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.
