Manifold Pressure Question

[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