Aerochargers are better than a twin screw!! hahaha ;-)

[Greg Hermann]

At 9:46 AM 10/15/01, Stephen Webb wrote:
>> Yes, a turbo makes more backpressure--but it DOES let the overall package
>> operate at a higher expansion ratio. A mechanically driven supercharger
>
>Could someone explain (or point me to a reference that does) what
>expansion ratio is / means, and how a turbo affects this property
>differently than a supercharger?
>
>Thanks
>
>-Steve

Expansion ratio in a NA engine is essentially the opposite of the
compression ratio. There can be some variation between the two, due to
twiddles with cam timing and such (try looking up the "Miller cycle").
But--the point is that an engine with, say a 9 to 1 compression ratio will
have roughly a 9 to 1 expansion ratio on its power stroke.

The expansion ratio is what _REALLY_ governs the thermal efficiency of the
engine--the working fluid  (gas) reaches its peak temperature at (roughly)
TDC, after the ignition event, and then the expansion ratio determines how
much heat can be extracted from the gas before the exhaust valve opens and
the rest of the energy in the gas is essentially rejected to the
"environment".

Things work just as described above in a supercharged engine insofar as
extraction of useful work from the working gas.

In a turbo motor, things are different. Downstream of the exhaust valve,
the working fluid undergoes further useful expansion as it passes through
the turbine. If the (absolute) pressure ratio across the turbine is, say,
2.5 to 1, it's relatively easy to figure out what the volume expansion
ratio across the turbine is. Of course if you have before and after turbine
EGT's AND pressures, you can figure out the true volume expansion ratio
across it exactly, including turbine efficiency.

The effect is geometric, so multiply the expansion ratio across the turbine
by the expansion ratio in the motor to arrive at the expansion ratio across
the entire system. Regardless of the fact that you are incurring a pumping
loss in the motor due to the higher backpressure on it, you are STILL
getting more total useful work (flywheel HP plus compressor HP) out of a
given amount of  fuel burned (or amount of air breathed) because of the
higher overall expansion ratio which exists in the motor/turbo system.

Greg



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