Manifold Pressure Question

[DC8 Flyer]

..NVM

 

[VNugget]

Almost. Remember when we lower the RPM we lower the rate at which the mass of air enters the cylinder (2700 RPM - 45 times a second 2200 RPM = 33 times a second). We dont change the mass we simply change how many times it enters the cylinder over a given time period. Since Horsepower is work over time, the lower the rate at which the fuel is burned lowers the work over time equation. So we are changing power, otherwise we could take off at 2200 RPM and have the same power as 2700 RPM.

Hmm... come to think of it, you're right. I forgot about that.

 

[Donsa320]

Almost. Remember when we lower the RPM we lower the rate at which the mass of air enters the cylinder (2700 RPM - 45 times a second 2200 RPM = 33 times a second). We dont change the mass we simply change how many times it enters the cylinder over a given time period. Since Horsepower is work over time, the lower the rate at which the fuel is burned lowers the work over time equation. So we are changing power, otherwise we could take off at 2200 RPM and have the same power as 2700 RPM.

Just keep in mind I am talking about the same airplane at the same point in space and time just one instance is 2700 RPM and the other is 2200 RPM, throttle and mixtures are the same only difference is RPM by prop control.

Whoops, sorry, you would get the same MAP change if you did it in flight with the engine not running, just windmilling in idle cutoff. Anytime you are at Part Throttle the throttle plate restriction is the reason the MAP changes with RPM changes. Avbug is totally correct. It has nothing to do with engine power. The engine is just an airpump in this situation.

~DC

 

[DC8 Flyer]

..NVM

 

[Donsa320]

Nope, sorry. All the MP gauge measures is absolute pressure at the intake valve (thats where the sensor is). If the engine is windmilling, ie no power, there is a very slow rate of "suction" thus the largely scaled MP gauge would read atmospheric pressure (absolute pressure in this case).

You guys need to get off this idea that just because the engine sucks air in, that it is creating a vacuum, if that where true the engine would be starving itself because in a vacuum there is no air.

You are stubborn, aren't you. I have done this exercise for over 50 years and whatever you are trying to say is WRONG! Man, wrong beyond my comprehension. Asquared, Avbug, the Marine guy...anybody... help. <bg>

Seriously, go get a constant speed equipped airplane and try this stuff out. Whatever you are reading in books is not cutting it!

~DC

 

[DC8 Flyer]

..NVM

 

[avbug]

Donsa,

He's quite wrong, but can't apparently see the forest through the trees.

First of all, the point at which manifold pressure is sampled varies with the engine installation. I've never seen a manifold pressure "sensor," though most all installations from small four cylinder horizontally opposed engines to large twenty eight cylinder radial installations use direct tube installations (bourdon tube). This may sample at any point in the intake manifold downstream of any boost device, and may sample at the cylinder, or at any other select point in the intake manifold. (That's why it's called manifold pressure, not intake-at-a-single-cylinder pressure...but you probably already knew that).

Second, the engine is indeed an "air pump." In a direct drive system, propeller RPM is crankshaft RPM, and it matters not in a normally aspirated engine weather the engine is driving the propeller or the propeller is driving the engine with respect to indicated manifold pressure. Indeed, shutting off the engine and letting the engine windmill with a closed throttle will indicate manifold pressure below barometric...how far below is entirely dependent upon engine RPM.

Shut off the engine and let it windmill with the throttle plate closed, you'll see the same low manifold pressure that you see at idle with the engine running. Pull the nose up, slow the prop, note manifold pressure climb as engine RPM decreases. Not because of a change in airflow velocity through the induction system but purely as a result of a decay in engine RPM...the air pump is slowing down...just like slowing down the proverbial vacum cleaner motor.

 

[DC8 Flyer]

..NVM

 

[gern_blanston]

You two are talking about two different phenomena, engine running, engine windmilling. The windmilling engine rise and fall in MP is due to expansion since the velocity of the air through the manifold is very slow 300 to 400 RPM.

The intake manifold doesn't care whether the engine's 'running' or 'windmilling', just that it's 'pumping'.

Now lets get back to the OP's original topic. Why does MP rise when he does a prop control check on runnup. Because with the throttle plate fixed as the air pump (piston) slows down the speed at which the volume of air is moving through the manifold decreases, raising its pressure.

Doesn't the volume of air entering the intake manifold decrease in proportion to the RPM change, minimizing any velocity change? It IS a positive-displacement pump, after all.

I'm gonna' have to jump in with Avbug and Donsa. I don't think there's enough of a velocity change to make that much difference. You're the physics whiz, so what's the formula for Pressure Delta -vs- Velocity Delta? I mean what sort of velocity change would be required to drop the pressure 60%, like going from 29" to 10" of manifold pressure when you go from full throttle to idle?
I love math, so let's see the numbers. Thanks.

 

[USMCmech]

You two are talking about two different phenomena, engine running, engine windmilling. The windmilling engine rise and fall in MP is due to expansion since the velocity of the air through the manifold is very slow 300 to 400 RPM.

Windmilling or running, the MAP will vary with trottle movement. A fact that many pilots don't realize. I routinely cover this when I do a BFR (what will happen if the engine quits, prop keeps spinning, ect.).

Now lets get back to the OP's original topic. Why does MP rise when he does a prop control check on runnup. Because with the throttle plate fixed as the air pump (piston) slows down the speed at which the volume of air is moving through the manifold decreases, raising its pressure.

AvBug, you really need to stop and think about what you are saying and read a book. Just because something sucks air in, doesnt mean it lowers the pressure entering or raises just because its a pump, there are Physics laws for the changes in pressure and you as a pilot should know them. I've given you multiple examples, formulas, Fluid Dynamics laws, its your turn to come with something other than "I said so". Anyone who flys an airplane with variable pitched props and a manifold pressure gauge know as you pull the props back the MP rises, there is only one way to describe it, and its not because the engine is a vacuum cleaner sucking air, therefore the pressure rises because the engine isnt sucking as hard. What does that explain? Look at a vacuum system and how it works, given suction to maintain an airstream velocity. Same as your precious vacuum cleaner analogy but pressure falls as engine speed is increased so it is vented overboard.

http://www.avweb.com/news/columns/182081-1.html

Deakin speaketh the truth.


Througout the intake system, from the trottle plate aft, the MAP is a measure of how much suction the "downward" moving piston is creating. There is very little change in total velocity in this system (the venturi is fwd of the throttle plate). The air pressure is trying to colapse the intake manifold (think about sucking a milkshake through a straw) because the piston is creating a vacum and the throttle is not allowing enough air to fill it.

 

[DC8 Flyer]

..NVM

 

[gern_blanston]

Uhhh. The volume per cylinder per revolution doesn't change, but we're decreasing the rpm, remember? So the volume of air per unit of time has decreased. I think that's the point you're missing.

 

[DC8 Flyer]

..NVM

 

[DC8 Flyer]

..NVM

 

[USMCmech]

Maybee it's just me since I'm just a dumb$hit mechaninc who never went to no college. But this really don't make no sense.

Just because the piston is sucking doesnt mean its creating a vacuum, the volume beind displaced is being filled with air at a restricted rate. This rate is slow enough to that not enough of ambient pressure air can fill the cylinder so the air that does get in expands to fill its container, and keeping with thermodynamic and fluid dynamic laws, the pressure decreases.

A piston sucking DOES make a vacum.

Otherwise no air would flow into the now larger cylinder.


Now you are correct, if we restrict the amount of air that can flow into that cylinder, then that smaller volume will expand and have a lower pressure.

What we have here is that we are talking about the same thing, except you are makeing this more complicated than it needs to be.

The piston sucks in air.

The closesd throttle plate only allows in a small amount of air.

The intake manifold has a lower pressure.

If you reduce the RPMs then the pistons aren't sucking as hard, therefore pressure rises.

 

[DC8 Flyer]

..NVM

 

[Donsa320]

I am by no means calling you dumb, you know more about engines than I ever could. The point I am trying to make, is that just because the piston is going down and drawing air in that doesnt make the pressure go down. The pressure goes down BECAUSE the air is traveling faster through the manifold. You could blow air into the manifold and get the same result. Take the manifold off the cylinder and blow air through from the intake end you will see a drop in MP because of the bernoulli effect.

The faster the piston moves down the faster the air is drawn through the manifold, we agree on this I think. Just take it the step further and apply the laws of physics to what happens to a fluid as its velocity is increased.

If the piston was making a vacuum you would see zero on the MP gauge, no air in a vacuum. The piston is displacing volume causing a pressure differential. Maybe thats being too nit picky on word usage but it seems the idea is that just because the piston goes down, it creates a void and pressure drops, when that is not what is happening when we just pull the props back.

Lordy, I see you guys are still at it. USMC just did a good summary and DC-8 it looks like you have come around a little.

Now it looks like DC-8 has gradually accepted that the engine is just an air pump and MAP is not dependant on the engine even operating. MAP (Manifold Air Pressure) is dependant upon the position of the throttle plate and the RPM of the engine, whether it is windmilling or operating.

Some examples: In flight, at a high enough cruise speed that the constant speed propeller can maintain the set RPM, move the mixture to cut-off. Observe the MAP. There will be NO change if the RPM does not change. Lesson; Manifold Air Pressure is NOT am indicator of engine power. Lesson 2; RPM is not an indicator of engine power either. I point these facts out to light-twin engine pilots especially.
Next, at lower airspeed where the RPM may decrease on a failed engine, one may see the MAP actually increase on the non operating engine. Again a misleading indicator of power loss or failure leading to misidentification of the failed engine.
These are some practical applications of the indications we are talking about.
The MAP rise during the propeller governor check or when reducing RPM during power reductions from Take-Off to Climb or from Climb to Cruise help illustrate this.
That otta get ya going again! <bg>

~DC

 

[gern_blanston]

So what about the volume change? If you reduce the RPM from 20%, the volume of air being drawn into the engine for any given amount of time decreases 20%, wouldn't it?

 

[DC8 Flyer]

..NVM

 

[DC8 Flyer]

..NVM

 

[gern_blanston]

We're not talking about cylinder pressure though, we're talking manifold pressure.
With a 7.5% decrease in RPM, don't you get a 7.5% decrease in the volume of air pulled through the intake manifold if the throttle plate doesn't move?
(OK, I realize that air's actually 'pushed' through the intake manifold due to the differential pressure caused by the increased displacement in all 6 cylinders on their intake strokes.)

 

[gern_blanston]

And the intake manifold doesn't know whether the engine's running or not, just that air's being transferred through it.

 

[MVSW]

This is a really funny post. I was reading this and I read about 25 different reasons why MP goes down when you decrease RPM!!LOL

 

[Donsa320]

This is a really funny post. I was reading this and I read about 25 different reasons why MP goes down when you decrease RPM!!LOL

Now, that is sad.....because MAP goes UP with a decrease in RPM, that was what all this was about. <vbg> Shows you what complicated explanations can do.

~DC

 

[Donsa320]

Lesson; Manifold Air Pressure is NOT am indicator of engine power. Lesson 2; RPM is not an indicator of engine power either. I point these facts out to light-twin engine pilots especially.
Correct Again! But only if you are only using one or the other, the two combined tell you engine power;
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DC-8

My last word on the subject...you are WRONG again. You can have Manifold Pressure and RPM and NO power, they are both INDIRECT indicators of power, whether alone or together. Any windmilling engine will show you this. Please, get out and fly and try it out, Jeez! You do fly do you not? You claim an ATP but I guess it was not in piston power. These facts are true in both normally aspirated and geared supercharger engines. From Apaches to DC-3s, Convairs and bigger. From O-320s to R-4360s and R3350s, I have plenty of time with all of them.
When you get to Turbo Superchargers there is a difference since exhaust energy is required to get some boost. If you want to go there, do it with someone else.
Good Day Sir.

 

[Donsa320]

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

Yes. You are pulling air out of the intake manifold into the cylinders at a slower rate as the RPM decreases so air coming in over the throttle plate from the atmosphere can increase the Manifold pressure closer to ambient.
Do you like it? (grin)

 

[Denizen]

Prt

Donsa, I know you bowed out of this conversation, However a quick ??

In the large engines you mentioned, I recall Power Recovery Turbines or some such from my A&P books. Only having seen them in museums, I forget how they work, Maybe you guys can explain them...they were/are pretty neat...

Thanks

 

[DC8 Flyer]

..NVM

 

[DC8 Flyer]

Yes. You are pulling air out of the intake manifold into the cylinders at a slower rate as the RPM decreases so air coming in over the throttle plate from the atmosphere can increase the Manifold pressure closer to ambient.
Do you like it? (grin)

You are still not explaining it! You said it "You are pulling air out of the intake manifold into the cylinders at a slower rate". What law jumps out at that statement?

 

[DC8 Flyer]

..NVM