Manifold Pressure Question

[A Squared]

.......definately not how it was explained to me.

Yeah, probably so. Seems there's more than a few instructors out there spreading bad info. At least you won't be now. Use multiple sources, check them against each other. be skeptical.

 

[avbug]

A Squared is correct...and the rest of you had better do some more studying. Ouch.

Think of your engine as a vacum cleaner....it's a suction machine. What happens when you put your hand over the end of the hose on a vacum cleaner? It starts to scream and whine, and the pressure inside the hose goes down. Same suction, but you've blocked it off and as it's still sucking, the pressure drops.

This has nothing to do with bernoulli or airflow velocity vs. pressure. The engine is sucking, you're blocking it with the throttle plate, and pressure drops.

As A Squared noted, if you keep your throttle position constant and decrease the RPM's with the propeller control, you're slowing down the vacum cleaner...you're slowing down the engine, and it's producing less suction. If you're at a given throttle setting, you're not moving the throttle plate, so as the engine slows, there is less "suction" on the manifold beteen the throttle plate and each cylinder...therefore manifold pressure rises.

Pull the mixture to idle now, and you'll see a further rise in manifold pressure...right to current barometric pressure. If you're at sea level on a standard day, you'll get 29.92 inches of manifold pressure with the mixture at cutoff and the engine no longer turning. In a normally aspirated engine, the most manifold pressure you're going to get with the throttle wide open is barometric. If you're at sea level, then you're looking at nearly thirty inches, but if you're at five thousand feet you're looking closer to twenty five inches manifold pressure max.

If you want more than that, then you need turbocharging or another form of induction boosting.

Slowing the engine down with the propeller control during runup isn't boosting anything or adding power. It's reducing engine RPM and subsequently power, but showing an increase in manifold pressure because the engine is producing less "suction" at lower RPM's in the induction manifold between the throttle plate and the cylinders.

 

[DC8 Flyer]

A Squared is correct...and the rest of you had better do some more studying. Ouch.

Think of your engine as a vacum cleaner....it's a suction machine. What happens when you put your hand over the end of the hose on a vacum cleaner? It starts to scream and whine, and the pressure inside the hose goes down. Same suction, but you've blocked it off and as it's still sucking, the pressure drops.

This has nothing to do with bernoulli or airflow velocity vs. pressure. The engine is sucking, you're blocking it with the throttle plate, and pressure drops.

As A Squared noted, if you keep your throttle position constant and decrease the RPM's with the propeller control, you're slowing down the vacum cleaner...you're slowing down the engine, and it's producing less suction. If you're at a given throttle setting, you're not moving the throttle plate, so as the engine slows, there is less "suction" on the manifold beteen the throttle plate and each cylinder...therefore manifold pressure rises.

Pull the mixture to idle now, and you'll see a further rise in manifold pressure...right to current barometric pressure. If you're at sea level on a standard day, you'll get 29.92 inches of manifold pressure with the mixture at cutoff and the engine no longer turning. In a normally aspirated engine, the most manifold pressure you're going to get with the throttle wide open is barometric. If you're at sea level, then you're looking at nearly thirty inches, but if you're at five thousand feet you're looking closer to twenty five inches manifold pressure max.

If you want more than that, then you need turbocharging or another form of induction boosting.

Slowing the engine down with the propeller control during runup isn't boosting anything or adding power. It's reducing engine RPM and subsequently power, but showing an increase in manifold pressure because the engine is producing less "suction" at lower RPM's in the induction manifold between the throttle plate and the cylinders.

Im sorry AB, youre a little off base on this one. This IS a velocity versus pressure problem. The manifold system is a closed system, ie one entrance and one exit. On normally aspirated engines the volume of air going into the manifold is relatively constant, no boosting. So if leave the throttle plate alone and slow the prop down with prop control I slow down the vaccum cleaner, increasing relative pressure.

Just like your analogy, take a vaccum cleaner and put a pressure gauge on the hose, and measure the atmospheric pressur in the room. With the vaccum cleaner on pressure in the hose will be lower because of velocity.

Most examples of bernoulli show the pipe with a constriction (venturi), but you can take that same formula and make the variables velocity instead of area and you get different pressures.

 

[avbug]

I'm pretty busy with little time to answer, but you're incorrect.

Clamp your hand over a vacum cleaner, note the absolute pressure in the hose. Lift your hand off, note hte pressure. It rises. Same as opening the throttle. Lift your hand partially off, note an intermediate pressure.

Put your hand back over the hose, same as closing the throttle. Note the pressure. Same as before. Now slow down the speed of the vacum impeller. Same as decreasing engine RPM. Less suction, higher manifold pressure.

It has nothing to do with airflow velocity through the induction system and an attendant pressure rise or drop.

 

[DC8 Flyer]

I'm pretty busy with little time to answer, but you're incorrect.

Clamp your hand over a vacum cleaner, note the absolute pressure in the hose. Lift your hand off, note hte pressure. It rises. Same as opening the throttle. Lift your hand partially off, note an intermediate pressure.

Put your hand back over the hose, same as closing the throttle. Note the pressure. Same as before. Now slow down the speed of the vacum impeller. Same as decreasing engine RPM. Less suction, higher manifold pressure.

It has nothing to do with airflow velocity through the induction system and an attendant pressure rise or drop.

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.

If you can point to another Fluid Dynamics Law that states otherwise, Bernoulli is the only law that explains the action of fluids and pressure changes, for constant energy and density anyway.

Here is the equation, simplified to prove it.

P1 + V(one)squared = P2 + V(two)squared. (that is bernoullis equation simplified to remove all the constants)

Take the same airplane where P1 is 2700 RPM and P2 is 2200 RPM. We know the pressures, manifold pressure. What we dont know is the velocity. You argue that velocity and pressure have nothing to do, but I will show below how the higher pressure (lower RPM) has the slower velocity.

2700 RPM = 26" MP
2200 RPM = 30" MP

26 + V(one)squared = 30 + V(two)squared

V(one)squared = 4 + V(two)squared

This shows that the 2200 RPM setting has a slower velocity, thus higher pressure, in the induction system. This is true, since Bernoullis principle states that in a closed system air travelling at a lower velocity, with the same density, kinetic engergy, etc, must have a higher pressure.

Its all conservation of energy stuff. Now, unless you can point to some other law of Fluid Dynamics, and I havent been able to find one in my books, that states otherwise, this is what is happening in the system.

It is true that the engine is just a big air pump, but that air is still has to obey basic laws of physics as it travels through the manifold and bernoulli is it.

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.

 

[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