,204 mm (47.4 in).
The Bugatti Veyron has a total of ten radiators.
It has a drag coefficient of 0.41 (normal condition) and 0.36 (after lowering to the ground), and a frontal area of 2.07 square metres (22.3 sq ft). This gives it a CdA ft² value of 8.02.
According to Volkswagen Group, the DIN rated motive power output, approved by TÜV Süddeutschland, of the final production Veyron engine produces 1,001 metric horsepower (736 kW; 987 bhp) and generates 1,250 newton metres (922 ft·lbf) of torque. The figure has been confirmed by Bugatti officials to actually be conservative, with the real total being 1020 bhp or more.
The top speed was verified by James May on Top Gear for the November 2006 issue, again at Volkswagen Group’s private Ehra-Lessien test track, where the final-production car hit 407.9 km/h (253.5 mph), which equated to almost one-third of the speed of sound at sea level. As the Bugatti Veyron approached the top speed during the test, May said that “the tyres will only last for about fifteen minutes, but it’s okay because the fuel runs out in twelve minutes”. He also gave an indication of the power requirements: at a constant 155 mph, the Veyron is using approximately 270 metric horsepower (200 kW; 270 bhp); the next 100 mph requires an additional 730 metric horsepower (540 kW; 720 bhp). Jeremy Clarkson, driving a Veyron from Italy to London, noted that at top speed, the engine consumes 10,000 imperial gallons (45,000 L) of air per minute (as much as a human breathes in four days). With a 0 to 60 time of 2.4 seconds, the Veyron was the fastest legal street car between the years 2005 and 2007. Once back in the Top Gear studio, May was asked by co-presenter Jeremy Clarkson what the Veyron felt like to drive at 407 km/h (253 mph), May replied that it was “totally undramatic”, and very stable at speed.
German inspection officials recorded an average top speed of 408.47 km/h (253.81 mph)[19] during test sessions on the Ehra-Lessien test track on 19 April 2005. The Bugatti website still refers to the Veyron as the fastest production vehicle of all time even though this title has since been taken by the SSC Ultimate Aero TT.
The car’s everyday top speed is listed at 350 km/h (220 mph). When the car reaches 220 km/h (140 mph), hydraulics lower the car until it has a ground clearance of about 9 cm (3.5 in.). At the same time, the wing and spoiler deploy. This is the “handling mode”, in which the wing helps provide 3,425 newtons (770 lbf) of downforce, holding the car to the road, and helping the Bugatti Veyron perform 1.34 g forces on a 300 foot skidpad.[13] The driver must, using a special key (the “Top Speed Key”), toggle the lock to the left of his seat in order to attain the maximum (average) speed of 407 km/h (253 mph). The key functions only when the vehicle is at a stop, when a checklist then establishes whether the car and its driver are ready to enable ‘top speed’ mode. If all systems are go, the rear spoiler retracts, the front air diffusers shut and the ground clearance, normally 12.5 cm (4.9 in), drops to 6.5 cm (2.6 in).
The Veyron’s brakes use cross drilled, radially vented carbon fibre reinforced silicon carbide (C/SiC) composite discs, manufactured by SGL Carbon, which have a much greater resistance to brake fade when compared with conventional cast iron discs. The lightweight aluminium alloy monobloc brake calipers are made by AP Racing; the fronts have eight titanium pistons and the rear calipers have six pistons. Bugatti claims maximum deceleration of 1.3 G on road tyres. As an added safety feature, in the event of brake failure, an anti-lock braking system (ABS) has also been installed on the handbrake.
Prototypes have been subjected to repeated 1.0 G braking from 312 km/h (194 mph) to 80 km/h (50 mph) without fade. With the car’s acceleration from 80 km/h (50 mph) to 312 km/h (194 mph), that test can be performed every 22 seconds. At speeds above 200 km/h (120 mph), the rear wing also acts as an airbrake, snapping to a 55-degree angle in 0.4 seconds once brakes are applied, providing an additional 0.68 G (4.9 m/s²) of deceleration (equivalent to the stopping power of an ordinary hatchback). Bugatti claims the Veyron will brake from 400 km/h (250 mph) to a standstill in less than 10 seconds.
The Bugatti Veyron is a car built around an engine. Essentially, Bugatti made the decision to blow the doors off the supercar world by creating a 1,000-horsepower engine. Everything else follows from that resolution.
So let’s start with the engine. How would you begin the design process for an engine this powerful? If you have know how a car engines works, you know that if you want to create a 1,000-horsepower engine, it has to be able to burn enough gasoline to generate 1,000 horsepower. That works out to about 1.33 gallons (5 liters) of gasoline per minute.
We need a 16-liter engine to burn 1.33 gallons of gas per minute. That actually makes sense — the engine in the Dodge Viper is 8.0 liters in displacement and produces 500 hp.
But there’s a problem: A 16-liter V-8 engine would be very large. And the pistons would be massive, so there would be no way it could turn at 6,000 rotations per minute (rpm). It might turn at a maximum of 2,000 rpm, meaning that you would need an immense 48-liter engine to generate 1,000 hp. Clearly an engine that big is impossible in a passenger car.
So how did Bugatti fit 1,000 horsepower into a passenger car?
Bugatti did two things to create a compact engine capable of producing 1,000 hp.
The first and most obvious thing is turbocharging.
The Bugatti Veyron’s 16-cylinder monster engine produces 1,001 horsepower for a top speed of more than 250 mph. And it’s a passenger car. Check out the Bugatti. Amazing isn`t it? If you have know how a turbocharger works, you know that one easy way to make an engine more powerful without making the engine bigger is to stuff more air into the cylinders on each intake stroke. Turbochargers do that. A turbo pressurizes the air coming into the cylinder so the cylinder can hold more air. If you stuff twice as much air in each cylinder, you can burn twice as much gasoline. In reality, it’s not quite a perfect ratio like that, but you get the idea. The Bugatti uses a maximum turbo boost of 18 PSI to double the output power of its engine. Therefore, turbocharging allows Bugatti to cut the size of the engine from 16 liters back down to a more manageable 8 liters. To generate that much air pressure, the Bugatti requires four separate turbochargers arranged around the engine.
The second thing Bugatti engineers did, both to keep the RPM redline high and to lower lag time when you press the accelerator, was to double the number of cylinders.
The Bugatti has a very rare 16-cylinder engine.
There are two easy ways to create a 16-cylinder engine.
The latter technique is, in fact, the way Bugatti created its first 16-cylinder cars in the early 20th century.For the Veyron, Bugatti chose a much more challenging path. Essentially, Bugatti merged two V-8 engines onto one another, and then let both of them share the same crankshaft. This configuration creates the W-16 engine found in the Veyron. The two V’s create a W.
The special features of the Bugatti W-16 engine are amazing. For example:
Everything about the engine is superlative.And it is remarkably compact. It measures just 710 mm (27 inches) long, 889 mm (35 inches) wide and 730 mm (28.7 inches) high. This is the beauty of Bugatti’s W-16 approach — the engineers managed to fit 1,000 hp into a reasonably sized package.
Transmission
The transmission is unique, in particular because it has to harness about twice as much torque as any previous sports-car transmission. It has:
This computer-controlled system is identical to the sort of system found in a Formula 1 car or a Champ car. There is no clutch pedal or shift lever for the driver to operate — the computer controls the clutch disks as well as the actual shifting. The computer is able to shift gears in 0.2 seconds. It would be almost impossible for all of the torque available from the W-16 engine to flow out to just two wheels without constant wheel-spin. Therefore, the Veyron has full-time all-wheel drive. By applying the engine’s power to all four wheels through a computer-controlled traction-control system, the car is able to harness all of the engine’s horsepower, even at full acceleration.
Body Design
According to one of the Veyron’s designers, the biggest challenge in creating the Veyron was the aerodynamics.
How do you keep a 250-mph passenger car on the road?
An F-1 car or a Champ car can travel at 250 mph or more, but they have a uniquely designed body, a single driver lying in a reclining position, just an inch or so of ground clearance and an aero-package made up of large wings to generate massive downforce. The Bugatti, on the other hand, is trying to look like a normal car and seat two passengers. The Veyron’s dimensions help to some extent. The car is 79 inches (200 cm) wide, 176 inches (447 cm) long and only 48 inches (122 cm) high. Keep in mind that a Hummer 2 is 81.2 inches wide. The Bugatti is extremely wide for its height. The underside of the Veyron, like an F-1 car, is streamlined and venturi-shaped to increase downforce. There is also a wing in the back of the Veyron (see below) that extends automatically at high speed to increase downforce and keep the car glued to the road. According to Popular Science: Hypercar, “With the moving tail spoiler we’ve got enough downforce now, about 100 kg (221 pounds) at the rear and 80 kg (177 pounds) at the front at top speed.”
The Veyron uses two snorkel-like devices one on either side of the engine to manage airflow. The Veyron has three reasons for managing airflow:
The engine of the Veryon sits behind the driver, so roof-mounted snorkels, the rear-deck vents and side-mounted scoops bring air to the engine and rear brakes.
The size of the engine and transmission, along with the four-wheel-drive system and the four drive shafts, along with the opulence of the passenger compartment (discussed in the next section) and the car’s oversized dimensions, all add weight. Even though the body is sculpted in carbon fiber to minimize its mass, the car weighs in at about 4,300 pounds (1,950 kg). For comparison, a Dodge Viper weighs about 1,000 pounds (454 kg) less.
Even the tires for the Veyron are unique. They’re specially designed by Michelin to handle the stress of driving at 250 mph. The tires need to be sticky like a race car’s and able to handle 1.3 G’s on the skidpad. However, they also need to last longer than the 70 or so miles of a typical race tire.
Michelin therefore created completely new tires to handle the Veyron’s unique requirements. In the rear, the tires are 14.4 inches (36.6 cm) wide. Specifically, the tires measure 245/690 R 520 A front and 365/710 R 540 A rear, where 245 and 365 are the width in millimeters (9.5 and 14.4 inches respectively). The rims are 520 mm and 540 mm in diameter (approximately 20 inches). These tires, in other words, are massive — the rears are the widest ever produced for a passenger car.
The tires use the Michelin PAX system. Their pressure is monitored automatically, and they can run flat for approximately 125 miles (201 km) at 50 mph (80 kph). According to Michelin, the run-flat detection system “plays an integral role in active safety in PAX System. Its role is to inform you of a loss of pressure, either gradual or sudden.” Once warned of an air leak by the PAX system, you can reduce your speed and head toward a tire repair center.
One advantage of the PAX system and its run-flat ability is that it eliminates the need for a spare tire.
The Interior The Veyron seats two in lavish style. The interior is swathed almost completely in leather — the dash, seats, floor and sides are all leather. Only the instruments and a few metal trim pieces interrupt the leather experience.
The car also surrounds its occupants with every sort of electronic nicety, including a remarkable stereo system, navigation system, etc.
Is all of this worth a million bucks? Who knows. But regardless, the Veyron represents a remarkable technological achievement.
The Veyron is also likely to represent the far end of the automotive performance spectrum for some time to come. To create a car much faster will require adding even more weight, and delivering even more power to the wheels. The added weight means diminishing returns in the power-to-weight domain. Additional power means more wheelspin.
Look at a Champ car and consider how radical its appearance is compared to a passenger car. Consider also that a Champ car does not go much faster than the Veyron. The Veyron probably approaches the outer limits of the passenger car envelope, and we are unlikely to see much beyond the Veyron in terms of performance.
This is, in other words, as good as it gets.
Well, here is a video of how fast the Veyron can actually fly.
References: http://en.wikipedia.org/wiki/Bugatti_Veyron
http://auto.howstuffworks.com/bugatti.htm
VTEC, Variable Valve Timing and Lift Electronic Control is a valve train system developed by Honda to improve the volumetric efficiency of a four-stroke internal combustion engine. This system uses two camshaft profiles and electronically selects between the profiles. It was invented by Honda R&D engineer Ikuo Kajitani. It can be said that VTEC, the original Honda variable valve control system, originated from REV (Revolution-modulated valve control) introduced on the CBR400 in 1983 known as HYPER VT EC. VTEC was the first system of its kind, though other variable valve timing and lift control systems have been produced by other manufacturers (MIVEC from Mitsubishi, VVTL-i from Toyota, VarioCam Plus from Porsche, VVL from Nissan, etc).
i-VTEC
(intelligent-VTEC) introduced continuously variable camshaft phasing on the intake cam of DOHC VTEC engines. The technology first appeared on Honda’s K-series four cylinder engine family in 2001 (2002 in the U.S.). In the United States, Honda first debuted the technology on the 2003 Honda Civic Si EP3 with the economy version.
Valve lift and duration are still limited to distinct low- and high-RPM profiles, but the intake camshaft is now capable of advancing between 25 and 50 degrees (depending upon engine configuration) during operation. Phase changes are implemented by a computer controlled, oil driven adjustable cam gear. Phasing is determined by a combination of engine load and rpm, ranging from fully retarded at idle to somewhat advanced at full throttle and low rpm. The effect is further optimization of torque output, especially at low and midrange RPM.
The K-Series motors have two different types of i-VTEC systems implemented. The first is for the performance motors like in the RSX Type S or the TSX and the other is for economy motors found in the CR-V or Accord. The performance i-VTEC system is basically the same as the DOHC VTEC system of the B16A’s; both intake and exhaust have 3 cam lobes per cylinder. However the valvetrain has the added benefit of roller rockers and continuously variable intake cam timing. Performance i-VTEC is a combination of conventional DOHC VTEC with VTC.
The economy i-VTEC is more like the SOHC VTEC-E in that the intake cam has only two lobes, one very small and one larger, as well as no VTEC on the exhaust cam. The two types of motor are easily distinguishable by the factory rated power output: the performance motors make around 200 hp (150 kW) or more in stock form and the economy motors do not make much more than 160 hp (120 kW) from the factory.
The new SOHC i-VTEC implementation is an entirely new implementation that was introduced on the 2006 Honda Civic’s R-series four cylinder SOHC engines. This implementation uses the so-called “fuel economy cam” and “high output cam” on one of the two intake valves of each cylinder (another intake valve is fixed). The “fuel economy cams” are designed to retard the closure of one intake valve and are activated between 1000-3500RPM and under low load condition. When “fuel economy cams” are activated, the intake valve closes well after the piston has started moving upwards in the compression stroke. During this time, the drive-by-wire throttle valve is open wider than normal. Due to the delayed closing of intake valve, a part of the intake mixture that has entered the combustion chamber is forced out again into the intake manifold. That way, the engine “emulates” a lower displacement than its actual one (its operation is also similar to an Atkinson cycle engine, with uneven compression and combustion strokes), which reduces pumping losses thus reducing fuel consumption and increases its efficiency. VTEC-off on the R18A means it can be considered to be running “high output cams”. When the right conditions are achieved for fuel economy, VTEC engages the 2nd set, the ‘low’ or ‘economy’ cams. Thus VTEC-on on the R18A means it is running low cams.
According to Honda, this measure alone can reduce pumping losses by 16%. Under heavier loads, the engine switches back into its “high output cams”, and it operates like a regular 4 stroke Otto cycle engine. This implementation of i-VTEC was initially introduced in the R18A1 engine found under the bonnet of the 8th generation Civic, with a displacement of 1.8 L and an output of 140 PS (100 kW; 140 hp). Recently, another variant was released, the 2.0 L R20A2 with an output of 150 PS (110 kW; 150 hp), which powers the EUDM version of the all-new CRV. SOHC i-VTEC
With the continued introduction of vastly different i-VTEC systems, one may assume that the term is now a catch-all for creative valve control technologies from Honda.
In 2003, Honda introduced an i-VTEC V6 (an update of the J-series) that includes Honda’s cylinder deactivation technology which closes the valves on one bank of (3) cylinders during light load and low speed (below 80 km/h (50 mph)) operation. The technology was originally introduced to the US on the Honda Odyssey minivan, and can now be found on the Honda Accord Hybrid, the 2006 Honda Pilot, and the 2008 Honda Accord.
i-VTEC VCM was also used in 1.3L 4-cylinder engines used in Honda Civic Hybrid.
It is a version of i-VTEC with direct injection.
It was first used in 2003 Honda Stream.
This is how an ivtec engine works
And this is the Honda Hybrid System
Reference: www.wikipedia.org
www.world.honda.com
For years, the trusty seat belt provided the sole form of passive restraint in our cars. There were debates about their safety, especially relating to children, but over time, much of the country adopted mandatory seat-belt laws. Statistics have shown that the use of seat belts has saved thousands of lives that might have been lost in collisions.
Air bags have been under development for many years. The attraction of a soft pillow to land against in a crash must be very strong — the first patent on an inflatable crash-landing device for airplanes was filed during World War II! In the 1980s, the first commercial air bags appeared in automobiles.
Since model year 1998, all new cars have been required to have air bags on both driver and passenger sides. (Light trucks came under the rule in 1999.) To date, statistics show that air bags reduce the risk of dying in a direct frontal crash by about 30 percent. Newer than steering-wheel-mounted or dashboard-mounted bags, but not so widely used, are seat-mounted and door-mounted side air bags. Some experts say that within the next few years, our cars will go from having dual air bags to having six or even eight air bags. Having evoked some of the same controversy that surrounded seat-belt use in its early years, air bags are the subject of serious government and industry research and tests.
In this article, you’ll learn about the science behind the air bag, how the device works, what its problems are and where the technology goes from here.
Before looking at specifics, let’s review our knowledge of the laws of motion. First, we know that moving objects have momentum (the product of the mass and the velocity of an object). Unless an outside force acts on an object, the object will continue to move at its present speed and direction. Cars consist of several objects, including the vehicle itself, loose objects in the car and, of course, passengers. If these objects are not restrained, they will continue moving at whatever speed the car is traveling at, even if the car is stopped by a collision.
Stopping an object’s momentum requires force acting over a period of time. When a car crashes, the force required to stop an object is very great because the car’s momentum has changed instantly while the passengers’ has not — there is not much time to work with. The goal of any supplemental restraint system is to help stop the passenger while doing as little damage to him or her as possible.
What an air bag wants to do is to slow the passenger’s speed to zero with little or no damage. The constraints that it has to work within are huge. The air bag has the space between the passenger and the steering wheel or dash board and a fraction of a second to work with. Even that tiny amount of space and time is valuable, however, if the system can slow the passenger evenly rather than forcing an abrupt halt to his or her motion.
There are three parts to an air bag that help to accomplish this feat:
The inflation system is not unlike a solid rocket booster.The air bag system ignites a solid propellant, which burns extremely rapidly to create a large volume of gas to inflate the bag. The bag then literally bursts from its storage site at up to 200 mph (322 kph) — faster than the blink of an eye! A second later, the gas quickly dissipates through tiny holes in the bag, thus deflating the bag so you can move.
Even though the whole process happens in only one-twenty-fifth of a second, the additional time is enough to help prevent serious injury. The powdery substance released from the air bag, by the way, is regular cornstarch or talcum powder, which is used by the air bag manufacturers to keep the bags pliable and lubricated while they’re in storage
The idea of using a rapidly inflating cushion to prevent crash injuries had a long history before the U.S. Department of Transportation called for the equipment to be adapted for automobiles in the 1980s. The first patent on an inflatable crash-landing device for airplanes was filed during World War II.
Early efforts to adapt the air bag for use in cars bumped up against prohibitive prices and technical hurdles involving the storage and release of compressed gas. Researchers wondered:
They needed a way to set off a chemical reaction that would produce the nitrogen that would inflate the bag. Small solid-propellant inflators came to the rescue in the 1970s.
In the early days of auto air bags, experts cautioned that the new device was to be used in tandem with the seat belt. Seat belts were still completely necessary because air bags worked only in front-end collisions occurring at more than 10 mph (6 kph). Only seat belts could help in side swipes and crashes (although side-mounted air bags are becoming more common now), rear-end collisions and secondary impacts. Even as the technology advances, air bags still are only effective when used with a lap/shoulder seat belt!
It didn’t take long to learn that the force of an air bag can hurt those who are too close to it. Researchers have determined that the risk zone for driver air bags is the first 2 to 3 inches (5 to 8 cm) of inflation. So, placing yourself 10 inches (25 cm) from your driver air bag gives you a clear margin of safety. Measure this distance from the center of the steering wheel to your breastbone. If you currently sit less than 10 inches away, you can adjust your driving position in the following ways:
The rules are different for children. An air bag can seriously injure or even kill an unbuckled child who is sitting too close it or is thrown toward the dash during emergency braking.Experts agree that the following safety points are important:
Expansion Chamber (Tuned Exhaust Pipe)
This article will show you how an expansion chamber (also known as a tuned pipe)
actually works through the use of a step-by-step explanation and a few graphics.
What it does
An expansion chamber which has been properly matched to an engine will effectively
supercharge the mixture of gas and air inside the cylinder. This is done by pulling
extra mixture into the cylinder and by pressing wasted mixture back in through the
exhaust port.
How it works
As the descending piston opens the cylinder exhaust port, the action of the exhaust
pressure wave rushing through the tuned pipe at the speed of sound initiates a
sequence of events that enhance the engines torque and horsepower output.
The positive pressure pulse leaves the exhaust port
As the mixture is combusted inside the cylinder, the piston is forced downward and
opens the exhaust port. The force of the exhaust pressure leaving the cylinder creates
a positive pressure pulse as it moves out into the expansion chamber.
The time between exhaust port opening and transfer port opening is called “exhaust
lead”. Exhaust lead allows the high-pressure exhaust gases in the cylinder to blowdown
(reducing pressure) before the transfer port opens. If this doesn’t happen,
exhaust particles may back-flow into the crankcase and contaminate and heat the
incoming fuel/air charge, thereby robbing the engine of power. Positive pressure pulse
leaves the exhaust port forcing exhausted gas and air into he chamber
Negative pressure waves
The inertia of the out flowing exhaust particles out into the divergent code creates
negative pressure waves and a strong partial vacuum (about minus 7psi) near the still
opening exhaust port shortly after the transfer port has opened. In addition to the
sucking out tail-end exhaust gases, these suctions cause fresh air/fuel mixture to be
sucked through the transfer port into the combustion chamber.
Negative pressure is created as the waves expand into the divergent cones
Positive pressure waves
The remaining energy in the negative pressure wave continues through the pipe and is
reflected off the convergent cone at the rear of the unit and returns to the engines
exhaust port. During this point, the negative pressure will actually end up pulling
extra fresh gas and air into the header of the expansion chamber.2
Positive pressure waves echo back from the convergent cone, towards the engine
Supercharging through the exhaust port
Finally just as the transfer ports inside the cylinder are closing, the returning positive
pressure wave compresses the extra fuel and air mixture back into the cylinder
through the exhaust port. The cylinder now contains the extra mixture for the next
combustion—effectively supercharging the engine and increasing efficiency and
performance.
Positive pressure forces over scavenged mixture back into the cylinder
The Resonator
When a wave hits the hole, part of it continues into the chamber and part of it
is reflected. The wave travels through the chamber, hits the back wall of the
muffler and bounces back out of the hole.
The length of this chamber is calculated so that this wave leaves the resonator
chamber just after the next wave reflects off the outside of the chamber.
Ideally, the high-pressure part of the wave that came from the chamber will
line up with the low-pressure part of the wave that was reflected off the
outside of the chamber wall, and the two waves will cancel each other out.
In reality, the sound coming from the engine is a mixture of many different
frequencies of sound, and since many of those frequencies depend on the
engine speed, the sound is almost never at exactly the right frequency for this
to happen.
The resonator is designed to work best in the frequency range where the
engine makes the most noise; but even if the frequency is not exactly what the
resonator was tuned for, it will still produce some destructive interference.
Some cars, especially luxury cars where quiet operation is a key feature, have
another component in the exhaust that looks like a muffler, but is called a
resonator.
This device works just like the resonator chamber in the muffler — the
dimensions are calculated so that the waves reflected by the resonator help
cancel out certain frequencies of sound in the exhaust.
There are other features inside this muffler that help it reduce the sound level
in different ways.
The body of the muffler is constructed in three layers: Two thin layers of
metal with a thicker, slightly insulated layer between them. This allows the
body of the muffler to absorb some of the pressure pulses.
Also, the inlet and outlet pipes going into the main chamber are perforated
with holes. This allows thousands of tiny pressure pulses to bounce around in
the main chamber, canceling each other out to some extent in addition to being
absorbed by the muffler’s housing.
Resonators are like little mufflers, and are usually the “straight through”
type. They are added at the end of the exhaust system to take care of any
noise that has made it through the muffler. The muffler quiets the noise of
the exhaust by “muffling” the sound waves created by the opening and
closing of the exhaust valves. When an exhaust valve opens, it discharges
the burned gases at high pressures into the exhaust pipe, which is at low
pressure. This type of action creates sound waves that travel through the
flowing gas, moving much faster than the gas itself (up to 1400 m.p.h.),
that the muffler must silence. It generally does this by converting the
sound wave energy into heat by passing the exhaust gas and its
accompanying wave pattern, through perforated chambers of varied
sizes. Passing into the perforations and reflectors within the chamber
forces the sound waves to dissipate their energy.
ENGINE DYNAMOMETER
Chassis dynamometer
A chassis dynamometer measures power delivered to the surface of the “drive roller”
by the drive wheels. The vehicle is often parked on the roller or rollers, which the car
then turns and the output is measured. Modern roller type chassis dyne systems use
the Salvisberg roller, which improved traction and repeatability over smooth or
knurled drive rollers. On a motorcycle, typical power loss at higher power levels,
mostly through tire flex is about 10% and gearbox chain and other power transferring
parts are another 2% to 5%. Other types of chassis dynamometers are available that
eliminate the potential wheel slippage on old style drive rollers and attach directly to
the vehicle’s hubs for direct torque measurement from the axle. Hub mounted dynos
include units made by Dynapack and Rototest. These dynes should read about 10% to
15% higher than a “rear wheel” chassis dyne. Chassis dynos can be fixed or portable.
Modern chassis dynamometers can do much more than display RPM, Horsepower,
and Torque. With modern electronics and quick reacting, low inertia dyne systems,
it’s now possible to tune to best power and the smoothest runs, in real-time. It’s also
common to, on a retail level, with a wideband 02 Sensor, graphed along with RPM, to
“tune to an air fuel ratio”. Some, like Dynojet and others can also add vehicle
diagnostic information to the dyno graph as well. This is done by gathering data
directly from the vehicle’s PCM via OBD communication. Because of frictional and
mechanical losses in the various drive train components, the measured rear wheel
brake horsepower is generally 15-20 percent less than the brake horsepower measured
at the crankshaft or flywheel on an engine dynamometer.
Other sources, after researching several different “engine” dyno software packages,
found that the engine dyno user can integrally add “frictional loss” channel factors of
+10% to +15% to the flywheel power, raising the claim that 20% to 25% or even
more power is actually lost between the crankshaft at high power outputs.
Power steering is a system for reducing the steering effort on cars by using an external power source to assist in turning the wheels. Power steering was invented in the 1920s by Francis W. Davis and George Jessup in Waltham, Massachusetts. Chrysler Corporation introduced the first commercially available power steering system on the 1951 Chrysler Imperial under the name Hydraguide. Most new vehicles now have power steering, although in the 1970s and 1980s it was the exception rather than the rule, at least on European cars. The trend to front wheel drive, greater vehicle mass and wider tires means that modern vehicles would be extremely difficult to manoeuvre at low speeds (e.g. when parking) without assistance.
Most power steering systems work by using a belt driven pump to provide hydraulic pressure to the system. This hydraulic pressure is generated by a rotary-vane pump which is driven by the vehicle’s engine. As the speed of the engine increases, the pressure in the hydraulic fluid also increases, hence a relief valve is incorporated into the system to allow excess pressure to be bled away.
While the power steering is not being used, i.e. driving in a straight line, twin hydraulic lines provide equal pressure to both sides of the steering wheel gear. When torque is applied to the steering wheel, the hydraulic lines provide unequal pressures and hence assist in turning the wheels in the intended direction.
Some more modern implementations of hydraulic systems also include an electronic pressure valve which can reduce the hydraulic pressure of the power steering lines as the vehicle’s speed increases (Variable assist power steering).
In the DIRAVI system invented by Citroën, the force turning the wheels comes from the car’s high pressure hydraulic system and is always the same no matter what the road speed is. As the steering wheel is turned, the wheels are turned simultaneously to a corresponding angle by a hydraulic ram. In order to give some artificial steering feel, there is a separate hydraulically operated system that tries to turn the steering wheel back to center position.
As long as there is pressure in the car’s hydraulic system, there is no mechanical connection between the steering wheel and the road wheels. This system was first introduced in the Citroën SM in 1970, and was known as ‘VariPower’ in the UK and ‘SpeedFeel’ in the U.S.
While DIRAVI is not the mechanical template for all modern power steering arrangements, it did innovate the now common benefit of speed adjustable steering. The force of the centering device increases as the car’s road speed increases.
Electric Power Steering, such as those found on the Chevrolet Cobalt, Acura NSX, Saturn VUE V6, 2nd gen Toyota MR2 and on most FIAT and Lancia cars, uses electric components. Sensors detect the motion and torque of the steering column and a computer module applies assistive power via an electric motor. This allows varying amounts of assistance to be applied depending on driving conditions. Most notably on FIAT group cars the amount of assistance can be regulated using a button named “CITY” that switches between two different assist curves (boost curve), while on Volkswagen/Audi group cars, the amount of assistance is automatically regulated depending on vehicle speed.
In the event of component failure, a mechanical linkage such as a rack and pinion serves as a back-up in a manner similar to that of hydraulic systems. The software in the computer module enables the flexibility of “tuning” the characteristics of the electric power steering system to suit the preference of the vehicle designers. The “feel” is often set a bit on the light side so a criticism commonly expressed is a lack of steering “feel”.
Electric power steering is limited to smaller vehicles. This is because the 12 volt electrical system is limited to 80 amps of current which, in turn, limits the size of the motor to less than 1 kilowatt. (12.5 volts times 80 amps equals 1000 watts) Vehicles such as trucks and SUVs require a larger power output. A new 42 volt electrical system standard may enable use of electric power steering on larger vehicles.
Electric systems have a slight advantage in fuel efficiency (almost 1 MPG) because there is no hydraulic pump constantly running, whether assistance is required or not, and this is the main reason for their introduction.
Servotronic offers speed-dependent power steering, in which the amount of servo assist depends on road speed and thus provides even more comfort and convenience for the driver. The amount of power assist is greatest at low speeds, for example when parking the car. The greater assist makes it easier to maneuver the car. At higher speeds, an electronic sensing system gradually reduces the level of power assist. In this way, the driver can control the car even more precisely than with conventional power steering. Servotronic is used by a number of automakers including Audi, BMW, and Porsche. Servotronic is a trademark of AM General[1].
So called “hybrid” systems use the same hydraulic assist technology as standard systems, with the hydraulic pressure being provided by an electric motor instead of a belt driven one. Those systems can be found in Volkswagen, Audi, Peugeot, SEAT, Skoda, Suzuki, MINI and some Mazda cars.
Because ABS has been popular since the mid-80s, I suppose most of you have already known its theory. Anyway, for the sake of those new joining car enthusiasts, I think it would be better to describe it briefly here.
You might think that optimal braking is implemented by completely locking all the wheels. No, law of physics tells us that the coefficient of friction between the ground surface and a static object is always greater than a moving object. If the tyres are sliding on the road surface, the friction between road and wheel will not be maximum. Therefore, the maximum braking occurs when the wheels are braked up to the level that the wheels just do not slide.
To ensure the shortest stopping distance, ABS applies intermittent braking in very high frequency. This avoid complete lock up of wheels, thus gives the name “Anti-Lock Braking System”.
Another advantage of ABS is letting the driver to keep controlling the car during braking. Before ABS appeared, cars lock up during braking, thus unable to be steered to avoid collision. With ABS, while slowing down the car, the driver can simultaneously try to steer away from the obstacle in front.
To implement anti-lock braking, ABS system employs speed sensors for individual wheels. If the wheel speed detected differs from the vehicle speed, that means the wheel is sliding, thus the computer will signal the corresponding brake to loose until sliding disappear. The computer will also compare the speed of all wheels, if one or more of them run considerably faster than others, that means the car is losing control, it will apply more brake to that wheel to correct the driving path.
Let me share with you the little bit information I gathered. ABS was originated in aeroplanes. It was developed in order to shorten the distance necessary for landing. It did not appeared in road cars until 1966, when Jensen FF (the first 4WD road car) installed a system developed by Dunlop. That system, called Maxaret, did not employ computer as well as wheel speed sensors. It just employed electronic sensors to avoid locking the disc brakes. Anyway, road testers immediately found its superiority over conventional brakes.
What’s next. Sorry, my information becomes incomplete since then. The following is the information bits I got :
Not only enhance braking, ABS sensors, computer and hydraulic pump also serve as the hardwares for Traction Control, Electronic Stability Control and Artificial LSD (read these topics in the following paragraphs). If not ABS is so popular, these new technology might not have appeared.
Anti-Lock Brake Image Gallery
In this article, the last in a six-part series on brakes, we’ll learn all about anti-lock braking systems — why you need them, what’s in them, how they work, some of the common types and some associated problems.
The theory behind anti-lock brakes is simple. A skidding wheel (where the tire contact patch is sliding relative to the road) has less traction than a non-skidding wheel. If you have been stuck on ice, you know that if your wheels are spinning you have no traction. This is because the contact patch is sliding relative to the ice By keeping the wheels from skidding while you slow down, anti-lock brakes benefit you in two ways: You’ll stop faster, and you’ll be able to steer while you stop.
There are four main components to an ABS system:
Speed Sensors The anti-lock braking system needs some way of knowing when a wheel is about to lock up. The speed sensors, which are located at each wheel, or in some cases in the differential, provide this information.
Valves There is a valve in the brake line of each brake controlled by the ABS. On some systems, the valve has three positions:
Pump Since the valve is able to release pressure from the brakes, there has to be some way to put that pressure back. That is what the pump does; when a valve reduces the pressure in a line, the pump is there to get the pressure back up.
Controller The controller is a computer in the car. It watches the speed sensors and controls the valves.
ABS at Work There are many different variations and control algorithms for ABS systems. We will discuss how one of the simpler systems works.
The controller monitors the speed sensors at all times. It is looking for decelerations in the wheel that are out of the ordinary. Right before a wheel locks up, it will experience a rapid deceleration. If left unchecked, the wheel would stop much more quickly than any car could. It might take a car five seconds to stop from 60 mph (96.6 kph) under ideal conditions, but a wheel that locks up could stop spinning in less than a second.
The ABS controller knows that such a rapid deceleration is impossible, so it reduces the pressure to that brake until it sees an acceleration, then it increases the pressure until it sees the deceleration again. It can do this very quickly, before the tire can actually significantly change speed. The result is that the tire slows down at the same rate as the car, with the brakes keeping the tires very near the point at which they will start to lock up. This gives the system maximum braking power.
When the ABS system is in operation you will feel a pulsing in the brake pedal; this comes from the rapid opening and closing of the valves. Some ABS systems can cycle up to 15 times per second.
An anti-lock braking system (ABS) is a system on motor vehicles which prevents the wheels from locking while braking. The purpose of this is to allow the driver to maintain steering control under heavy braking and, in some situations, to shorten braking distances (by allowing the driver to hit the brake fully without the fear of skidding or loss of control). Disadvantages of the system include increased braking distances under certain conditions and the creation of a “false sense of security” among drivers who do not understand the operation and limitations of ABS.
Since it came into widespread use in production cars (with “version 2″ in 1978), ABS has made considerable progress. Recent versions not only handle the ABS function itself (i.e. preventing wheel locking) but also traction control, brake assist, and electronic stability control, amongst others. Not only that, but its version 8.0 system now weighs less than 1.5 kilograms, compared with 6.3 kg of version 2.0 in 1978.
Anti-lock braking systems were first developed for aircraft in 1929 by the French automobile and aircraft pioneer Gabriel Voisin, as threshold braking an airplane is nearly impossible. An early system was Dunlop’s Maxaret system, introduced in the 1950s and still in use on some aircraft models, in 1936 the German Companies Bosch and Mercedes-Benz pioneered the first electronic version for use on Mercedes Benz cars.[citation needed] This version which was made of more than 1000 analogue electronic parts was still fairly slow.
A fully mechanical system saw limited automobile use in the 1960s in the Ferguson P99 racing car, the Jensen FF and the experimental all wheel drive Ford Zodiac, but saw no further use; the system proved expensive and, in automobile use, somewhat unreliable. However, a limited form of anti-lock braking, utilizing a valve which could adjust front to rear brake force distribution when a wheel locked, was fitted to the 1964 Austin 1800.
ABS brakes on a BMW motorcycle
The first true electronic 4-wheel multi-channel ABS was co-developed by Chrysler and Bendix for the 1971 Imperial. Called “Sure Brake”, it was available for several years and had a satisfactory performance and reliability record. Ford also introduced anti lock brakes on the Lincoln Continental Mark III and the Ford LTD station wagon, called “Sure Trak”. The German firms Bosch and Mercedes-Benz had been co-developing anti-lock braking technology since the 1930s; They first appeared in trucks and the Mercedes-Benz S-Class. ABS Systems were later introduced on other cars and motorcycles.
The anti-lock brake controller is also known as the CAB (Controller Anti-lock Brake).
A typical ABS is composed of a central electronic unit, four speed sensors (one for each wheel), and two or more hydraulic valves on the brake circuit. The electronic unit constantly monitors the rotation speed of each wheel. When it senses that any number of wheels are rotating considerably slower than the others (a condition that will bring it to lock[1]) it moves the valves to decrease the pressure on the braking circuit, effectively reducing the braking force on that wheel. The wheel(s) then turn faster and when they turn too fast, the force is reapplied. This process is repeated continuously, and this causes the characteristic pulsing feel through the brake pedal.
The sensors can become contaminated with metallic dust and fail to detect wheel slip; this is not always picked up by the internal ABS controller diagnostic.
One step beyond ABS are modern ESC systems. Here, two more sensors are added to help the system work: these are a wheel angle sensor, and a gyroscopic sensor. The theory of operation is simple: when the gyroscopic sensor detects that the direction taken by the car doesn’t agree with what the wheel sensor says, the ESP software will brake the necessary wheel(s) (up to three with the most sophisticated systems) so that the car goes the way the driver intends. The wheel sensor also helps in the operation of CBC, since this will tell the ABS that wheels on the outside of the curve should brake more than wheels on the inside, and by how much.
^ The electronic unit needs to determine when some of the wheels turn considerably slower than any of the others because when the car is turning the two wheels towards the center of the curve inherently move slightly slower than the other two – which is the reason why a differential is used in virtually all commercial cars.
A 2003 Australian study[2] by Monash University Accident Research Centre found that ABS: •Reduced the risk of multiple vehicle crashes by 18 percent, but that it •Increased the risk of run-off-road crashes by 35 percent.
On high-traction surfaces such as bitumen, or concrete many (though not all) ABS-equipped cars are able to attain braking distances better (i.e. shorter) than those that would be easily possible without the benefit of ABS. Even an alert, skilled driver without ABS would find it difficult, even through the use of techniques like threshold braking, to match or improve on the performance of a typical driver with an ABS-equipped vehicle, in realworld conditions. ABS reduces chances of crashing, and/or the severity of impact. The recommended technique for non-expert drivers in an ABS-equipped car, in a typical full-braking emergency, is to press the brake pedal as firmly as possible and, where appropriate, to steer around obstructions. In such situations, ABS will significantly reduce the chances of a skid and subsequent loss of control.
In gravel and deep snow, ABS tends to increase braking distances. On these surfaces, locked wheels dig in and stop the vehicle more quickly. ABS prevents this from occurring. Some ABS calibrations reduce this problem by slowing the cycling time, thus letting the wheels repeatedly briefly lock and unlock. The primary benefit of ABS on such surfaces is to increase the ability of the driver to maintain control of the car rather than go into a skid — though loss of control remains more likely on soft surfaces like gravel or slippery surfaces like snow or ice. On a very slippery surface such as sheet ice or gravel it is possible to lock multiple wheels at once, and this can defeat ABS (which relies on detecting individual wheels skidding). Availability of ABS relieves most drivers from learning threshold braking.
But part of the answer is that on HEAVY snow, locked wheels can be useful because they gather up a “wedge” of snow which helps to slow the vehicle. ABS allows this wedge to clear every time the wheels are unlocked. The same can apply on sand in some conditions.
Note, however, that this somewhat simplistic test compares ABS with locked wheels. A good driver with a car with a decently designed braking system, designed to minimize the chances of accidentally locking the brakes during a “panic stop”, would fare better under these conditions.
A June 1999 NHTSA study found that ABS increased stopping distances on loose gravel by an average of 22 percent [1].
Other tests shows results that differ from those above when braking on ice. An independent test, with a 1989 Dodge Omni, a small economy car, and a 1995 Pontiac Grand Am equipped with ABS (Mid Sized family Vehicle) The Pontiac matched or had shorter stopping distances on the glare ice, despite being heavier. However, since the vehicles, brakes and tires were different, this is not a completely valid comparison.
When activated, some earlier ABS systems caused the brake pedal to pulse noticeably. As most drivers rarely or never brake hard enough to cause brake lockup, and a significant number rarely bother to read the car’s manual, this may not be discovered until an emergency. When drivers do encounter an emergency that causes them to brake hard and thus encounter this pulsing for the first time, many are believed to reduce pedal pressure and thus lengthen braking distances, contributing to a higher level of accidents than the superior emergency stopping capabilities of ABS would otherwise promise. Some manufacturers have therefore implemented Mercedes-Benz’s “brake assist” system that determines that the driver is attempting a “panic stop” and the system automatically increases braking force where not enough pressure is applied. Nevertheless, ABS significantly improves safety and control for drivers in most on-road situations.
The ABS equipment may also be used to implement traction control on acceleration of the vehicle. If, when accelerating, the tire loses traction with the ground, the ABS controller can detect the situation and take suitable action so that traction is regained. Manufacturers often offer this as a separately priced option even though the infrastructure is largely shared with ABS. More sophisticated versions of this can also control throttle levels and brakes simultaneously.
ABS brakes are the subject of some widely cited experiments in support of risk compensation theory, which support the view that drivers adapt to the safety benefit of ABS by driving more aggressively.
The two major examples are from Munich and Oslo. In both cases taxi drivers in mixed fleets were found to exhibit greater risk-taking behaviour when driving cars equipped with ABS, with the result that collision rates between ABS and non ABS cars were not significantly different.
Given the required reliability it is illustrative to see the choices made in the design of the ABS system. Proper functioning of the ABS system is considered of the utmost importance, for safeguarding both the passengers and people outside of the car. The system is therefore built with some redundancy, and is designed to monitor its own working and report failures. The entire ABS system is considered to be a hard real-time system, while the subsystem that controls the selfdiagnosis is considered soft real-time. As stated above, the general working of the ABS system consists of an electronic unit, also known as ECU (electronic control unit), which collects data from the sensors and drives the hydraulic control unit, or HCU, mainly consisting of the valves that regulate the braking pressure for the wheels.
The communication between the ECU and the sensors must happen quickly and at real time. A possible solution is the use of the CAN bus system, which has been and is still in use in many ABS systems today (in fact, this CAN standard was developed by Robert Bosch GmbH, for connecting electronic control units!). This allows for an easy combination of multiple signals into one signal, which can be sent to the ECU. The communication with the valves of the HCU is usually not done this way. The ECU and the HCU are generally very close together. The valves, usually solenoid valves, are controlled directly by the ECU. To drive the valves based on signals from the ECU, some circuitry and amplifiers are needed (which would also have been the case if the CAN-bus was used).
The sensors measure the position of the tires, and are generally placed on the wheel-axis. The sensor should be robust and maintenance free, not to endager it’s proper working, for example an inductive sensor. These position measurements are then processed by the ECU to calculate the wheel-spin.
The hydraulic control unit is generally located right next to the ECU (or the other way around), and consists of a number of valves that control the pressure in the braking circuits. All these valves are placed closely together and packed in a solid block. This makes for a very simple layout, and is thus very robust.
The central control unit generally consists of two microcontrollers, both active simultaneously, to add some redundancy to the system. These two microcontrollers interact, and check each other’s proper working. These microcontrollers are also chosen to be power-efficient, to avoid heating of the controller which would reduce durability. The software that runs in the ECU has a number of functions. Most notably, the algorithms that drive the HCU as a function of the inputs, or control the brakes depending on the recorded wheel spin. This is the obvious main task of the entire ABS-system. Apart from this, the software also needs to process the incoming information, e.g. the signals from the sensors. There is also some software that constantly tests each component of the ABS system for its proper working. Some software for interfacing with an external source to run a complete diagnosis is also added. As mentionned before the ABS system is considered hard real-time. The control algorithms, and the signal processing software, certainly fall in this category, and get a higher priority than the diagnosis and the testing software. The requirement for the system to be hard real-time can therefore be reduced to stating that the software should be hard real-time. The required calculations to drive the HCU have to be done in time. Choosing a microcontroller that can operate fast enough is therefore the key, preferably with a large margin. The system is then limited by the dynamic ability of the valves and the communication, the latter being noticeably faster. The control system is thus comfortably fast enough, and is limited by the valves.
Anti-lock braking systems use different schemes depending on the type of brakes in use. We will refer to them by the number of channels — that is, how many valves that are individually controlled — and the number of speed sensors.
This system provides individual control of the front wheels, so they can both achieve maximum braking force. The rear wheels, however, are monitored together; they both have to start to lock up before the ABS will activate on the rear. With this system, it is possible that one of the rear wheels will lock during a stop, reducing brake effectiveness.
This system operates the same as the rear end of a three-channel system. The rear wheels are monitored together and they both have to start to lock up before the ABS kicks in. In this system it is also possible that one of the rear wheels will lock, reducing brake effectiveness.
This system is easy to identify. Usually there will be one brake line going through a T-fitting to both rear wheels. You can locate the speed sensor by looking for an electrical connection near the differential on the rear-axle housing.
How Cruise Control Systems Work
Introduction
Cruise control is an invaluable feature on American cars. Without cruise control, long
road trips would be more tiring, for the driver at least, and those of us suffering from
lead-foot syndrome would probably get a lot more speeding tickets.
Cruise control is far more common on American cars than European cars, because the
roads in America are generally bigger and straighter, and destinations are farther
apart. With traffic continually increasing, basic cruise control is becoming less useful,
but instead of becoming obsolete, cruise control systems are adapting to this new
reality — soon, cars will be equipped with adaptive cruise control, which will allow
your car to follow the car in front of it while continually adjusting speed to maintain a
safe distance.
What It Does
The cruise control system actually has a lot of functions other than controlling the
speed of your car. For instance, the cruise control pictured below can accelerate or
decelerate the car by 1 mph with the tap of a button. Hit the button five times to go 5
mph faster. There are also several important safety features — the cruise control will
disengage as soon as you hit the brake pedal, and it won’t engage at speeds less than
25 mph (40 kph).
The system pictured below has five buttons: On, Off, Set/Accel, Resume and Coast. It
also has a sixth control — the brake pedal, and if your car has a manual transmission
the clutch pedal is also hooked up to the cruise control.
The on and off buttons don’t actually do much. Hitting the on button does not
do anything except tell the car that you might be hitting another button soon.
The off button turns the cruise control off even if it is engaged. Some cruise
controls don’t have these buttons; instead, they turn off when the driver hits the
brakes, and turn on when the driver hits the set button.
The set/accel button tells the car to maintain the speed you are currently
driving. If you hit the set button at 45 mph, the car will maintain your speed at
45 mph. Holding down the set/accel button will make the car accelerate; and
on this car, tapping it once will make the car go 1 mph faster.
If you recently disengaged the cruise control by hitting the brake pedal, hitting
the resume button will command the car to accelerate back to the most recent
speed setting.
Holding down the coast button will cause the car to decelerate, just as if you
took your foot completely off the gas. On this car, tapping the coast button
once will cause the car to slow down by 1 mph.
The brake pedal and clutch pedal each have a switch that disengages the
cruise control as soon as the pedal is pressed, so you can shut off the cruise
control with a light tap on the brake or clutch.
How It’s Hooked Up
The cruise control system controls the speed of your car the same way you do — by
adjusting the throttle position. But cruise control actuates the throttle valve by a
cable connected to an actuator, instead of by pressing a pedal. The throttle valve
controls the power and speed of the engine by limiting how much air the One of the
cables is connected to the gas pedal, the other to the vacuum actuator In the
picture above, you can see two cables connected to a pivot that moves the throttle
valve. One cable comes from the accelerator pedal, and one from the actuator. When
the cruise control is engaged, the actuator moves the cable connected to the pivot,
which adjusts the throttle; but it also pulls on the cable that is connected to the gas
pedal — this is why your pedal moves up and down when the cruise control is
engaged.
The
electronical
lycontrolled
vacuum
actuator
that
controls the
throttle
Many cars use actuators powered by engine vacuum to open
and close the throttle. These systems use a small, electronically-controlled valve to
regulate the vacuum in a diaphragm. This works in a similar way to the brake booster,
which provides power to your brake system.
Controlling the Cruise Control
The brain of a cruise control system is a small computer that is normally found under
the hood or behind the dashboard. It connects to the throttle control seen in the
previous section, as well as several sensors. The diagram below shows the inputs and
outputs of a typical cruise control system.
A good cruise control system accelerates aggressively
to the desired speed without overshooting, and then
maintains that speed with little deviation no matter
how much weight is in the car, or how steep the hill
you drive up. Controlling the speed of a car is a
classic application of control system theory. The
cruise control system controls the speed of the car by
adjusting the throttle position, so it needs sensors to
tell it the speed and throttle position. It also needs to
monitor the controls so it can tell what the desired
speed is and when to disengage.
The most important input is the speed signal; the
cruise control system does a lot with this signal. First,
let’s start with one of the most basic control systems
you could have — a proportional control.
Proportional Control
In a proportional control system, the cruise control
adjusts the throttle proportional to the error, the error
being the difference between the desired speed and
the actual speed. So, if the cruise control is set at 60
mph and the car is going 50 mph, the throttle position
will be open quite far. When the car is going 55 mph,
the throttle position opening will be only half of what
it was before. The result is that the closer the car gets
to the desired speed, the slower it accelerates. Also, if you were on a steep enough
hill, the car might not accelerate at all.
PID Control
Most cruise control systems use a control scheme called proportional-integralderivative
control (a.k.a. PID control). Don’t worry, you don’t need to know any
calculus to make it through this explanation — just remember that:
The integral of speed is distance.
The derivative of speed is acceleration.
A PID control system uses these three factors — proportional, integral and derivative,
calculating each individually and adding them to get the throttle position.
We’ve already discussed the proportional factor. The integral factor is based on the
time integral of the vehicle speed error. Translation: the difference between the
distance your car actually traveled and the distance it would have traveled if it were
going at the desired speed, calculated over a set period of time. This factor helps the
car deal with hills, and also helps it settle into the correct speed and stay there. Let’s
say your car starts to go up a hill and slows down. The proportional control increases
the throttle a little, but you may still slow down. After a little while, the integral
control will start to increase the throttle, opening it more and more, because the longer
the car maintains a speed slower than the desired speed, the larger the distance error
gets.
Now let’s add in the final factor, the derivative. Remember that the derivative of
speed is acceleration. This factor helps the cruise control respond quickly to changes,
such as hills. If the car starts to slow down, the cruise control can see this acceleration
(slowing down and speeding up are both acceleration) before the speed can actually
change much, and respond by increasing the throttle position.
Adaptive Cruise Control
Two companies are developing a more advanced cruise control that can automatically
adjust a car’s speed to maintain a safe following distance. This new technology, called
adaptive cruise control, uses forward-looking radar, installed behind the grill of a
vehicle, to detect the speed and distance of the vehicle ahead of it.
Adaptive cruise control is similar to conventional cruise control in that it maintains
the vehicle’s pre-set speed. However, unlike conventional cruise control, this new
system can automatically adjust speed in order to maintain a proper distance between
vehicles in the same lane. This is achieved through a radar headway sensor, digital
signal processor and longitudinal controller. If the lead vehicle slows down, or if
another object is detected, the system sends a signal to the engine or braking system to
decelerate. Then, when the road is clear, the system will re-accelerate the vehicle back
to the set speed.
The 77-GHz Autocruise radar system made by TRW has a forward-looking range of
up to 492 feet (150 meters), and operates at vehicle speeds ranging from 18.6 miles
per hour (30 kph) to 111 mph (180 kph). Delphi’s 76-GHz system can also detect
objects as far away as 492 feet, and operates at speeds as low as 20 mph (32 kph).
Adaptive cruise control is just a preview of the technology being developed by both
companies. These systems are being enhanced to include collision warning
capabilities that will warn drivers through visual and/or audio signals that a collision
is imminent and that braking or evasive steering is needed.
Advantages and disadvantages
Cruise control has many advantages but also some serious vices.
Some of those advantages include:
Its usefulness for long drives across sparsely populated roads. This usually
results in better fuel efficiency.
Some drivers use it to avoid unconsciously violating speed limits. A driver
who otherwise tends to unconsciously increase speed over the course of a
highway journey may avoid a speeding ticket. Such drivers should note,
however, that a cruise control may go over its setting on a downhill which is
steep enough to accelerate with an idling engine.
However, cruise control can also lead to accidents due to several factors, such as:
The lack of need to maintain constant pedal pressure, which can help lead to
accidents caused by highway hypnosis or incapacitated drivers; future systems
may include a dead man’s switch to avoid this.
When used during inclement weather or while driving on wet or snow- and/or
ice-covered roads, the vehicle not equipped with Electronic_Stability_Control
could go into a skid. Stepping on the brake — such as to disengage the cruise
control — often results in the driver losing control of the vehicle.
ABSTRACT:
Cryogens are effective thermal storage media which, when used for automotive purposes, offer significant advantages over current and proposed electrochemical battery technologies, both in performance and economy. An automotive propulsion concept is presented which utilizes liquid nitrogen as the working fluid for an open Rankine cycle. When the only heat input to the engine is supplied by ambient heat exchangers, an automobile can readily be propelled while satisfying stringent tailpipe emission standards. Nitrogen propulsive systems can provide automotive ranges of nearly 400 kilometers in the zero emission mode, with lower operating costs than those of the electric vehicles currently being considered for mass production. In geographical regions that allow ultra low emission vehicles, the range and performance of the liquid nitrogen automobile can be significantly extended by the addition of a small efficient burner. Some of the advantages of a transportation infrastructure based on liquid nitrogen are that recharging the energy storage system only requires minutes and there are minimal environmental hazards associated with the manufacture and utilization of the cryogenic “fuel.”
INTRODUCTION:
Under present regulations, a Zero Emission Vehicle is one that does not produce any tailpipe pollutants, regardless of the emissions produced in the manufacture of the vehicle or in generating the electricity to recharge its energy storage system. Some of the available technologies for storing energy that meet these qualifications are electrochemical batteries, fuel cells, and flywheels. Improvements in each of these energy storage systems continue to be made; however, only the electrochemical battery has reached a high enough state of development to be considered useful and practical in a large EV fleet. Among the different battery concepts being developed, the lead-acid battery still appears to offer the best compromise of performance, utility, and economy for transportation applications.
A significant fraction of the lead produced each year is used in lead-acid batteries for automobiles and a typical EV requires 20-30 conventional batteries to have a useful range. The growing public awareness of the health hazards arising from elevated concentrations of lead in the environment has resulted in a steady decrease in the amounts of lead used in industry and personal products over the years. In addition, it has been found that the pollution control practices at the mine heads and ore smelters have not prevented serious degradation of their surroundings. Thus, it is highly probable that the environmental impact of the increased mining and refining of lead ore, to meet the needs of a transportation infrastructure based on lead-acid batteries, could completely negate the benefits expected from the elimination of tailpipe emissions. It appears that liquid nitrogen (LN2) can be used in a zero-emission propulsion system that is as effective and probably more economical to operate than the high performance battery systems currently under development.
THEORY BEHIND CRYOCAR:
The basic idea of the LN2 propulsion system is to utilize the atmosphere as a heat source and a cryogen as a heat sink in a thermal power cycle. This is in contrast to typical thermal engines which utilize an energy source at temperature significantly above ambient and use the atmosphere as a heat sink. In both cases the efficiency of conversion of thermal energy of the source to work (W) is limited by the Carnot efficiency, h = W/Qh = 1 - Tl/Th, where Qh is heat input, Tl is the sink temperature, and Th is the temperature of the heat source. By using liquid nitrogen as the cryomobile energy sink (Tl = 77 K) this ideal thermal efficiency is impressively high (74%) with an atmospheric heat source at Th = 300 K. The key issues are the ability to design a practical energy conversion system that can take advantage of this high efficiency and the available energy of the cryogen while still being cost competitive with alternative EVs.
Power Cycle
The Rankine cycle is among the most attractive choices for approximating Carnot performance, when using a fixed temperature heat source and sink.
Fig. 2 Temperature-entropy diagram of Rankine cycle using LN2 for working fluid.
We have focused on directly using the nitrogen itself as the working fluid, wherein the liquid is compressed with a cryogen pump, heated and vaporized by heat exchange with the atmosphere, and then expanded in a piston-cylinder engine. State 1 is the cryogenic liquid in storage at 0.1 MPa and 77 K. The liquid is pumped up to system pressure of 4 MPa (supercritical) at state 2 and then enters the economizer. State 3 indicates N2 properties after its been preheated by the exhaust gas. Further heat exchange with ambient air brings the N2 to 300 K at state 4, ready for expansion. Isothermal expansion to 0.11 MPa at state 5 would result in the N2 exhaust having enough enthalpy to heat the LN2 to above its critical temperature in the economizer, whereas adiabatic expansion to state 6 would not leave sufficient enthalpy to justify its use. The specific work output would be 320 and 200 kJ/kg-LN2 for these isothermal and adiabatic cycles, respectively, without considering pump work. While these power cycles do not make best use of the thermodynamic potential of the LN2, they do provide specific energies competitive with those of lead-acid batteries.
WORKING:
The LN2 propulsion system for a ZEV operates very much like a conventional steam engine while taking advantage of machinery designed for sub ambient temperature applications. Instead of using a steam jacket to minimize the heat loss through the steam cylinder walls, a LN2 expander will have circulating fluid to maintain the wall temperature as high as possible to enhance heat transfer during the power stroke.
A schematic of a Rankine cycle propulsion system using only ambient air to heat the cryogenic working fluid is shown in Fig. 1. An insulated “fuel” tank contains LN2 at atmospheric pressure and a temperature of 77 K. A cryogenic pump draws LN2 from the bottom of the tank and compresses it to the operating pressure of the system.
Pressurized LN2 then passes through the heat exchanger, which is optimized as a LN2 vaporizer and N2 super heater, to raise the gas temperature to just below ambient conditions. The gaseous N2 is then injected into the cylinder as the piston approaches top dead center. It is possible for multiple expansion strokes and reheats to be utilized, at the expense of mechanical complexity, to approach quasi-isothermal performance. If the exhaust gas has sufficient enthalpy it passes through an economizer before being vented to atmosphere. The nitrogen condensation phase closing the Rankine cycle occurs at stationary air liquefaction plants.
This zero emission propulsion concept offers many environmental advantages over internal combustion engines and electrochemical battery vehicles. It has low operating costs, ample propulsive power, and reasonable round trip energy efficiency. We refer to this ZEV as the “cryomobile.”
Application to Automobile Propulsions:
Peak injection pressures of 4 MPa and peak cycle temperature of 300 K are used . Estimates of the mass and volume of the LN2 required for a given range are based on a vehicle that requires the same amount of road power during freeway cruise i.e., 7.8 kW at 97 km/h (60 mi/h). The “fuel” operating costs are based on the economics of supplying LN2 from a plant optimized for its production (2.6¢ per kg-LN2), as discussed in Section II. For the specified cruise conditions, the propulsion system having an isothermal expansion process will consume LN2 at a rate of 25 gm/sec and have an operating cost of approximately 2.4¢ per kilometer. The corresponding storage volume and mass of LN2 required to provide a maximum range of 300 km is 400 liters (106 gal) and 280 kg, respectively. Thus the cryogenic storage tank can readily fit within the trunk volume of a conventional automobile.
Not only are the operating costs of the cryogen propulsion systems lower than those for the EV concepts discussed above, they are also competitive with the advanced battery systems currently being developed. The LN2 storage system compares favorably with the lead-acid battery on a per mass basis. Indeed, if near-isothermal performance can be achieved, the specific energy characteristics become particularly attractive. In addition, since the fully loaded LN2 tank would comprise about 25% of the total vehicle mass (30% is typical mass ratio for lead-acid battery systems), the cryomobile performance should increase as the cryogen is consumed.
Components Of LN2 Vehicle:
Expander:
The maximum work output of the LN2 engine results from an isothermal expansion stroke. Achieving isothermal expansion will be a challenge, because the amount of heat addition required during the expansion process is nearly that required to superheat the pressurized LN2 prior to injection. Thus, engines having expansion chambers with high surface-to-volume ratios are favored for this application. Rotary expanders such as the Wankel may also be well suited. A secondary fluid could be circulated through the engine block to help keep the cylinder walls as warm as possible. Multiple expansions and reheats can also be used although they require more complicated machinery.
Vehicle power and torque demands would be satisfied by both throttling the mass flow of LN2 and by controlling the cut-off point of N2 injection, which is similar to how classical reciprocating steam engines are regulated. The maximum power output of the propulsion engine is limited by the maximum rate at which heat can be absorbed from the atmosphere. The required control system to accommodate the desired vehicle performance can be effectively implemented with either manual controls or an on-board computer. The transient responses of the LN2 power plant and the corresponding operating procedures are topics to be investigated.
Heat Exchanger
The primary heat exchanger is a critical component of a LN2 automobile. Since ambient vaporizers are widely utilized in the cryogenics and LNG industries, there exists a substantial technology base. Unfortunately, portable cryogen vaporizers suitable for this new application are not readily available at this time. To insure cryomobile operation over a wide range of weather conditions, the vaporizer should be capable of heating the LN2 at its maximum flow rate to near the ambient temperature on a cold winter day. For an isothermal expansion engine having an injection pressure of 4 MPa, the heat absorbed from the atmosphere can, in principle, be converted to useful mechanical power with about 40% efficiency. Thus the heat exchanger system should be prudently designed to absorb at least 75 kW from the atmosphere when its temperature is only 0°C.
To estimate the mass and volume of the primary heat exchanger, it was modeled as an array of individually fed tube elements that pass the LN2 at its peak flow rate without excessive pressure drop. Each element is a 10 m long section of aluminum tubing having an outside diameter of 10 mm and a wall thickness of 1 mm. They are wrapped back and forth to fit within a packaging volume having 0.5 m x 0.4 m x 0.04 m dimensions and are arrayed in the heat exchanger duct as shown. Incoming air will pass through a debris deflector and particulate filter before encountering the elements. An electric fan will draw the air through the ducts when the automobile is operating at low velocities or when above normal power outputs are required.
The atmospheric moisture will be removed relatively quickly as the ambient air is chilled over the first few tube rows, leaving extremely dry air to warm up the coldest parts at the rear of the heat exchanger where the LN2 enters. Surface coatings such as Teflon can be used to inhibit ice build up and active measures for vibrating the tube elements may also be applied. However, these approaches may not be necessary since high LN2 flow rates are only needed during times of peak power demand and the heat exchanger elements are much longer than necessary to elevate the LN2 temperature to near ambient at the lower flow rates required for cruise. Thus, the frosted tube rows may have ample opportunity to de-ice once the vehicle comes up to speed.
Cryogen Storage Vessel
The primary design constraints for automobile cryogen storage vessels are: resistance to deceleration forces in the horizontal plane in the event of a traffic accident, low boil-off rate, minimum size and mass, and reasonable cost. Crash-worthy cryogen vessels are being developed for hydrogen-fueled vehicles that will prevent loss of insulating vacuum at closing speeds of over 100 km/h. Moderately high vacuum (10-4 torr) with super insulation can provide boil-off rates as low as 1% per day in 200 liter (53 gal) containers. Using appropriate titanium or aluminum alloys for the inner and outer vessels, a structurally reinforced dewar could readily have a seven-day holding period.
Range Extension and Power Boosting
Range extension and performance enhancement can be realized by heating the LN2 to above ambient temperatures with the combustion of a relatively low pollution fuel such as ethanol or natural gas. The augmentation of power output is most apparent for the adiabatic expansion engine. To evaluate the performance enhancement potential for an isothermal engine, the high temperature N2 is assumed to polytropically expand to the end state reached when the engine is operating isothermally at ambient temperature. In this particular propulsive cycle an extra superheat of 200°C results in only a 30% increase in specific power. Thus the advantage of operating above ambient temperature depends, in part, on how isothermal the expansion process can be made to be.
There is also the intriguing possibility of storing energy for boosting power or extending range by applying a medium that undergoes a phase change to the final super heater segment of the heat exchanger system. Ideally the phase change material would be slowly “recharged” as it absorbs heat from the atmosphere while the vehicle is parked and during cruise when peaking power is not required. Fast recharging with electric heaters may also be considered. We recognize that this added complexity must compete in mass and compactness with the alternative of just carrying more LN2.
While extremely cold weather would degrade performance of a LN2 propulsion system, this would not diminish the cryomobile’s advantage over most battery EVs since their performance is severely compromised in cold weather. Below freezing, air temperatures are extremely warm to the cryogen “fuel” and the moisture content of the atmosphere is significantly diminished. Thus it is anticipated that sufficient enthalpy can still be drawn from the air to provide ample power without incurring a detrimental icing penalty. If necessary, an auxiliary combustor can be added that would allow continuous use of the vehicle on the very coldest days.
COMPARISION WITH EVs:
It is useful to compare the performance capabilities of the LN2 propelled vehicle with two other EV concepts: General Motor’s new electric car called the “Impact” and a Honda CRX that was converted to operate with an advanced electric propulsion system. The Impact is an optimized EV that utilizes advanced lead-acid batteries, special tires for low rolling resistance, and a streamlined body having a very low drag coefficient. This two-seater car contains 400 kg of batteries to get a maximum range of 240 km between recharges and it has an effective operating cost of 5¢/km (8¢/mi). The modified CRX uses a pack of 28 lead-acid batteries, which have a total mass of 500 kg and occupy a volume of 240 liters (63 gal). The maximum range for this EV at cruise is 180 km and its effective operating cost is 28¢/km (45¢/mi). If the CRX were modified with an LN2 propulsion system and the cryogen storage vessel occupied the same volume as the lead-acid batteries, then this vehicle would have the same range while saving 300 kg in energy storage mass.
