Manifold Pressure Question

[flyifrvfr]

Can someone please tell me why manifold pressure increases when you decrease RPM'S when you are doing a run-up

 

[jspilot]

you are reducing the engine power which in turn reduces the air flow through the intake which increases manifold pressure....slower velocity - higher pressure......well at least it sounds good!!

 

[Yellow Snow]

you are reducing the engine power which in turn reduces the air flow through the intake which increases manifold pressure....slower velocity - higher pressure......well at least it sounds good!!

Sounds backwards to me.

 

[UnAnswerd]

Can someone please tell me why manifold pressure increases when you decrease RPM'S when you are doing a run-up

It doesn't actually increase, technically. You see, anyitme the engine is running, the pressure within the manifold will be lower than atmospheric pressure. We tend to think of it as a "vacuum". But "vacuum" isn't really the best term, because the pressure inside the manifold is not negative, it's just lower than atmospheric pressure.

When you close the throttle, the downward moving pistons are pulling this "vacuum" against a restriction. The actual pressure in the manifold decreases, but the pressure differential or so-called "vacuum" within the manifold increases.

 

[Yellow Snow]

It doesn't actually increase, technically. You see, anyitme the engine is running, the pressure within the manifold will be lower than atmospheric pressure. We tend to think of it as a "vacuum". But "vacuum" isn't really the best term, because the pressure inside the manifold is not negative, it's just lower than atmospheric pressure.

When you close the throttle, the downward moving pistons are pulling this "vacuum" against a restriction. The actual pressure in the manifold decreases, but the pressure differential or so-called "vacuum" within the manifold increases.

Keep trying you're closing in on it. Think about turbo charged engine though might have to rethink this.

 

[UnAnswerd]

Keep trying you're closing in on it. Think about turbo charged engine though might have to rethink this.

I still think he has it backwards. He claimed the manifold pressure increased with lower RPMS. We know that's not true with a naturally aspirated engine.

A turbo??? Well lets see. The turbo uses exhaust gasses to spin a turbine, which in turn forces more air into the manifold than the vacuum created by the pistons could otherwise provide. This means that when you increase RPM, you will also increase the manifold pressure as the turbine spins faster. When you let off, the turbine spins slower, and the manifold pressure decreases.

I say that it doesn't matter whether an engine is turbo-charged or not. The manifold pressure, in the exact meaning of the word, shouldn't be increasing with a decrease in RPM. Of course, I'm dealing with the normal situation, wherein closing the throttle decreases the RPM. Since RPM can in fact decrease with a constant throttle setting, it could be possible to have a higher manifold pressure with a slower RPM. Let's see:

You are climbing at full-throttle in a "cruise climb" with the props forward. You decide to switch to Vy, so you pitch up the nose and climb at a higher rate. The RPM decreases, but the throttle is still wide open. Since the throttle is wide open, the pressure within the manifold is probably close to atmospheric pressure. As the RPM decreases, less of a "vacuum" is created within the manifold because the pistons are moving slower. Therefore, the actual pressure within the manifold does indeed increase even though the throttle is wide open. Though I have little exposure to constant-speed props, I believe it would be possibe to have an increase in manifold pressure if the decrease in RPM was caused by pulling the propeller control back.

 

[UnstableAviator]

When you reduce the engine RPM, you reduce the engine's demand of air. Since the throttle position hasn't changed, the air will in effect "back up" in the intake tract thereby causing a pressure increase. It is just like Bernoulli's principle in reverse to the way we commonly think of it; Slower moving air exerts a higher pressure.

 

[five-alive]

Think about it, turn the engine off, you get maximum manifold pressure (local pressure). Start the engine, and the pressure will decrease for a given throttle position. Changing the rpm lies between these points and gives you a sliding scale.

This is why in order to set say 23" and 2300 RPM you first set the MP to 22.5" and then lower the RPMs, the pressure should rise to 23" or so.


sounds backwards but isnt

 

[UnAnswerd]

When you reduce the engine RPM, you reduce the engine's demand of air. Since the throttle position hasn't changed, the air will in effect "back up" in the intake tract thereby causing a pressure increase. It is just like Bernoulli's principle in reverse to the way we commonly think of it; Slower moving air exerts a higher pressure.

This is true.

In conclusion, I'd say it depends on what is exactly causing the reduction in RPM. If it is caused by closing the throttle, the manifold pressure will decrease with a fixed prop. If it is caused by other factors such as engine loading, the manifold pressure will increase so long as the throttle is held constant, fixed prop or not.

 

[Yellow Snow]

But wait...

If you have a given RPM and increase the throttle this lets more air into the engine right. But the manifold pressure also rises. There is no back-up of air, the engine is actually increasing its power output and taking in more air. This doesn't jive with your theory of less air higher man pressure.

Speak amongst yourselves.

 

[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.

 

[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