Designing Exhaust w/ VR6: Finding Velocity

[mpaige1]

So, I'm still working on designing an exhaust. The engine I'm using is the VR6, non-turbo, etc. I want to design the exhaust only under full throttle. This way I can over-design the exhaust rather than under-designing it. I'm really not sure how to find the velocity of the exhaust as it exits the exhaust manifold. Can someone just explain what information I need to have or direct me to a reference?

 

[Curl]

Find how quick the piston is moving up to push the air out.
If it is below 100m/s you can assume incompressible fluid.
Find the diameter of the exhaust port. Now you know the air must move through this hole in this amount of time. The "average" speed will be the volumetric flow rate divided by area (simple). It will be fully turbulent so just most of the gas will be moving at this velocity... well, except very far down the pipe. You didn't say how long you want your duct, or if there are twists in it, but this is how you go about doing this type of problem.

 

[S_Happens]

That's assuming the cylinder pressure is at atmospheric (or exhuast system) pressure, which is not likely, especially at full throttle. Initially there will be higher flow than simply that displaced by the cylinder.

It's much more complicated than "over-design" vs "under-design." In reality you want to work around a certain rpm to support the strenths that the rest of the engine already has based on valve events, intake design, etc.

Is this still the project you already posted. If so, have you made any progress or are you still looking for all the same information? I'm only asking because this one is pretty vague, and that one seemed to be specific to max rpm (I skimmed).

 

[russ_watters]

It doesn't matter what the piston is doing: as soon as the exhaust gets into the pipe the velocity and pressure should be plenty low enough to assume incompressibility. The tough part to me would be figuring out how much of a burden a little backpressure is on your engine and choosing a pressure to design to. Another assumption that will assure oversizing is assuming the gases don't cool as they exit...of course finding the initial temp may be tough too.

 

[AlephZero]

I don't see how you could start to answer that question without modelling the aerodynamics of the exhaust system. The back pressure at the manifold end could vary by orders of magnitude, depending on how the resonances of the exhaust interact with the engine RPM.

Even in something as "low powered" as a wind instrument blown by a human, the aerodynamic conditions inside the instrument can include compressibility and cavitation effects because of the resonances in the system.

 

[mpaige1]

That's assuming the cylinder pressure is at atmospheric (or exhuast system) pressure, which is not likely, especially at full throttle. Initially there will be higher flow than simply that displaced by the cylinder.

It's much more complicated than "over-design" vs "under-design." In reality you want to work around a certain rpm to support the strenths that the rest of the engine already has based on valve events, intake design, etc.

Is this still the project you already posted. If so, have you made any progress or are you still looking for all the same information? I'm only asking because this one is pretty vague, and that one seemed to be specific to max rpm (I skimmed).

Ya, it's the same project as before. This is pretty much what I need to figure out:

1) Velocity at end of manifold and end of tailpipe
2) Temperature at end of manifold. I'll assume this temperature throughout the exhaust because I am not going to use different materials from one end to the other and the material obviously will need to hold up to the maximum temperature I find anyways.
3) After all of this is found, I need to find the pressure and thermal stresses exerted on the exhaust pipe.

If this is going to be too complicated to accomplish, should I just assume some numbers for these variables and leave it at that?

I am modeling this in Pro/E and we would just have to analyze stresses on the exhaust itself and the stresses exerted on the valve in a y-bar exhaust. We are only taking into consideration thermal and pressure stresses but if it was possible I wanted to go more in-depth.