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Originally Posted by sperry
I remember reading somwhere that the gasses are nowhere near done expanding due to the heat of combustion until they're into the headers and on their way up the up-pipe. The expansion of the exhaust gasses isn't instantaneous. However I would agree that the rate of expansion is decreasing from the instant the combustion event occures.
Also, if you have an expanding gas volume between the piston and the turbo's turbine blades, doesn't the math start looking a lot like PV=nRT? Granted it's not totally closed, but it's also not just an open pipe.
Thirdly, what about the exhaust pulses? There is energy in the form of a compression wave traveling down the pipe. I know lots of N/A cars take advantage of this and tune the pipe length/diameter such that the resonance frequency of the pipe matches the opening of the exhaust valves on another cylinder, allowing the low-pressure part of the waveform to assist in extracting exhaust from the cylinder. I think the turbine in the exhaust might limit the usefulness of that effect on a turbo car, unless you went with one of those funky rear mounted turbos.
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Muddy waters. Pressure in the cylinder > pressure in the headers, so yes, the gases "expand" as they exit the cylinder, but that is pressure equalization, not really heat based expansion. Once your heat source is gone, all you have left is volume, and pressure.
And as the exhaust valve opens, the volume of space available to that specific exhaust charge mass increases. As it increases, the mass cools, and pressure drops. Being a gas, it continues along the headers seeking the ultimately lower pressure of the end of the exhaust. And yes, i am using PV=nRT sort of, but only in principal, because it doesn't perfectly hold because it is an open system.
Yes, everything pulses as each cylinder fires in order, but I think on a turbocharged car, you are probably just trying to keep pressure even as opposed to trying to have a scavenging effect as you might on a non-turbo car.