douglassmt

douglassmt

Well Known Member
I'm just starting on CS propeller airplanes and making the transition to that third little knob on the panel in preparation for flying my RV. As the concept of manifold pressure was explained to me, it immediately occurred to me that it's not a pressure, it's a vacuum. As you open the throttle, it increases the air flow (and fuel) to the engine, but you quickly learn that the max manifold "pressure" you can achieve (on non-turbo engines) decreases with altitude and cannot exceed 30"hg at sea level. So manifold "pressure" is the difference between atmospheric pressure at your altitude and the "pressure" in the intake manifold. If MP was a pressure in the conventional sense (positive), air would flow out of the intake manifold back to the atmosphere - bad deal. It has to be a vacuum for the engine to suck air in.

Am I wrong?

This might just be this engineer annoyed at incorrect terminology, but it seems to me it would explain better if proper terms were used.
 
You are correct. Check out the John Deakin article "Manifold Pressure Sucks" article on Avweb. (no kidding)

Erich
 
Manifold Pressure is Absolute Pressure

douglassmt said:
I'm just starting on CS propeller airplanes and making the transition to that third little knob on the panel in preparation for flying my RV. As the concept of manifold pressure was explained to me, it immediately occurred to me that it's not a pressure, it's a vacuum. As you open the throttle, it increases the air flow (and fuel) to the engine, but you quickly learn that the max manifold "pressure" you can achieve (on non-turbo engines) decreases with altitude and cannot exceed 30"hg at sea level. So manifold "pressure" is the difference between atmospheric pressure at your altitude and the "pressure" in the intake manifold. If MP was a pressure in the conventional sense (positive), air would flow out of the intake manifold back to the atmosphere - bad deal. It has to be a vacuum for the engine to suck air in.

Am I wrong?

This might just be this engineer annoyed at incorrect terminology, but it seems to me it would explain better if proper terms were used.
Click to expand...

For those used to thinking in terms of vacuum, manifold pressure might be a bit confusing at first. Conceptually there really is no such thing as a vacuum, since a vacuum is just a lack of gas pressure.

For our aircraft engines, the manifold pressure reads in absolute pressure. So, for example if your engine is not running the manifold pressure gauge will read the pressure of the atmosphere, which is approximately 30 inches of mercury at sea level. When the engine is running and the throttle is closed such as at idle, you will see a low pressure on the manifold gauge. The pressure is in fact lower than atmospheric pressure, but don't think of it as a vacuum. In this condition, the throttle plate on the carburetor (or fuel injection servo) is nearly closed, but the engine is attempting to draw air through it, thus there is a large pressure drop. Since the manifold pressure instrument measures the pressure in the intake manifold between the carburetor or servo and one of the engine cylinders, this pressure that is lower than atmosphere is what you read on your instrument, maybe 10-12 inches of mercury. When you open up the throttle to full throttle for take off, you should see approximately atmospheric pressure or just slightly less (due to some losses of the intake system), thus at see level you might see 28 inches or so of pressure. In a turbo charged plane, the turbocharger is actually pressurizing the intake manifold above atmospheric pressure at full power, thus you might see something like 36 inches of pressure at full power, while atmospheric pressure may still be only 30 inches or so at see level, thus in this case the turbocharging system is boosting the manifold pressure 6 inches of mercury above the pressure of the atmosphere.

Skylor
 
douglassmt said:
I'm just starting on CS propeller airplanes and making the transition to that third little knob on the panel in preparation for flying my RV. As the concept of manifold pressure was explained to me, it immediately occurred to me that it's not a pressure, it's a vacuum. As you open the throttle, it increases the air flow (and fuel) to the engine, but you quickly learn that the max manifold "pressure" you can achieve (on non-turbo engines) decreases with altitude and cannot exceed 30"hg at sea level. So manifold "pressure" is the difference between atmospheric pressure at your altitude and the "pressure" in the intake manifold. If MP was a pressure in the conventional sense (positive), air would flow out of the intake manifold back to the atmosphere - bad deal. It has to be a vacuum for the engine to suck air in.

Am I wrong?

This might just be this engineer annoyed at incorrect terminology, but it seems to me it would explain better if proper terms were used.
Click to expand...

Just change your frame of reference to zero pressure, not atmospheric pressure. Then, there is no vacuum, just various positive pressures.

It's the same concept as absolute zero. All temperatures above zero K are positive. Common folk use Farenheit or Celcius and talk about "negative temperature", which does not exist. It's all in the frame of reference.

Neither does centrifugal force, but that's a different discussion....

Vern
 
Lack of Centripetal Force?

Semantics, I suppose, but:

Centrifugal Force - The False Force​
An evil word has worked its way into our daily vocabulary, and with it, an incorrect understanding of the way physics works. "Centrifugal Force" ( Latin for "center fleeing") is often used to describe why mud gets spun off a spinning tire, or water gets pushed out of the clothes during the spin dry cycle of your washer. It is also used to describe why we tend to slide to the outer side of a car going around a curve. It is a common explanation...the only problem is all of it is absolutely wrong!!! Centrifugal force does not exist...there is no such thing...it is a ghost we tend to blame odd behavior on.
Take for example this common situation. You are riding in a car going around a curve. Sitting on your dashboard is a cassette tape. As you go around the curve, the tape moves to outside edge of the car. Because you don't want to blame it on ghosts, you say "centrifugal force pushed the tape across the dashboard."--wwrroonngg!! When we view this situation from above the car, we get a better view of what is really happening. The animation below shows both views at the same time. The top window shows you the bird's eye view of the car and the tape, while the bottom window shows you the familiar view from the passenger.
casette.gif
The car tires on the road have a enough static friction to act as centripetal force which forces the car to go around the curve. The tape on the slippery dashboard does not have enough friction to act as a centripetal force, so in the absence of a centripetal force the tape follows straight line motion. The car literally turns out from underneath the tape, but from the passenger's point of view it looks as though something (a ghost force?) pushed the tape across the dashboard. If the car you are riding in has the windows rolled down, then the tape will leave the car (or does the car leave the tape?) as it follows its straight line path. If the windows are rolled up, then the window will deliver a centripetal force to the tape and keep it in a circular path.
Any time the word Centrifugal Force is used, what is really being described is a Lack-of-Centripetal Force.

For More: http://www.regentsprep.org/Regents/physics/phys06/bcentrif/default.htm
 
vlittle said:
It's the same concept as absolute zero. All temperatures above zero K are positive. Common folk use Farenheit or Celcius and talk about "negative temperature", which does not exist. It's all in the frame of reference.


Vern
Click to expand...

I once hear a TV weatherman say "It's zero degrees outside this morning. No temperature at all!" :rolleyes:
 
douglassmt said:
I'm just starting on CS propeller airplanes and making the transition to that third little knob on the panel in preparation for flying my RV. As the concept of manifold pressure was explained to me, it immediately occurred to me that it's not a pressure, it's a vacuum.
Click to expand...

Another way to look at this is that the manifold pressure is actually higher than the cylinder pressure during the intake stroke. Otherwise air couldn't enter the cylinder. You could perhaps think of the cylinders as "vacuum" pumps, but really they just have lower pressure.
 
Good comments

Thanks all for the good comments. At least for me, it helps to bounce this around a bit.

Skylor, as for MP being absolute pressure, I agree - sort of. As someone commented, all pressure is actually absolute, just like the temp. scale. However, when we use gauge pressures for reference (or F/C temp. scales) we (by default) develop and recognize terms like vacuum (negative gauge pressure) and below zero (negative F/C temperature) even though they are meaningless in the absolute sense. So when we are using a gauge to measure the pressure in the intake manifold relative to atmospheric, and the pressure in the intake is less than atmospheric, that is a vacuum and we should call it a vacuum. My only point, really, is that for someone unfamiliar with how engines work and trying to learn, it doesn't make sense to call it a "pressure". At least for me.

Obviously just one of those conventions we've gotten used to that will probably never change. But we can try, like convincing someone of the fallacy of centrifugal force.
 
douglassmt said:
So when we are using a gauge to measure the pressure in the intake manifold relative to atmospheric...
Click to expand...

But when are you doing that? Manifold pressure gauges in aircraft measure absolute manifold pressure, not manifold pressure relative to atmosphere.

True, this manifold pressure will be less than absolute atmospheric pressure, unless you are turbocharged or supercharged or getting significant ram air effect in your induction system.

But on the other hand it will be greater than cylinder pressure during the intake stroke, and that's why engine power is in part a function of absolute manifold pressure, and that's why absolute manifold pressure is useful to measure.

--Paul
 
Insurance claim

All this talk of absolute vs. relative motion and pressure reminds me of a supposedly true statement written on an auto insurance claim form:

"The telephone pole was approaching. I was attempting to swerve out of its way when it struck my front end."
 
Ahhh..

OK, I assumed that the MP gauge was a conventional gauge, comparing abs. pressure in one location to abs. pressure in another. If it is a true abs. pressure gauge, that would make more sense. It would explain why it reads local atmospheric/barometric pressure when the engine is not running and decreases upon start and further upon engine rpm increase.

It's still a suction in the manifold, but that helps explain why it's called manifold "pressure" instead of vacuum. I stand corrected.
 
Preflight check?

Having worked on a couple vacuum chambers for industry, I'm aware there is no such thing as 100% leak proof. Thus, I wonder how long it takes for a typical MP gauge to read incorrectly? Certainly years would be expected.

Does anyone check their MP gauge prior to engine start to ensure it's reading ~30 inches or whatever your barometric pressure happens to be? If it's not, it's broken.
 
skylor said:
Since the manifold pressure instrument measures the pressure in the intake manifold between the carburetor or servo and one of the engine cylinders, this pressure that is lower than atmosphere is what you read on your instrument, maybe 10-12 inches of mercury.
Click to expand...

Sorry, but no.

The manifold pressure gauge does not compare the differential pressure between the manifold and an engine cylinder (or any other part of the engine). It compares manifold pressure against a hard vacuum, and the readout is in inches of mercury (or psi, or kg/cm2 or whatever) of ABSOLUTE pressure. One atmosphere at sea level on our blue mud-ball planet is 14.70 psia (pounds per square inch absolute) or 29.92 inches of mercury as compared to the relatively hard vacuum of space a few hundred miles above.

The key here is the difference between GAUGE pressure and ABSOLUTE pressure. Absolute pressure is the total pressure of whatever you are measuring, referenced against a hard vacuum. Gauge pressure is the difference between whatever you are measuring, and the environment the gauge is immersed in. These pressures are commonly used in the engineering world as PSIG and PSIA, but most of the rest of the world has no clue about PSIA and so they only have experience with GAUGE pressure, which just gets shortened to psi. Strictly speaking, psi is a surface loading, not a pressure.
 
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WW2

I've heard that old WW2 British aircraft like Lancasters and Spitfires have manifold pressure gauges that show pressure relative to the atmosphere. So climb power could be +2 psi, and cruise could be -3psi [examples of course]. Next time I'm in hamilton, I'll have to look into the Lancaster cockpit.
 
airguy said:
Sorry, but no.

The manifold pressure gauge does not compare the differential pressure between the manifold and an engine cylinder (or any other part of the engine). Snip
Click to expand...

I never said that it did but I can see how you interpreted my statement as that. I should have said that "the manifold pressure instrument measures the pressure at a point in the intake manifold that is between the carburetor or servo and one of the engine cylinders, this pressure that is lower than atmosphere is what you read on your instrument, maybe 10-12 inches of mercury.
 
skylor said:
I never said that it did but I can see how you interpreted my statement as that. I should have said that "the manifold pressure instrument measures the pressure at a point in the intake manifold that is between the carburetor or servo and one of the engine cylinders, this pressure that is lower than atmosphere is what you read on your instrument, maybe 10-12 inches of mercury.
Click to expand...

Ahhh, quite right then, sorry about that...
 
Excellent Discussion!

Just to throw another wrench into this kind of discussion, electricity actually flows from negative to positive. If electricity was pressure, you would think that it would flow from a positive pressure to a negative pressure. This is what was thought when the symbols were made up. Electricity actually flows from a surplus of electrons to a lack of electrons. When we say current flows from positive to negative it is called "Conventional current or Hole flow".
 
Nictech said:
Just to throw another wrench into this kind of discussion, electricity actually flows from negative to positive. If electricity was pressure, you would think that it would flow from a positive pressure to a negative pressure. This is what was thought when the symbols were made up. Electricity actually flows from a surplus of electrons to a lack of electrons. When we say current flows from positive to negative it is called "Conventional current or Hole flow".
Click to expand...

And if you notice, the arrow on the diode and the arrow on transistor's emitters all point in the direction of current flow, from positive to negative. Ask Bell Labs why they did this!
 
elippse said:
And if you notice, the arrow on the diode and the arrow on transistor's emitters all point in the direction of current flow, from positive to negative. Ask Bell Labs why they did this!
Click to expand...

Actually, the flow of current is just a classical physics view of the quantum wavefunction of the probabilistic distribution of charge in a conductor as it moves along the time axis, and is observed by a fellow named Heisenberg. Please remember this when wiring your circuits or you will let the smoke out!:)
 
Probabilistic?

Actually, the flow of current is just a classical physics view of the quantum wavefunction of the probabilistic distribution of charge in a conductor as it moves along the time axis, and is observed by a fellow named Heisenberg. Please remember this when wiring your circuits or you will let the smoke out!
Click to expand...

Are you sure? Even Heisenberg was uncertain!:confused:
 
What I learned

My last electrical schooling was two years ago and we were taught about two different conventions
Electron Flow, which is negative to positive
Conventional Flow, which is positive to negative.

As long as you hook up your diodes and other electronics the correct way, it really doesn't matter which convention you use.
 
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