Manifold Pressure and RPMs

[oktex88]

Why does the Manfiold Pressure increase when you decrease the RPMs using the prop lever on a constant speed prop?

For example if I set my Manifold Pressure to 15" and bring the RPMs from 2700 to 2500 using the prop level then my Mainfold Pressure goes up to 18". Why does this happen, becase the engine should still be producing the same amount of power?

 

[Clocks]

Man it's been so freaking long since I've had to think about this stuff...so I'll throw this out and probably look ridiculous...since the engine needs less air at 2500RPM the velocity in the intake manifold is reduced and the pressure rises. Sort of the reverse of what happens when you start the engine (the manifold pressure goes from reading atmospheric pressure to a much lower number, because the velocity of the air increases).

 

[USMCmech]

MP = how hard the air is being sucked out of the intake manifold.

RPM = how many breaths the engine is taking.


As you reduce RPM the engine is taking fewer breaths, therfore the air is not being sucked out as quickly, and the pressure rises.

 

[oktex88]

So it's like Bernoulli's principle then? The speed of the air going through the engine is decreasing when I reduce the RPMs so therefore the pressure will rise (aka rise in the MP).

 

[stratoduck]

it is related to the ideal gas law:

PV=nRT

for the most part, nRT is staying constant. n is a constant, R is measurement of density (or related, been a while since i was in school), and T is the temperature.

your real variables are P pressure and V volume. when you reduce the engine RPM, you are reducing V the volume (amount) of air entering the engine, therefore you need a corresponding rise in P pressure to keep the equation balanced.

 

[FrenchPilot]

MP = how hard the air is being sucked out of the intake manifold.

RPM = how many breaths the engine is taking.


As you reduce RPM the engine is taking fewer breaths, therfore the air is not being sucked out as quickly, and the pressure rises.

:yeahthat:

Read this, it will answer all your questions about MP. Its called "Manifold Pressure Sucks"
Quite interesting . Enjoy http://www.advancedpilot.com/downloads/prep.pdf

 

[tgrayson]

it is related to the ideal gas lawV=nRT

You can use the ideal gas law if you want, but it's a change in "n" that's causing a pressure change. "n" is a measure of the number of air molecules in a constant volume of space. When you close the throttle, you're restricting the intake of air into the manifold, and the engine is removing the air at a faster rate than it's supplied, resulting in fewer molecules in the manifold. This reduces the pressure in accordance with the gas law.

 

[fish314]

You can use the ideal gas law if you want, but it's a change in "n" that's causing a pressure change. "n" is a measure of the number of air molecules in a constant volume of space. When you close the throttle, you're restricting the intake of air into the manifold, and the engine is removing the air at a faster rate than it's supplied, resulting in fewer molecules in the manifold. This reduces the pressure in accordance with the gas law.

:yeahthat:. He was confusing "n" which is the number of molecules (in moles) with "R" which is the ideal gas law constant.

There are some other versions of the Ideal gas law. If you divide the number of molecules by the volume, you basically get the density of the gas so you sometimes see it as P=rho*R*T. You'll need a slightly different R, however in this formula because density is really the weight of all of the molecules in a volume of air divided by the volume, rather than the number of molecules. That "R" is the original "R" divided by the molecular weight of the gas in question. Some books write R/M others just write "R", where they've already divided the "M" into it.