Manifold Pressure Question

[USMCmech]

Clamping my hand over the hose is the same as closing the throttle plate, which the OP was not doing. He is asking about MP rise due to RPM reduction. Keep in mind I am talking about a carb system not fuel injected.

Carb or fuel injected there is no difference.

If I cover the end of a vaccum cleaner the pressure drops because it becomes a vaccum, all the air is sucked out. Thats not what happens when you move the prop levers to a lower RPM, you simply slow the velocity of the air down as it is entering the manifold

Reduceing the RPM by changine the pitch of the prop would be equivilant to reduceing the speed of the vaccum cleaner motor in the above scenerio.

Since there is less suction, the pressure in the hose rises.

 

[DC8 Flyer]

Carb or fuel injected there is no difference.

There is a little difference, little better fuel control keeps the engine working about the same, so not as much of a MP change with pure prop change.






Reduceing the RPM by changine the pitch of the prop would be equivilant to reduceing the speed of the vaccum cleaner motor in the above scenerio.

Thats what I have been trying to say! You reduce the speed of the vacuum cleaner by putting a load on the motor not by reducing the airflow, to simulate prop angle increase.

Since there is less suction, the pressure in the hose rises.

Thanks!

 

[avbug]

Which is exactly what I said. Airflow velocity is irrelevant, as is fuel injection or carburetion.

By reducing engine RPM for a given throttle plate setting, one has reduced the suction value of the engine (the engine being an air pump). Accordingly, manifold pressure rises. This has nothing to do with airflow velocity, but everything to do with less suction on the part of the engine. The manifold pressure merely rises somewhat, and will continue to do so as the engine RPM is reduced. When the engine RPM is reduced to the point of being shut down and has zero speed and subsequently zero suction, manifold pressure will be ambient air pressure.

Clearly this is not a velocity issue, and is not a function of pressure drop or rise due to airflow velocity. You seem to believe that as airflow increases in the induction system, the pressure drops. Pressure drops due to suction from the engine, not bernoulli's principle, and not due to venturi effect. Accordingly, your insinuation that carburetion vs. fuel injection plays some part, is also flawed.

Pressure rise with reduction in RPM isn't at all due to reduction of airflow velocity in the induction manifold...but due to a reduction in the suction provided by the engine as it's RPM is reduced, relative to the throttle plate opening...and without regard to either fuel injection or carburetion.

 

[DC8 Flyer]

Which is exactly what I said. Airflow velocity is irrelevant, as is fuel injection or carburetion.

By reducing engine RPM for a given throttle plate setting, one has reduced the suction value of the engine (the engine being an air pump). Accordingly, manifold pressure rises. This has nothing to do with airflow velocity, but everything to do with less suction on the part of the engine. The manifold pressure merely rises somewhat, and will continue to do so as the engine RPM is reduced. When the engine RPM is reduced to the point of being shut down and has zero speed and subsequently zero suction, manifold pressure will be ambient air pressure.

Clearly this is not a velocity issue, and is not a function of pressure drop or rise due to airflow velocity. You seem to believe that as airflow increases in the induction system, the pressure drops. Pressure drops due to suction from the engine, not bernoulli's principle, and not due to venturi effect. Accordingly, your insinuation that carburetion vs. fuel injection plays some part, is also flawed.

Pressure rise with reduction in RPM isn't at all due to reduction of airflow velocity in the induction manifold...but due to a reduction in the suction provided by the engine as it's RPM is reduced, relative to the throttle plate opening...and without regard to either fuel injection or carburetion.

Ok, find the Fluid Dynamics law that states that then. Both my engineering (aeronautical) and my aviation training say Bernoulli, but you say not. All I want is the law and the eqaution.

 

[USMCmech]

I'm with Avbug on this one.

All suction in the intake system is caused by a large piston moving "down" in it's cylinder thus drawing in air.


The only place a venturi is used is in the carburator which has nothing to do with airflow, but rather drawing fuel into the carb.

 

[DC8 Flyer]

..NVM

 

[VNugget]

Constant throttle setting = constant power = constant mass flow, at lest in theory. If you have constant mass flow, then Bernoulli's pressure/velocity relationship must hold true, unless you just decide to throw physics out the window. Anyone that decides to do that had better have a **CENSORED****CENSORED****CENSORED****CENSORED** good reason.

 

[DC8 Flyer]

Constant throttle setting = constant power = constant mass flow, at lest in theory. If you have constant mass flow, then Bernoulli's pressure/velocity relationship must hold true, unless you just decide to throw physics out the window. Anyone that decides to do that had better have a **CENSORED****CENSORED****CENSORED****CENSORED** good reason.

Exactly! Except take the "flow" out of constant mass flow, since the slower RPM, lowered the flow, but the mass is the same, voila, pressure rise.

 

[nosehair]

All you have simply done is slown down the rate at which air is entering

....anyway, I liked his way with words....

 

[VNugget]

Exactly! Except take the "flow" out of constant mass flow, since the slower RPM, lowered the flow, but the mass is the same, voila, pressure rise.

No, that's the whole point, the the mass flow stays the same regardless of RPM. If the mass flow canged, then the power would also change. (We still are talking about constant power, and only playing with the prop governor, right?) Remember that X air molecules combust with Y fuel molecules to release Z energy, over some given time