Manifold Pressure Question

[UnstableAviator]

Yellow Snow,

You are correct in your description, but you don't seem to understand the relationship between the throttle position, manifold pressure, and RPM. Manifold pressure can be changed by the RPM or the throttle.

In the example you cited, the throttle is opened therefore letting all avaliable outside air pressure to "rush" into the engine. Barring the restrictions present in any induction system, this wide-open throttle MP will be the outside atmospheric pressure. It doesn't matter what your RPM is, since there is no restriction you will always have the full avaliable MP. So basically, anytime you open the throttle (past idle) you'll see an increase in MP. (Provided you aren't at a partial throttle setting at higher altitudes)

There is no back-up of air, the engine is actually increasing its power output and taking in more air.

Actually, the air is "backing up". If the RPM has not changed, as in a constant speed prop, there is no increase in engine demand for air. You are simply removing the restriction and allowing all avaliable air pressure to better fill the cylinders, making more power. It was demanding the same volume of air at the lower throttle setting, but we weren't allowing it in.

 

[Yellow Snow]

I understand fully that the throttle and the prop lever both affect man press. The original question was why does the man press rise when you reduce the RPM with the prop lever during the runup. I am simply trying to answer the question that was asked. Everyone is dancing around it but not answering the simple question. The physics of an induction system doesn't answer his question.

The answer is related to the coarser prop setting and the drag and resistance created by it. The engine has to actually work harder to turn a prop set at a higher angle of attack. Man pressure is simply a measure of the amount of power the engine is putting out.

 

[A Squared]

OK, the way I read the original question is this: when you decrease the RPM by moving the propeller control to low RPM (without changing the throtle position) why doed the manifold pressure increase? Admitidly the original question is ambiguous, but this is the only interpretation of the question that also fits with reality.

It is not because the engine is "working harder".

the reason is this: the engine is an air-pump, When it is turning, it is pumping air. this creates lower pressure in the manifold, for a given throttle setting, the faster it turns, the faster it pumps, the lower the pressure in in the manifold, the slower it turns for a given throttle setting, the higher the pressure. At the extreme slow end (0 RPM, engine stopped), the pressure in the manifold is at the max it ever will be, 30", the same as the atmosphere. (assumes normally aspirated engine) About the only way you can vary the RPM, without changing the throttle setting is with the prop control. Yeah, you can control it some with the mixture, but this doesn't give you very precise control over rpm.

edit:


Just to add a little more here, UnstableAviator, FIve Alive and I are all saying essentially the same thing, although wording it differently. This is the correct answer to your question. Yellow_snow is completely off base with the idea that the MP is higher becuse the engine is working harder. Here's an excellent article which explains this very well. http://www.avweb.com/news/columns/182081-1.html

Personally, I don't care for the use of the term Vacuumn, to describe what really is pressure that is lower than ambient , as it is imprecise at best, but if hte term makes it easier to visualize, it's useful.

 

[Yellow Snow]

I have been corrected. Gonna have to discuss this one with my instructor, definately not how it was explained to me.

 

[DC8 Flyer]

Lets see if I can simply this. At full throttle and full rpm lets say you have 26 inches of MP and 2700 RPM. Using simple numbers, the air is flowing through the manifold at 100 MPH. Now, not touching the throttles, you pull the props back to 2200 RPM. The butterfly valve is in the same position, the only thing you have done is forced oil pressure out of the hub. When you slowed the RPMs down you slowed down the flow of air into the manifold to 50 MPH.

Just like airflow over a wing, aka bernoulli, in a closed system, faster moving air has lower pressure than slower moving air. All you have simply done is slown down the rate at which air is entering that particular piston that the MP sensor is on.

 

[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.