ExhausTEC FEATURES

• ExhausTEC is incorporated in Silencer to induct optimum

volume of air-fuel mixture inside combustion chamber at mid

range engine

revolutions i.e. @ 3000-4000 rpm for improving low end torque and

drive-ability.

ExhausTEC – Advantages and Benefits

Advantages

• Improved volumetric efficiency.

• Improved & optimized low end torque.

• Improved Scavenging.

Benefits

• Less frequent gear shifting in city driving conditions.

• Less frequent clutch operation.

• Subsequent increase in fuel efficiency (Mileage).

• Better engine performance – Power & Pick-up.

• Improved drive-ability especially at low and mid range engine

R.P.M.

ExhausTEC – The unique ExhausTEC

technology allows you to rev up when your

heart feels like and the engine will pick up

instantaneously irrespective of the gear you

are in. It improves engine torque even at low

rpms and is optimized to get maximum

performance from the engine. Gives a feeling

of abundant latent power at any stage of

riding, which ensures effortless pulling for

any load conditions.

• So wats the cost of installing the ExhausTEC on pulsar dtsi v1??

IT WILL COST AROUND 3600/- , FROM AUTHORISED

DEALER.THE PORFORMENCE WILL INCREASE A BIT, ENGIN

NOTE WILL CHANGE AND MARGINALINCREASE IN EFFICINCY.

• its exhaust TEC = Torque Expansion Chamber 80 % of

the exhaust is allowed to leave from the Bike n rest 20%

is accumulated n sent back into the chamber which hits

the pistons to give u that extra torque n push/ jerk when u

throttle. only makes a little difference in the initial pick

up of the pulsar, which every1 brags about

Why would spent exhaust gases be sent into the exhaust

chamber? What purpose does it serve?

The final stroke is the exhaust stroke where the exhaust valve

opens as the piston moves from BDC to TDC. How will the gases

enter the chamber?? The exhaust valve would close as soon the

piston comes down as the intake valve opens.

i am not really 100% sure abt the principle behind the

ExhausTEC. but it has something to do with the generation of a

secondary pressure wave which helps the silencer to scavenge the

spent gases fully from the cylinder, so that during the intake

stroke there is more volume for the fresh charge. more fresh

charge, more BANG and more power.

Induction of optimum volume of

air-fuel mixture i.e. Improved

volumetric efficiency.

ExhausTEC

ExhausTEC stands for Exhaust Torque Expansion Chamber, a Bajaj Auto trademark.

The technology involves use of a small chamber connected to the exhaust pipe of the

engine to modify the back-pressure and the swirl characteristics, with an aim to improve

the low-end performance of the bikes. This was attempted in response to the issue of a

reported lack of low-end response in Bajaj’s single-cylinder four-stroke engines. The

ExhausTEC technology is claimed to be highly effective in improving the overall

engine response, especially the low-end torque characteristics. This enhanced

performance is claimed to come at no loss of top-end performance or engine

smoothness.

Expansion chamber

An Expansion chamber is an exhaust system used on a two-stroke cycle engine to

enhance its power output by improving its volumetric efficiency. It makes use of the

energy left in the burnt exhaust exiting the cylinder to aid the filling of the cylinder for

the next cycle. It is the two-stroke equivalent of the tuned pipes (or headers) used on

four-stroke cycle engines.

How it works

The high pressure gas exiting the cylinder initially flows in the form of a “wavefront” as

all disturbances in fluids do. The exhaust gas pushes its way into the pipe which is

already occupied by gas from previous cycles, pushing that gas ahead and causing a

wave front. Once the gas flow itself stops, the wave continues on by passing the energy

to the next gas down stream and so on to the end of the pipe. If this wave encounters

any change in cross section or temperature it will reflect a portion of its strength in the

opposite direction to its travel. For example a high pressure wave encountering an

increase in area will reflect back a low pressure wave in the opposite direction. A high

pressure wave encountering a decrease in area will reflect back a high pressure wave in

the opposite direction. The basic principle is described in wave dynamics. An expansion

chamber makes use of this phenomenon by varying its diameter (cross section) and

length to cause these reflections to arrive back at the cylinder at the desired times in the

cycle.

Parts of an expansion chamber

There are three main parts to the expansion cycle.

Blowdown

When the descending piston first exposes the exhaust port on the cylinder wall, the

exhaust flows out powerfully due to its own pressure without assistance from the

expansion chamber and so the diameter/area over the length of the first portion of the

pipe is constant or near constant with a divergence of 0 to 2 degrees which preserves

wave energy. This section of the system is called the “head pipe” (the exhaust port

length is considered part of the head pipe for measurement purposes). By keeping the

head pipe diameter near constant, the energy in the wave is preserved because there is

no expansion until needed later in the cycle. In any case the flow leaving the cylinder

during most of the blowdown process is sonic or supersonic and therefore no wave

could travel back into the cylinder against that flow.

Transfer

Once the exhaust pressure has fallen to near atmospheric level the piston uncovers the

transfer ports. At this point energy from the expansion chamber can be used to aid the

flow of fresh mixture into the cylinder. To do this the expansion chamber is increased in

diameter so that the out going high pressure wave reflects a negative pressure wave

back toward the cylinder. This negative pressure arrives in the cylinder during the

transfer cycle and greatly increases the flow of fresh mixture into the cylinder and can

even suck fresh mixture out into the headpipe. This part of the pipe is called the

divergent (or diffuser) section and it diverges at 6 to 12 degrees. It may be made up of

more than one diverging cone depending on requirements.

Port blocking

When the transfer is complete the piston is on the way back up on its compression

stroke but the exhaust port is still open, an unavoidable problem with the two stroke

design. To help prevent the piston pushing fresh mixture out the open exhaust port a

strong high pressure wave from the expansion chamber is timed to arrive during the

compression stroke. The port blocking wave is created by reducing the diameter of the

chamber. This is called the convergent section (a.k.a baffle cone or section). The

outgoing high pressure wave hits the narrowing convergent section and reflects back a

high pressure wave to the cylinder which arrives in time to block the port during the

compression stroke and can push back into the cylinder any fresh mixture drawn out

into the head pipe. The convergent section is made to converge at 8 to 90 degrees

depending on requirements.

Combined with the high pressure wave there is a general rise in pressure in the chamber

caused by deliberately restricting the outlet with a small tube called the stinger. The

stinger restricts flow out of the chamber to cause higher pressure during the

compression cycle and empties the chamber during the compression/power stroke to

ready it for the next cycle. The stingers length and inside diameter are selected to match

the engines requirements. (The inside diameter has the greatest effect and so is the most

sensitive of the two.)

Complicating Factors

The operation of expansion chambers in practice is not as straightforward as described

above. Waves traveling back up the pipe encounter the divergent section in reverse and

reflect a portion of their energy back out. Temperature variations in different parts of

the pipe cause reflections and changes in the local speed of sound. Sometimes these

secondary wave reflections can inhibit the desired goal of more power.

It is useful to keep in mind that although the waves traverse the entire expansion

chamber over each cycle, the actual gasses leaving the cylinder during a particular cycle

do not. The gas flows and stops intermittently and the wave continues on to the end of

the pipe. The hot gasses leaving the port form a “slug” which fills the header pipe and

remains there for the duration of that cycle. This causes a high temperature zone in the

head pipe which is always filled with the most recent and hottest gas. Because this area

is hotter, the speed of sound and thus the speed of the waves that travel theough it are

increased. During the next cycle that slug of gas will be pushed down the pipe by the

next slug to occupy the next zone and so on. The volume this “slug” occupies constantly

varies according to throttle position and engine speed. It is only the wave energy itself

that traverses the whole pipe during a single cycle. The actual gas leaving the pipe

during a particular cycle was created two or three cycles earlier.

Expansion chambers almost always have turns and curves built into them to

accommodate their fit within the engine bay. Gasses and waves do not behave in the

same way when encountering turns. Waves travel by reflecting and spherical radiation.

Turns causes a loss in the sharpness of the wave forms and therefore must be kept to a

minimum to avoid unpredictable losses.

Calculations used to design expansion chambers take into account only the primary

wave actions. This is usually fairly close but errors can occur due to these complicating

factors.

How Expansion chambers are made

There are three main methods of fabricating expansion chambers.

Hand formed

Flat sheet metal is rolled into cones and round sections, which are then welded together

section by section. Although time consuming, it is usually the method chosen for

development of a new design due to its flexibility, accuracy and low tooling costs.

Hydro forming

Two flat representations of the required finished pipe are cut out of sheet metal. The

edges of the two identical flat cutouts are welded together forming a sandwich. On one

end of the pipe a fitting is welded and high-pressure water is pumped into the cavity

between the sheets. The pressure inflates the flat sheet into its final rounded shape. This

method can be quicker than hand forming and only slightly more costly in tooling,

however it requires a number of trials before a finished design as accurate as hand

formed or stamped can be produced. All curves must be made in a single plane so

cutting apart and re-welding is often required but the final product can be as good as a

stamped pipe if enough care is taken to be precise.

Stamping

Flat sheet metal is pressed between a male and female mold in the shape of the required

pipe. Each half of the pipe is stamped this way and the two halves are welded together.

Stamping requires expensive tooling and machinery and is used only for mass

production.

(Note-Functionally, expansion chambers need not be round in cross section but in

practice a round shape is the best acoustically and is the only shape which (at a

reasonable weight) can withstand the intense vibration and pounding without cracking.)

Summary

All these events need to be synchronized with the engine port timings and speed. An

expansion chamber “tuned” for 8,000 rpm will not deliver the proper wave timings at

4,000 or 11,000 rpm. In fact it is likely to incur a power loss outside its “tuned” range.

The length of the pipe determines at what time the waves arrive back at the cylinder.

Longer pipes require more time for the waves to traverse and so will be tuned to a lower

rpm than a shorter pipe. The shorter the pipe the higher the rpm it is tuned to.

The rate of convergence/divergence of the cones determines the duration of the wave

returned. A gentle taper give a long duration but weaker return wave while a steeper

taper gives a short but strong return wave. The longer the wave, the broader the RPM

range at which it is useful. This extra power band width is at the sacrifice of peak

torque.

The diameter of the center or dwell section determines the ratio of scavenging suction

to port blocking pressure as well as the over all energy recovery. The resulting volume

determines the maximum pressure rise with large volumes giving less pressure rise. The

fatter the pipe the harder it sucks but the weaker the blocking pressure. Thinner pipes

will scavenge less but block the port very strongly. The optimum diameter is related to

compression ratio, the quality of the transfer port layout and its scavenging efficiency.

A variety of devices are used to try to extend the tuned range of the expansion chamber.

Pipes that slide like a trombone adjust the timing to match the rpm changes of the

running engine. Devices that control the exhaust port timing to vary blowdown duration

as well as extending the tuned range of the expansion chamber. Valves that open at

certain speeds to absorb or dump waves arriving at undesirable times.

Another approach to altering the tuned RPM of an expansion chamber is to alter the

speed of the pressure waves inside the exhaust pipe. The speed at which pressure waves

travel is greatly affected by temperature: higher temperature means faster wave speed.

As a result, expansion chambers can be retuned for higher-than-design RPM resonance,

by increasing the average temperature of the exhaust gases inside the pipe. Techniques

to achieve this increase in gas temperature can include: insulating the pipe (thermal

wrap), restricting flow from the pipe (smaller stinger diameter), or by retarding the

ignition timing at the correct RPM (a later burn allows more heat to escape into the

pipe).

Conversely, a pipe can be retuned to work at a lower-than-design RPM range by

reducing the temperature of the exhaust gases. Injecting water or a water-alcohol mix

into the headpipe of an expansion chamber can reduce temperatures significantly,

enough to lower the tuned RPM of an exhaust system by as much as 1500- 2000 RPM.

The heat absorbed as the liquid changes into a gas is responsible for the drop in

temperature. As a result, the two stroke exhaust can be tuned to stay “on the pipe” over

a remarkably wide RPM range, if the designer takes advantage of all the tools available.

Retrieved from “http://en.wikipedia.org/wiki/Expansion_chamber”

Expansion Chamber

The expansion chamber can be considered as a simple low

pass filter. The transmission loss performance is

determined by the cross-sectional area ratio between the

inlet and outlet ducts and the volume. With the single

expansion chamber the transmission loss falls to zero when

a half wavelength or multiple thereof equates to the length

of the chamber.

Extending the inlet and outlet pipes into the volume will give a more abrupt impedance

change and hence a larger transmission loss.

The transmission loss is given by:

where

f = frequency [Hz]

fn = resonant frequency [Hz]

c = speed of sound [ms-1]

S = cross-sectional area of inlet and outlet ducts [m2]

Sc = cross-sectional area of expansion chamber [m2]

Lc = length of expansion chamber [m]

Due Tempi & Le Espansioni Theory

How Two Strokes and Expansion Chambers Work

(I think this fabulous animation was created by Joseph Schuster. It doesn’t show a Rumi

engine, but this is a similar design.)

A two stroke (“due tempi” in Italian) is an engine in which each piston moves 2 stokes

for each firing of the spark plug. Two stoke engines do not have valves or the

associated cams. The intake of air and fuel and exhaust are controlled by ports in the

cylinder walls that are covered and uncovered by the movement of the piston.

Compared to 4 stroke engines, 2 stroke engines are light (no valve train) and they fire

twice as often creating more power than a similar 4 stroke. Additionally, oil is mixed

with the fuel (called premix) and is used to lubricate the crankcase. The animation

above illustrates the engines function.

When the piston moves toward the spark plug (up), the following things happen:

Below the piston, air and fuel (along with the premixed oil) are sucked into the

crankcase through the intake port uncovered by the piston. The oil in the fuel lubricates

the big and small ends of the crank, cylinder walls and other bearings in the crankcase.

Above the piston the waste gasses are forced out the exhaust port, and after that port is

closed off by the rising piston the charge is compressed for ignition.

After ignition the piston moves away from the spark plug (down) and the following

things happen: The power generated by the ignition of the charge turns the crank.

Above the piston the spent charge starts to escape out the exhaust port as it is uncovered

by the piston. Below the piston the fuel and air mixture in the crankcase is being

compressed. The intake port opens next as the piston uncovers it and the air and fuel is

forced through the intake port into the combustion chamber. This charge being forced

into the combustion chamber also pushes the spent charge out the exhaust port.

Now if that all makes sense, it may be clear that there is inefficiency with the intake and

exhausts ports: both happen to be open at the same time during some of the process!

Why doesn’t the intake charge just slide across the piston and out the exhaust port?

Well, to some degree it does. Note that Rumi’s have a deflector on the piston crown; a

ridge that is designed to divert the charge some and help prevent the intake from just

sliding through the combustion chamber. But this is where the expansion chamber

exhaust comes into effect. The expansion chamber is designed so that at certain RPMs

the exhaust gasses resonate just right and help suck out the spent gasses and then reflect

back and compress the charge back into the combustion chamber! This is what two

stroke riders mean when they say the engine “comes onto the pipe” at the right RPM the

engine becomes much more powerful as the expansion chamber starts to help the

engine run much more efficiently. Obviously, this effect can greatly improve a two

stroke engine’s performance!

Note that the engine in the illustration above has a reed valve between the carburetors

and the crankcase. This valve prevents the intake charge from escaping out the

carburetors. Rumi’s do not have this valve and this explains why Rumi’s never rust

behind the engine: they are always covered by a thin film of oil that escapes the

carburetors’!