Quote:
Originally Posted by stonebreaker
You are trying to argue that forcing a gas through a smaller pipe is somehow easier than forcing it through a large pipe.
If that's the case, then why do turbo cars, whose turbos operate off of the delta P before and after the turbine, have huge exhausts? Even more to the point, turbo diesel trucks, which operate in the rpm range we are discussing, come with 3 and 4 inch exhausts from the factory.
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I'm not arguing that forcing a gas through a smaller pipe is easier at all, I'm arguing that the expansion of that gas will create a stacking effect as it cools, since density is proportionate to temperature. If the diameter of the pipe isn't small enough to maintain velocity, the exhaust pulse will slow as the gas expands and cools. Cooler gasses are necessarily more dense, and thus, could cause flow reversion if another pulse were to meet it. This is described as a "Stack Effect", which increases back pressure.
There are more forces involved than simple pressure and velocity.
Reread your question, then answer it for yourself.
Nevermind, I'll do it for you: Turbo compounding. It's a feed back loop, which means that the engine can have greater than 100% volumetric efficiency.
Since the intake is pressurized, there is a higher density of gas, which has more potential for expansion, and will require a larger volume of space to occupy for a high deltaP/low cF. The increased volume works in increasing levels with the turbo charger to create compounding, which continuously increases the mass of intake gasses under compression, to an extent determined by parameters outside the range of this discussion.
Take those same turbo engines and remove the turbo, keeping the same pipe, and tell me what happens to BSFC at low RPM.
The larger flow volume requires a larger pipe to maintain a higher deltaP with lower friction. I thought I already covered this?
You were right about bernoulli, that was my mistake. It's conservation of mass we're after here.