Warbird Information Exchange

It was definitely a cooker. Had it not been for an, almost terminal, upper respiratory infection that I was carrying around, I would have enjoyed it a
WHOLE lot more. Kept me in the motel all day Sunday.

Mudge the resiliant

ps. Give me some time with "Miracle Max" and I can be hot.
"It would take a miracle" (With apologies to Billy Crystal.)

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Land of the free because of the brave

Not to beat this subject to death......

Very simplistic -

Think of manifold pressure as the air pressure at the intake valve to the cylinder. The more air pressure, the larger the fuel/air mix that is forced into the cylinder, hence the more power it can produce.

A bigger 'charge' in the cylinder, the bigger the 'bang' when it ignites.

There is a problem, however, if too big of a 'bang' goes off at a low RPM - this leads to detonation (explosion of the fuel/air mix rather than rapid burning) which stresses the mechanical components of the engine (piston, connecting rod, crankshaft) and brings higher temperatures which leads to burned/melted pistons.

So a manifold pressure guage tells you how much 'bang' you are putting in the cylinder, the RPM gauge tells you if the engine is turning fast enough to accept it.

A supercharger/turbocharger simply packs more air into the induction system, allowing a) A bigger air/fuel mix for more power b) Allowing sea-level air pressure to be maintained at altitude which elimiates power loss due to 'thin air' c) A combination of both

Most engines (even small fixed-pitch propeller types) have charts which tell you how much horsepower the engine produces at certain MP/RPM combinations.

RPM alone does not indicate horsepower produced - you can sit in your car in the driveway a rev the engine to 5000 RPM, and it will only produce only enough HP to overcome the internal friction of the engine and the accessories belted to it. If you had a MP guage on the engine it would read a very low pressure, actually a vacuum as compared to outside air pressure.

Technically it is not the pressure at the inlet valve, ie the whole reasoning behind overlap and ram effect and induction tuning.

Mudge all reciprocating engines have inlet manifold pressure, the V, inline, radial has nothing to do with it.

In an NA engine the highest manifold pressure is when it is stopped.

In a running engine if you want it higher it has to be boosted with a pump of some sort.

If you open the throttle which lets in more air and put a load on the engine in essence slowing it down. Well like if you want to go faster in your car you press harder on the gas pedal. In an aircraft you have a certain RPM range to stay in. So if you give it more gas so to say, yet keep the rpm constant the inlet pressure goes up. Same goes if you vary a prop pitch your increasing the load. With out prop pitch the only way to vary the load is to climb the plane or dive it.

The engine produces the most power with the highest manifold pressure.
Why the more pressure/ air fuel you can pack in a cylinder the more power it will produce. But then there is that thing called detonation which is the limiting factor on how much pressure you can pack in there.
And of course other design limitations ie the construction of the engine and the loading on the parts.

Most all diesel engines run at idle at atmospheric pressure.
Were as most all car and aircraft engines idle at a good deal less than atmospheric, because the carburetor, or throttle body throttle plate restricts airflow to the manifold, thus a vaccum is created.

I guess I resaid what the guy above did, I had not read all of it before writing.

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

"Manifold Pressure Sucks" - a great article by John Deakin on the topic, complete with diagrams for those visual learners out there.

Great explanations by all around, but I will take exception to only one thing so far:



Only true at sea level! Altimeter settings are pre-compensated for elevation to give you what the air pressure would be at sea level... imagine they dug a hole down to sea level and lowered a barometer in it; that's the number you get. So at a 5000 foot airport on a standard day the altimeter setting would still be 29.92, but the ambient air pressure (and MP indication with the engine off) would be about 25 inches.

Also another thing that was said both by Bipe215 and written in Deakin's article... that the MP gauge shows suction and not pressure. While true in a way, I think it misses the point and only serves to further confuse the issue.

It's true that at many power settings the MP is lower than ambient (thus "suction" past the throttle plate), but that's irrelevant to the end result that MP (more properly MAP, or manifold absolute pressure) is simply the pressure being fed into the cylinders, thus determining power output. The ambient pressure, or the difference between the two, has no bearing. (before someone mentions it, let's agree to leave exhaust backpressure out of this discussion, OK? )

Let's talk about a supercharged engine and the various power settings it goes through on a typical flight... Say you do a takeoff at full rated 36 inches MAP, climb out at 30 inches, cruise at 25 inches, descent at 20, short final at 10-15, etc. Does it matter that some of those settings were above and some below ambient pressure (30 inches at sea level, and lower up at altitude)? No. Does it even matter what ambient is? No. The MAP (at a certain RPM) is simply the power setting, and that's that.

Then the Brits have to go make it all confusing again

So what about diesel aero engines then?

(Runs away and hides.)

_________________
James K

"Switch on the underwater landing lights"
Emilio Largo, Thunderball.

www.VintageAeroWriter.com

Taigh,
i think that the BMEP explanation was a slight bit off. BMEP is a measure of torque pressure, not cylinder pressure. On the R2800 the BMEP setting is an absolute means of measuring power out put and is more accurate than Manifold pressure as it is unaffected by atmoshperic conditions.Plus it is measured by a sensor on the foward nose case section which is a type of sensor that is against the propshaft to measure the torque.On a given winter day it is possible to overboost a large radial while still being within "spec" as manifold pressure limits are concerned. In our DC-3 we did routinely use the BARO check for engine health, 2000 rpm +- 50. If we didnt make that rpm, we had a cylinder offline for some reason. Been a while since i was involved in them but i think i got it correct, anyone else care to chime in?
Simon

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"Never in the field of human conflict was so much owed by so many to so few"

What Taigh said is spot on, as is what you said; there's no contradiction. BMEP and torque are directly related, by definition. Torque = BMEP x displacement, which is hopefully a constant . So there's no difference in saying you have a torque indicator, or BMEP indicator... in the end it's only a matter of what unit the instrument is marked in, and whether you stay below the specified limit.

To be more clear, in real-life installations, torque is what's being measured, but BMEP units are shown on the indicator, which is where the confusion comes from. The displayed BMEP is derived from the torque measurement divided by a constant, but what it IS is cylinder pressure. (hence the P in BMEP, brake mean effective pressure)

If they are unthrottled, then yes.

Atmospheric is what ever the pressure is in the atmosphere and that can be from sea level till you reach the outer most last drop of air near the edge of space, if that is what you are trying to imply.

Holy crap...I seem to have opened a can o' worms.

Now if we could only get two people to agree....

I'm actually in information overload.
Help...somebody use some 2 syllable words.

Mudge the brain cramped

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Land of the free because of the brave

Brake Mean Effective Pressure, is the average cylinder pressure. Meaning there are high peak pressures and then falling pressure, the mean is the average of those pressures, and is reffered to BMEP, if those various pressures were all combined to do the work against the top of the piston the BMEP is the figure that would be arrived at.

It can be a pretty safe bet for these discussions to branch out into a million tangents, as seen here. It would help to tell us the specific issues you're having trouble with.

In answer to your original question, manifold pressure is the air pressure in the intake manifold, after it's been boosted by the supercharger if so equipped, and regulated by the throttle. In other words, the final stage before it's fed to the cylinders.

Open throttle -> more manifold pressure -> more air in cylinders -> more power
Closed throttle -> less manifold pressure -> less air in cylinders -> less power

Simple as that, as far as as the basics go.

It gets more complicated with mixture, exhaust backpressure, prop loading and RPM, valve and ignition timing, etc., but if you keep all those constant and talk about throttle changes only, that's really as simple as it is.

The throttle changes how much air you let into the engine, and manifold pressure is a measure of that. Why does it matter? More air, burned with the appropriate amount of fuel, gives you more power. And you want to know how much power you're asking from the engine.

Thanks. I'm not implying anything, my brain shut down several pages ago!

IIRC, the Germans operated diesel powered aircraft as high-altitude intruders.

http://en.wikipedia.org/wiki/Junkers_Ju_86

They were turbo supercharged diesels, but 52,500 ft is no mean altitude!

_________________
James K

"Switch on the underwater landing lights"
Emilio Largo, Thunderball.

www.VintageAeroWriter.com

I've been out of town and just discovered this interesting thread. Love the technical stuff.

To defend the Brits, they use the term "boost" because their measurement of MP is gauge, not absolute. That means they measure the difference between the pressure in the manifold and the absolute ambient pressure. Their instrument will have a pressure line to the manifold, and another port on the instrument case (or a second line going outside of a pressurized cabin). So, if you are sitting in Leadville in August or Tuktuyuktuk in February, as long as the engine is off, the gauge says zero. When you are at "plus 5", the pressure in the manifold is five units above the air outside the airplane.

Not sure about all American engines, but the little flat ones I've worked with are all gauge as well. This means they indicate relative pressure. I think it would be very hard to set allowable/preferable operating parameters in terms of absolute pressure, since you would need a different "correct" value for every combination of pressure and temperature. I'm willing to be corrected though.

Now, why do we use "indicated airspeed" instead of real airspeed?

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Bill Walker
Canadian Military Aircraft Serials
www.ody.ca/~bwalker/

What do you mean by "real airspeed"? Are you referring to ground speed?