Exhaust length affecting power output ?

[soumyakd]

Can anyone please explain me as to how the change in length of exhaust lines affect the power output of an engine ? Was going through a forum regarding a recent change in exhaust lengths and how it boosted the performance of the engine. Is it due to back pressure accumulation in the lines ?

[Neno]

Can anyone please explain me as to how the change in length of exhaust lines affect the power output of an engine ? Was going through a forum regarding a recent change in exhaust lengths and how it boosted the performance of the engine. Is it due to back pressure accumulation in the lines ?

i think it's have something with exaust pipe, engine is always in same place in car. but you can change exaust pipe in her length, with shorter pipe engine work optimal but not producing as near much gasses from exaust, same is for revert situation. When exaust pipe is long, you suffocate pure engine power for more gasses. It's just matter of preferences if you want bigger power or gasses from exaust for coanda.

[matt21]

it has to do with pressure waves.

When the exhaust valve opens, a positive pressure wave is running down the exhaust pipe. At the open end of the exhaust this wave is reflected, but with opposite sign. So you have a negative pressure valve runnig up the exhaust towards the valve.
At the valve this wave can help drawing gas out of the cylinder.

Without variable length geometries you can tune your exhaust system for one rpm only. In general shorter exhaust pipe shift the resonance to higher rpm and vice versa.

The same effect is used at the inlet tract where you have a negative pressure wave when the valve opens.

You can also use helmholtz resonators to tune the system.

[Kiril Varbanov]

Extensive article on exhaust applications in different motorsport divisions.

[olefud]

it has to do with pressure waves.

When the exhaust valve opens, a positive pressure wave is running down the exhaust pipe. At the open end of the exhaust this wave is reflected, but with opposite sign. So you have a negative pressure valve runnig up the exhaust towards the valve.
At the valve this wave can help drawing gas out of the cylinder.

Without variable length geometries you can tune your exhaust system for one rpm only. In general shorter exhaust pipe shift the resonance to higher rpm and vice versa.

The same effect is used at the inlet tract where you have a negative pressure wave when the valve opens.

You can also use helmholtz resonators to tune the system.

This is pretty much my view except for “…help drawing gas out of the cylinder”. The inertia of the exhaust gas flow can maintain flow after ΔP drops. Pressure waves are acoustical and a different animal.

As I see it, the pressure waves are rarefied and dense areas rather like the peak and trough of waves on water. Rather than responding to pressure differences, acoustical waves position more and less dense pulses within the flow. Further, the combustion chamber and intake are part of the system. As stated the high and low density pulses change sign when passing from a pipe to a larger cross section.

The significant action occurs when the reflected negative pulse reaches the open exhaust valve and, again changing sign to a positive pulse or wave as it moves into the larger cross section cylinder, interacts with the intake port –the intake and exhaust valves both being open during overlap- sets up a positive intake pulse within the combustion chamber. This denser fuel mixture pulse or wave increases the volumetric efficiency and, along with the inertial exhaust flow, provides a useful fuel mixture charge before the piston actually starts to pump additional fuel mixture.

All this depends on being properly timed and having appreciable exhaust inertia which is why particularly lower RPM power suffers with a tuned cam and pipe combination.

[matt21]

This is pretty much my view except for “…help drawing gas out of the cylinder”.

I have to disagre here. The effect of reflecting pressure waves, either with same value or in verted, is used extensively in the tuning of an two-stroke engine, hence the use of expansion champers. By using these chambers you can use the effect for a wider range of rpm, instead for an exhaust pipe with fixed diameter and length. Still you have one optimum rpm, where the effect is the greatest.

For multi-cylinder engines, also the joint of the primary exhaust pipes is of grat importance. This joint acts as open end to the primary pipe and you have interaction between the single cylinders.

For two stroke chambers:


Further info:
http://www.motorcycle.com/how-to/how-tw ... -3423.html

[olefud]

This is pretty much my view except for “…help drawing gas out of the cylinder”.

I have to disagre here. The effect of reflecting pressure waves, either with same value or in verted, is used extensively in the tuning of an two-stroke engine, hence the use of expansion champers. By using these chambers you can use the effect for a wider range of rpm, instead for an exhaust pipe with fixed diameter and length. Still you have one optimum rpm, where the effect is the greatest.

For multi-cylinder engines, also the joint of the primary exhaust pipes is of grat importance. This joint acts as open end to the primary pipe and you have interaction between the single cylinders.

For two stroke chambers:
http://www.spudfiles.com/forums/files/2 ... st_852.jpg

Further info:
http://www.motorcycle.com/how-to/how-tw ... -3423.html

There are (at least) two ways the exhaust can affect power; inertial flow and acoustical pressure waves. Inertial exhaust flow is effective to scavenge residual exhaust gases and to initiate intake flow –though it can scavenge the intake flow too. However, inertial flow is not dependent on the exhaust pipe length.

Acoustical tuning is dependent on pipe length and involves high and low pressure waves or pulses. However, since acoustical pulses travel at Mach 1, flow between the high and low pressure pulses are sonically choked i.e. do not respond to ΔP.

Particularly in 2-cycle engines where the concurrent intake and exhaust gets tricky, expansion chambers can increase exhaust velocity to enhance scavenging. But the big bang comes from the pressure-wave tuning that, IMO, operate at the intake port to acoustically position a high density fuel charge in the combustion chamber. This is in part based on theory and in part on the thought that the exhaust alone can’t produce the substantial power increase gained from an acoustically tune pipe.

Expansion chambers can function as inertial enhancers and can also broaden the effective tuned length of a pipe by gradually increasing pipe diameter so an acoustical wave sees the enlarged cross section over the length of the expansion chamber/outlet cone.

IMV, there are two distinct exhaust phenomena, acoustical and inertial. Only the form depends on a tuned-length pipe.

[Tommy Cookers]

Without variable length geometries you can tune your exhaust system for one rpm only.

not really true
eg the 4:2:1 exhaust arrangement gives tuning for two diferent rpms (not always worthwhile, though)

EDIT
just read the reference given by Kiril Varbanov, it's PRICELESS !!
(also from this company's site the article comparing Nascar and F1 engines is priceless)

IMO the exhaust article vindicates posts of mine that cost me a lot of brain work
(and vindicates in the example of the quite recent Kawasaki MotoGP my advocacy of such years ago (a normal/flat crank with simultaneous firing pairs) when everyone was speculating about the (so-called) 'big bang' MotoGP Yamaha)

2 stroke 'expansion chambers' were surprising in having a very small tailpipe diameter
(hidden the last 30+ years by muffler elements)
there must be some desired velocity/pressure effect of the small dia ??

the 1925 Garelli 350 appears to have such a system ?
but it was then forgotten ? till the 1950s ?
(the dominant DKW supercharged 2 strokes of the late 30s had 'megaphone' type systems, they could be heard in early morning practice in the Isle of Man TT from Liverpool Bay about 60 miles away ! , the sound being efficiently reflected between the sea and the low level atmospheric thermoclines)