Quote:
Originally Posted by Sutts
If boost rises when rpm falls (at a fixed throttle setting) and boost falls as rpm rises then the sim is correct.
As mentioned earlier, an engine is just like a big suction pump, sucking air fuel mixture out of the inlet manifold and thereby reducing the pressure in that manifold (which shows on the boost gauge).
The higher the RPM (at a fixed throttle setting), the faster the air is pumped out of the manifold and the lower the manifold pressure becomes. On the other hand, when RPMs are reduced (at a fixed throttle setting), less air is being sucked out and the pressure rises.
When the engine isn't running, the suction pump stops and the pressure in the manifold returns to the atmospheric pressure at the current altitude being flown.
On a non-supercharged / non-turbo charged engine, the highest boost pressure available is the atmospheric pressure at the current altitude.
On a supercharged / turbo charged engine, the highest boost pressure available is the max pressure provided by the compressor at the current altitude.
That's my understanding anyway.
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It's true that pressure drops with increased RPM in a normal (aspirated) engine because of "suction". But you completely miss the other important factor:
the supercharger is not some magical box that gives its nominal boost output regardless of what the engine is doing. Its
mechanically driven by it, and the faster the engine RPMs, the faster the supercharger spins and the more boost is able to deliver. The pressure ratio the supercharger is able to achieve does not vary linearly with its RPM, and depends on the specific type of compressor installed. So you see? the pressure tends to go down because of increased RPMs, but the supercharger tends to deliever more boost because of this RPM increase. What is the predominant effect? difficult to say, depends on supercharger and intake design, and also if the engine is running at low or high RPM.
Read my post on top of page 5.
In the end we can't say
anything about the accuracy of the model, until we have specific engine test data or supercharger characteristics.