What makes an exhaust better ?

[flynfrog]

When traffic is in my way, say at a light, I just open and lock the clutch, and pedal my bicycle onto the sidewalk.

Albeit a nice ride, the bike you showed me is more of a bona fide motorcycle, and you could just jump over the cars.

But still, I wonder of the exhaust and what I can do other than put a yellow stripe on it somewhere, cuz if I learned anything from boy racers, yellow makes your car faster.

read what carlos posted then learn to braze.

[Giblet]

I figured someone here would be able to tell me if these expansion type exhausts can work on an application like I described, with a couple of serious bends to the pipe.

I appreciate the info that was posted, but I don't want to learn how to make 2 stroke exhaust systems and braze (weld?) before knowing if it can even work in this application.

Most times I have seen them the exhaust points rearward, but mine points to the front. I guess I could have the exhaust point phallus-like out the front, but that would be a little insane.

Anyone know?

[marcush.]

of course you can make the turn and still have the expansion box something like that should really make some noise:
http://www.silentsport.de/produkte/pics ... e_pipe.jpg

[Carlos]

Giblet - Because tubes bend ... and an expansion chamber is assembled in sections ... you could make it "over/under/sideways/down" or even a corkscrew ... it's a matter of egronomics and imagination.

[woohoo]

Ok, first, I am mostly interested in four stroke engine exhausts

Second; what I meant with bigger exhaust is:

Basicly what I thought was that if you have a an engine exhaust of 5square cm per cylinder (dont know how big they really are)
and you have four cylinder, you should have a hole of 20 square cm so the pressure in the pipe does not increase.

The away I understand it, the easier the exhaust flows out of the cylinders, the more efficient the engine (as there is less exhaust gases on the next bang, just clean O2 and petrol)

But here you post expansion chambers that send exhaust gases back to the engine ???

I now know less then I started with.

[autogyro]

Ok, first, I am mostly interested in four stroke engine exhausts

Second; what I meant with bigger exhaust is:

Basicly what I thought was that if you have a an engine exhaust of 5square cm per cylinder (dont know how big they really are)
and you have four cylinder, you should have a hole of 20 square cm so the pressure in the pipe does not increase.

The away I understand it, the easier the exhaust flows out of the cylinders, the more efficient the engine (as there is less exhaust gases on the next bang, just clean O2 and petrol)

But here you post expansion chambers that send exhaust gases back to the engine ???

I now know less then I started with.

Both two stroke and four stroke exhausts are just one part in a complex balance of things in a reciprocating IC engine.
Usually for a multi cylinder four stroke, the separate exhausts connected to the ports are joined together at certain defined lengths, dictated by other factors such as valve lift, overlap, duration, the expected working rpm, torque and bhp maximums and many other factors.
Joining the pipes is primarily to utilize the exhaust pulses from each cylinder to assist scavenge in others. The length, positions of joins and also the over all length of the final single pipe are all critical. Usually these measurements have been finalized for purpose using an engine dyno.
Sometimes it is a big problem fitting the manifold when it is installed in the vehicle and as usual compromises are made.
The expansion box on a two stroke exhaust is designed to achieve much the same result by bouncing the exhaust pulse back to assist scavenge and in some cases to help close flap valves.

[Carlos]

Woohoo - Yes you are right, I diverged the thread from 4 stroke exhaust systems to 2 stroke expansion chambers in an effort to help Giblet with specific, detailed information on designing and building expansion chambers and 2 stroke porting, although it is often of no consequence to offer aid, it is my nature, please excuse the transgression.

If I may offer you specific detailed 4 stroke exhaust system advice, I would recommend:
The Scientific Design of Intake and Exhaust Systems
http://www.bentleypublishers.com/automo ... ystems.htm
A reference book that has explained the basics and build instructions to at least 2 generations of 4 stroke enthusiasts.

Favourable reviews from Amazon.com USA:
"The information in this book is just as relavant now as it was when it was written back in 1972. Physics is physics, after all. The book gave me a much clearer understanding of header theory. My only complaint is that Smith and Morrison obviously wrote this book for engineers, and I'm not one. I had to struggle to understand some of the concepts, particularly on tri-y header design; but in the end, it was definitely worth it. The exhaust concepts are equally applicable to carbs and fuel injection. The only things lacking were crossovers and merge collectors, but I guess those things weren't invented in 1972. This book will give you a basic grounding in header design theory, and will enable you to discern a set of well-designed headers from a useless tangle of pipes. If you plan on building a set of custom headers, this book is definitely required reading. "

"I have designed race cars for years and I definitely recommend this book. I found information in this book that I have not seen published anywhere else. It explains how tri-y headers work. It explains the theory behind pulse tuning of exhaust and intakes. The intake pulse tuning is the basis of why Porsche created intake manifolds that effectively change between several intake runner lengths. These theories are even backed up by proven test results performed by engineers. There are even simple equations that are directly applicable to designing an intake or exhaust. This book is "old" but then so is the 4-stroke engine and although today's engines may seem "new" they are fundamentally the same as the "old" ones which makes the material in this book very worthwhile. If you want to know more than what intake or exhaust you should *buy* and are possibly thinking about creating your own intake or exhaust system, definitely buy this book. If you are looking for a book like this but even more technical, I suggest the 2 volumes titled "The Internal Combustion Engine in Theory and Practice" by Charles Fayette Taylor. "

[Giblet]

Sorry for the side track, and thanks for the great pics Carlos.

I plan to start design after Christmas, and with some extra work (making the hollow bike frame a gas tank).

4 stroke away.

[Edis]

Given that some people have mentioned helmholtz resonator, they have clearly misunderstood the basics of an exhaust system.

The most important part of an exhaust system is the exhaust manifold. When an exhaust valve is opening, the pressure inside the cylinder can at full load be very high. This will cause the formation of a pressure pulse in the exhaust port. This pressure pulse will then start traveling downstreams to the collector with an approximate speed equal to the local speed of sound. At the collector the pulse is subject to an expansion (increase in area), this will cause a refection of the pulse. However, since the pulse was subjected to an expansion the nature of the pulse will change from compression (pressure increase) to expansion (pressure decrease). This expansion pressure will travel upsteam back to exhaust port and designed correctly the expansion pulse will arrive at the exhaust port when the exhaust valve is about to close. This lowers the pressure in the exhaust port at the time both the exhaust valve and intake valve is open improving cylinder filling.

Two stroke engines have exhaust systems that use an increase in area followed by a decrease in area. The increase in area will result in a negative pressure refection pulse and the decrease in area a positive pressure reflection pulse. The idea is to use the negative pressure pulse to aid scavenging and then use the pressure pulse to push back the air/fuel mixture that have escaped the cylinder to the exhaust. This will have a 'supercharging' effect.

As can be seen on for instance some F1 exhaust manifolds, there can be a small step up in diameter before the collector. When the exhaust valve have just opened the flow will be sonic (even supersonic) due to the great pressure difference between the exhaust port and the cylinder. A small increase in pipe diameter will cause a refection pulse that will return to the exhaust port lowerering the pressure. The placement of this small increase in diameter is placed such that it arrives during blowdown extending the time the flow velocity remains high due to the pressure difference.

After a catalytic converter or a turbocharging the exhaust system is mainly about pressure drop and sound. Here a larger pipe diameter will result in a lower flow velocity and because of that a lower pressure drop, but it also tend to increase sound levels. This rear part of the exhaust system is also much about resonance - it's here Helmholts resonator comes in. You don't want a resonating exhaust system.

[F1_eng]

Can you explain how you get supersonic flow in the exhaust system?

[Edis]

Can you explain how you get supersonic flow in the exhaust system?

When the exhaust valve opens the flow past the valve will be sonic for about 50 crankshaft degrees due to the small area and the high pressure inside the cylinder. If you have an area increase after a sonic throat, supersonic velocities are also possible due to the release of kinetic and pressure energy.

[F1_eng]

The mach number at exhaust port gas tempratures is very high, which makes going supersonic at these temperatures extremely unlikely. The conditions required for supersonic flow in a nozzle are very exact, for example you need perfect geometry in which the flow can be considered to be 1 dimensional plus you need the pressure variance not be too great or you begin to have instabilities in the flow which serve to stay the dynamic flow behaviour away from 1D.
The conditioning would have to be a perfectly crafted nozzle geometry which is at steady-state.

You never see supersonic flow in exhaust ports because of the transient behaviour of the gas.

[Edis]

The mach number at exhaust port gas tempratures is very high, which makes going supersonic at these temperatures extremely unlikely. The conditions required for supersonic flow in a nozzle are very exact, for example you need perfect geometry in which the flow can be considered to be 1 dimensional plus you need the pressure variance not be too great or you begin to have instabilities in the flow which serve to stay the dynamic flow behaviour away from 1D.
The conditioning would have to be a perfectly crafted nozzle geometry which is at steady-state.

You never see supersonic flow in exhaust ports because of the transient behaviour of the gas.

The flow would only be supersonic is certain areas during a very limited time, but it's difficult to find reliable sources on the subject. But given that you can reach supersonic velocities around a throttle plate at idle, where the flow is mainly subsonic, it would hardly be surprising.

[raceman]

I am not good at drawing, therefore don't have tools to sketch any idea that may strike me I also hope I am not asking an idiotic question out of the blue..... I also not definitely know the regulations whether this is possible, but just will briefly state what I think about exhaust and downforce!

I request the drawing lads here to exploit my idea a little bit better with your drawing skills.....


is it possible to direct hot exhaust from an F1 engine to the rear wing where a low-weight heat-resistant or something-of-that-kind plates are fixed at a predetermined angle that directs the exhaust gases out through the rear wing and thus giving the rear end of an F1 car a little bit of downforce by the the shear force and high speed of the exhaust gas??? I mean, the force and speed of the exhaust will push those plates downwards and will kind of increase downforce to the rear end.

I don't know if I have stated my idea briefly enough to understand, but if not, pls help me correcting it.

[F1_eng]

That is definetely possible. One of the issues with the method is that when the driver backs off, the exhaust gas mass flow is reduced and so would the downforce from the rear wing which causes serious instability.
For example say the driver is going through a very high speed corner at full throttle with the exhaust gas significantly increasing rear downforce, he is not quite in the position he wants to be in the corner so he has to lift of the throttle briefly or he's understeering. The car would become very unsettled very quickly, centre of pressure would be quickly moving, completely changing the balance.
Also braking stability at the end of long straights can cause issues.