italia458,
what hot gasses?
In a naturally aspirated engine, you have expansion affecting the temperature in the manifold. In a charged engine, you (usually) have compression. What's more 'normal', as far as temperature in the manifold goes?
The relatively hot gasses that go into the intake manifold after passing through the intercooler. I can't see any difference between a charged or NA engine with regard to what happens in the intake manifold. In a charged engine compression happens in the turbine itself, before it goes into the manifold. I'm not sure what compression you're talking about in the manifold.
I meant 'normal' in that case, as in the same as a naturally aspirated engines since it isn't benefiting from the aid of the turbo yet.
In a constant volume, n is directly proportional to density.
It was just confusing since you made no mention of volume until later on - you had labeled (n) as Density. I understand what you did at the end.
Knowing this, and assuming that the temperature drop or increase you see due to expansion or compression from ambient to MAP is also in accordance with the ideal gas law means the hypothesis about the density/pressure interrelationship being the reason for the lower required MAP is not an explanation for the observed phenomenon. You will end up with the same temperature in the manifold regardless of ambient.
I don't understand that. If I have -20 degrees Celsius air in winter, it doesn't magically become the same temperature in the manifold as the 35 degrees Celsius air in summer! The change in temperature, I would say, would follow the ideal gas laws... but starting out at a lower temp means you will finish with a lower temp.
Maybe I'm not seeing what you're trying to say... any way of clarifying what you mean?
As for back pressure, I can't really see that proof is needed for the engine using power when it is pumping air against a pressure. I'm not really interested in debating whether it is the explanation or not. Dig up a few more references for comparison?
Reading over my post I can see I didn't clearly state my point... I agree it definitely does use power to pump against a pressure and it makes sense that it would require a lower MAP in this case. However, at SL (compared to 10,000'), the percentage of horsepower required to pump air out of the engine into an atmospheric pressure that's 4.6 PSI higher I think would be small. I do agree it would have an effect on MAP but I haven't seen any data on the amount of power required in this case so I can't really draw any definitive conclusions.
It's possible that both the lower back pressure and the fact that pressure decreases faster than density, with an increase in altitude, affect the required manifold pressure. But how much of each I'm not sure.
Also, you're quoting Aerodynamics for Naval Aviators out of context.
I didn't think I was. Here is the full paragraph. If you want to read more it's on page 138.
"Of course, the engine airflow is a function of RPM for two reasons. A higher engine speed increasesthe pumping rate and the volume flow through the engine. Also, with the engine driven supercharger or impeller, an increase in engine speed increases the supercharger pres- sure ratio. With the exception of near closed throttle position, an increase in engine speed will produce an increase in manifold pressure."
EDIT: I actually think what they were referring to was an engine outside of its governing range or one with a fixed pitch prop. If you increase the throttle, the RPM and MAP both increase. That makes sense now.