my brother was telling me about this phenomenon with putting a specific bulge in the exhaust pipe on say a motorcycle. if the bulge is the right size/shape, it will create back-pressure into the engine, creating a turbocharge effect.
our best guess at the way this happens is by the soundwave of the exhaust going back into the engine and compressing the air, allowing more fuel in.
is this correct?
This only works on two stroke engines, which leaves out most street motorcycles. The expansion chamber helps power in two ways, first as the exhaust expands into the expansion chamber it creates a vacuum that sucks the exhaust from the cylinder and (since the intake valve is open) also sucks in fresh air/fuel into the cylinder. Second when the exhaust pulse reaches the end of the expansion chamber it is reflected back so any unburned air/fuel in the exhaust is forced back into the cylinder.
I belive an exhaust pipe made for maximum power will only work over a narrow RPM range and cause the bike to loose power at other RPMs. This is fine for a race bike but it could make riding a lot less fun for recreational riding. (feel free to correct me on this)
There was thread on this not to long ago but I can’t seem to find it. Here is more info at How stuff works though – http://www.howstuffworks.com/question636.htm
ah yes, there is a direct pathway across the chamber that leaks out some fresh mixture. i knew that some mixture got out, i just never thought it possible to reuse this “lost” mixture. cool. it turned out to be a lot less of a phenomenon than what we thought about the sound wave though…
Anachronism’s post has a couple of red herrings in it. The part about the exhaust being forced back into the cylinder being the biggest.
The pipe your brother is describing is a “tuned” pipe for a two stroke engine. Nearly all two-stroke off-road bikes sport this type of exhaust. Four stroke engines have an analogous exhaust called a header.
How tuned intakes or exhausts work in general:
All these ideas do produce the extra power at a designed in rpm range. Depending on the design of the intake or exhaust system, it can be set up to give a modest boost or a fairly wide rpm band or a greater boost over a narrower band of rpm.
All of these types of intakes and exhaust need to be designed with the cam grind (four stroke) or port timing (two stroke), engine displacement, and desired power band in mind to make them work.
scotth: you are talking about air, what about unburned mixture? does this get shuttled back in via vacuum, or just the air?
Nothing gets shuttled back into the cylinder by the vacuum, the partial vacuum just helps draw the next exhaust charge out of they cylinder into the exhaust system.
OK, I had to look up what a red herring is and I am not sure how it applies to my post. While I am not an expert on engines I do know a bit about them. My answer was only intended to be basic and as far as I know it is correct. Perhaps I did not explain everything as well as I could have but I do know there many people on this board more knowledgeable than I who can clarify and expand on my answer.
If you are saying that the exhaust being forced back into the cylinder is untrue or unimportant (and from your response while I am typing this that does seem to be what your saying) all I can say is several web sites describe it that way including ‘how stuff works’. I don’t consider ‘How stuff works’ the final authority on anything, but they are usually right and I would believe almost anything I read there until it was proved otherwise, especially when every other web site I can find describes it the same way.
It looks like you just described the scavenging effect of the vacuum created by a tuned exhaust in more detail and slightly differntly than I did.
I know exhaust gas velocity is an important consideration for cylinder scavenging with four stroke engines but I was unaware that there was any kind of expansion chamber in (on?) headers.
Two strokes use pressurized fuel/air in the crankcase to fill the cylinder (with help from a tuned exhaust), I don’t see how this has anything to do with tuned anything.
It sounds like you are describing [http://www.howstuffworks.com/question517.htm”](” [url) ]Tuned Intake Runners for a 4 stroke engine, but I think these work quite differently than an expansion chamber.
Doohhh Heres the right link - Tuned Intake Runners
Easy, fellas!
Actually, scotth provided an excellent, if generic, explanation of the mechanics of tuned exhausts. It is completely true that the same principles apply to both 2-cycle and 4-cycle engines, although the exhaust designs that accomplish the desired effects vary necessarily from type to type.
The whole idea is to take advantage of the principles of fluid mechanics – hydraulics – to increase the flow of fuel mixture into an engine and of exhaust gases out of same. In the case of expansion chambers behind 2-cycle engines, the mechanism is similar to that of the venturi effect – high-to-low pressure. In the case of header(s) on a 4-cycle engine, the mechanism is transient pressure waves, or transient phenomena. The end result is the same, and the principles involved are remarkably similar.
hehe, put a tennis ball or a golf ball in the exhaust pipe… or any ball that fits in the pipe. fun for the whole family
Good, someone followed my post. That is what I was trying to do, provide the general mechanics of the all tuned systems like this and hoped from a general understanding of the physics, the mystery of the whole situation would dissappear.
And the specific red herrings:
This idea of reflecting the pulse to push unburned air/fuel back into the cylinder once it has made it into the exhaust is pretty off base.
and
True, but misleading or could just been worded better. Pretty much all two stroke engines used in vehicles use tuned pipe exhaust. The key is that the pipes are designed to put the power where it is appropriate for the application. Also, on dirt bikes in particular, a variety of pipes are available for the popular models that allow the user to pick a pipe suitable to the riding that will be done. Big, open, high speed track - pick a higher rpm, narrower band pipe. On a track with alot of slow parts, mud bogs, rocks, obstacles, etc - pick a pipe to put the power more in the middle or bottom of the rpm range.
Even my use of the word power is somewhat misleading there. Really I should be saying torque in most of those places.