Manifold Air Pressure - What is it??

[Helo]

Sorry to bother you guys, but as a PPL there's a pretty basic technical question that I never asked early on in my training, and having qualified am embarassed to ask about now .....

What exactly is a manifold, and how does the air pressure in it measure engine power? How, if at all, does this relate to engine RPM, which is what I vary to cool down my piston helicopter?

Thanks for your help.

[Field In Sight]

Manifolds are just a set of pipes that lead into "Induction" and out of "Exhaust" the engines cylinders.

The power an engine can produce is related to the RPM and manifold pressure.

The faster the RPM the more bangs per minute and therefore more power.

The induction manifold pressure is related to the amount of "charge" i.e. fuel/and air that can be stuffed in to the engine thus giving a BIGGER (more powerful) bang.

When the engine RPM is constrained i.e. Constant speed propeller or RPM contol of the main rotor the varying element of power is the Induction Manifold Pressure.

That's my understanding anyway.

[Kermit 180]

Manifolds are the pipes and ducts that take a fuel/air mixture to the cylinders from a carburetor or fuel control unit (induction manifold) and away from the cylinders to the exhaust (exhaust manifold). The MP guage (measured in inches Hg) measures pressure in the inlet manifolds downstream from the carburetor or fuel control unit.

With a Constant Speed Unit equipped aeroplane the manifold pressure (pressure of the mixture downstream from the carburettor or fuel control unit) is controlled using the throttle, and propeller RPM is controlled using a pitch control.

Not knowing much about helicopters, I would presume a reduction in manifold pressure, and therefore the amount of engine work being done, would bring about a reduction in temperature, although the addition of more fuel might help to cool the cylinders :confused: .

Hope this sort of makes sense and helps.

Kermie

[Tinstaafl]

MAP is short for 'Manifold Absolute Pressure', not "manifold air pressure". It is often referred to as 'Manifold Pressure' (MP).

The greater the pressure in the inlet manifold, the greater the amount of fuel/air mixture that will enter the cylinders. More fuel & air in the cylinders equals more power being produced. 'More fuel & air' in this case means measured by weight, not volume or pressure.

For all practical purposes though, the density of the mixture is constant, so the pressure in the manifold will be representative of how much mixture will go into the cylinders.

Closing the throttle causes the MAP to reduce because the throttle restricts how much mixture can flow into the manifold to replace the mixture that the cylinders have just sucked out. Hence the term 'throttle' - it literally throttles the engine.

Less mixture available in the manifold to enter the cylinders means a lower power output from the engine.

Opening the throttle removes some or all of the restriction, allowing more fuel/air mixture to enter the manifold to replace what the cylinders have sucked out, ready to enter the next cylinder that has an open inlet valve.

[DB6]

Put more simply, on aircraft with constant speed (variable pitch) propellers there are two power variables as already mentioned, manifold pressure and RPM (which is related to propeller pitch). To draw a rough analogy with a car the RPM - controlled by the RPM lever - can be thought of as the gears (high RPM/fine pitch = 1st gear) and the MAP as the accelerator (controlled by the throttle as described above). As to how this relates to angry palm trees, try posting on the rotary forum if you haven't already.

[IanSeager]

The word pressure can be misleading in this context. In normally aspirated engines the MAP will not (ever) be higher than atmospheric pressure. (Take a look at the MAP gauge before starting the engine)

With anything lower than full throttle you will have a degree of suction in the inlet manifold

Ian

[TOT]

I have looked at your profile , as you fly a H 269 helicopter this reply is specific for that helicopter.

In the case of manifold pressure, this is a misconception. As there is no turbo on the H 269 there is NEVER pressure in the inlet manifold, only vacuum. The word manifold , as applied to the H269 refers to the quadruple pipe connecting the fuel distibutor to the individual cylinders. The MAP (vacuum) is taken from a port midway between the fuel distributor and the inlet valves of the engine. When the engine is stopped the MAP will read atmospheric pressure,( eg 29.92 or 1013mb). When the engine is running, regardless of power being used , there will always be vacuum in the manifold. The engine was designed to produce 225 hp( reduced to 190hp ) . To do this this it requires a certain WEIGHT of air. When the engine is idling rotors engauged, the MAP will read approxiametley 10 -11 inches .At this moment the butterfly in the venturi will be open a small amount, the vacuum in the manifold will be quite high. When you are hovering using , lets say 24 inches the butterfly will be open MORE and vacum will be LOWER, however because of reverse gearing in the MAP gauge you are now able to use the gauge as an indicator to power being used.
ENGINE TEMPERATURE/POWER CHART.
You will be aware of the above chart on the heli panel.
As the engine requires a given weight of air to produce the design power the volume of air required will vary from day to day with temperature /height being flown.
In simple terms the chart says , on a colder day(more dense) maximum design power is reached at say 24.1inches, but on a warmer day the engine needs a greater volume ( less dense) say 26.0 inches - but still the same weight, therefore the butterfly will be open MORE on warmer day to allow the greater flow of air required.
The MAP has nothing to do with cooling of the engine after flight.

[Helo]

It never ceases to amaze me the power of such a discussion forum - my knowledge increases every time I log on!

Thanks for the replies, especially TOT since his was 100% specific tothe helicopter I fly.

Thanks again. :)

[Whirlybird]

Wow, I thought I understood this stuff, and I didn't know half of that. Helo, thanks for asking the question. And if you don't mind my highjacking your thread, I wonder if TOT could post an R22 specific answer too?

[Helo]

Hijack away (although I just might squawk 7500) ;)

[Tinstaafl]

TOT,

I beg to differ regarding your assertion that there is a vacuum in the inlet manifold.

There ispressure in the manifold. It may be less than atmospheric, but it is a pressure none the less. In a normally aspirated engine this somewhere in the range of approx. 10"Hg to 27"Hg, depending on power setting.

The only time it would be a vacuum would be if all gas was evacuated from the chamber in which case there would be no mixture to enter the cylinders.

[DB6]

If I give you another hair will you split that as well :D ?

[Flight Safety]

Here's a really good article on manifold pressure, from the avweb website.

MAP article (http://www.avweb.com/articles/pelperch/pelp0015.html)

[Flight Safety]

All of the posts so far relating to manifold pressure are correct, but I’d like to try a different perspective, which is to look at a reciprocating engine from an “air pump” perspective. I think understanding this particular perspective helps a lot in understanding exactly what manifold pressure is.

Let’s assume for this perspective, that whether gasoline enters the engine and is burned or not, is unimportant. Yes I know, the reason a reciprocating engine exists is to burn fuel and create power, but to see the “air pump” nature of an engine, let’s leave the fuel out at least for the first part of this discussion.

When a recip engine is rotated (whether by its own power burning fuel or by an external force) and the pistons go up and down and the valves open and close, the engine is literally behaving as an air pump. It sucks air into the cylinders (via the intake manifold) and pushes air out of the exhaust valves into the exhaust manifold, where the air exits the engine.

The main purpose of the intake manifold is to concentrate the intake of air into the engine at one point (for most engine designs) and then to distribute that air to all the cylinders. There are 2 main reasons for concentrating the air intact at one location (again for most engine designs). The first is to provide a means of measuring the exact amount of air entering the engine, so that the correct amount of fuel can be mixed with it. The second reason is to provide a means of controlling the amount of air that’s entering the engine as it pumps (this of course is the throttle).

The purpose of the exhaust manifold is to concentrate the exit of air (or burnt gases) from the engine. You have to do this since you don’t want those hot exhaust gases to go just anywhere.

The manifold pressure gauge senses the pressure (or vacuum) inside the intake manifold. The sensing point is located in the intake manifold between the cylinders and the throttle (which is a valve, normally a “butterfly” valve, at the central air intake point). Aside from the number and size of the cylinders and any increase in “air pumping” efficiency provided by the complexity of the valve gear (which are fixed when the engine is designed), there are only 2 things that control the amount of air being “pumped” by the engine, the engine RPM and the position of the throttle valve. Any given engine RPM (say 2700) determines the maximum amount (or potential amount) of air that the engine can pump (at that RPM), while the throttle exists to lower and control this maximum potential.

Other factors also determine the maximum amount of air (volume and weight) that an engine can “pump”. These include altitude, temperature, and barometer, which translate into absolute air density.

An interesting fact about normally aspirated recip engines (those without turbochargers or superchargers) is that at any given engine RPM and throttle setting (with all other factors being equal), they will “pump” virtually the same amount of air through the engine whether they are burning fuel or not. This is why the “air pump” perspective is so useful.

The manifold pressure gauge is really nothing more than an absolute barometer gauge, unlike the altitude corrected unit in the altimeter. When the engine is off, the manifold pressure gauge should read the absolute barometer at your location and altitude. Without fuel burning, let’s say that an engine is turning at a low RPM and the throttle is wide open. In this case the manifold pressure gauge should read nearly the same as the engine off reading, since the wide-open throttle allows normal atmospheric pressure inside the intake manifold without restriction. If the conditions were the same except that the throttle is closed (a closed throttle does allow a small amount of air to enter), then a partial vacuum would form inside the manifold as the “air pumping” capacity of the engine even at low RPM would exceed the air supply being allowed into the intake manifold. Now the manifold pressure gauge is reading an absolute atmospheric pressure inside the intake manifold that is much lower than the surrounding atmosphere.

Now let’s say this engine without fuel is turning at high RPM and the throttle is still closed. The manifold pressure would be even lower, as the increased “air pumping” capacity of the engine caused by the high RPM, is evacuating the intake manifold even faster than before, as the closed throttle is still only allowing a very small amount of air to enter into it. At high RPM with a wide-open throttle, the manifold pressure gauge would read near (but a little less than) the surrounding atmospheric pressure, since the high air flow rates inside the intake manifold are partially restricted by flow inefficiencies within the manifold itself and within the throttle body and/or carburetor.

Let’s say you wanted to create a condition where an engine turning at 2000 RPM has a manifold pressure of 16 inches. At 1000 ft. altitude let’s say the throttle must be opened about 1/3 to create the 16 inches of manifold pressure. At 5000 ft. altitude, the throttle would have to be opened even wider to create the same 16 inches of manifold pressure, since the pressure density of the surrounding air is lower at the higher altitude. In other words a greater volume of air would be needed since the air entering the engine is less dense (at a lower pressure) to begin with.

So now let’s add fuel back into this picture of an “air pumping” engine.

An important thing to keep in mind about recip engines is that fuel is mixed with the incoming air in a precise way. The air/fuel mixture has to be controlled to a very tight tolerance for normal combustion to occur, and this mixture can be varied slightly (and I mean SLIGHTLY) to a “rich” or “lean” condition where this is useful. This is the “mixture” control found on most recip aircraft. Since the air/fuel mixture is so tightly controlled, you can see that the power output of an engine is based primarily on the amount of air being allowed to be “pumped” through it.

In aircraft applications of recip engines, engine RPM must be controlled for various reasons, such as maintaining a constant rotor RPM in helos, maintaining a constant propeller RPM for constant speed props, and for other reasons. In those situations where fixed RPM settings are used, the power output of an engine is regulated by the throttle, and this in turn is registered by the manifold pressure gauge. In applications where a governor is used to control engine RPM, the throttle is moved more open or more closed to meet the changing power demands placed on the engine, in order to maintain a constant RPM. As the changing air supply needs (i.e. power needs) are being manipulated by the governor at the throttle, this would be reflected as changing manifold pressure readings.

Hope this helps.

(edited for additional clarity)

[Charlie Foxtrot India]

Thanks Flight Safety, that helps make it very clear...but it is inaccurate that a vacuum is created, and I think it's important to teach the reality, that an area of lower pressure than ambient is created. I've yet to see an MAP gauge read zero.
Sorry to be picky
:)

[Flight Safety]

CFI, that's ok, I don't mind.

I did use the phrase "partial vacuum", but "lower pressure than ambient" will work also. The main idea to understand is that the "pumping action" of the engine will lower the internal pressure of the intake manifold below that of the surrounding atmosphere, if the throttle is partially closed. The manifold pressure gauge will in turn read this as a absolute barometer value below the ambient pressure.

If a manifold pressure gauge ever read "0" then the engine would die, as it cannot run without some "atmospheric pressure" inside the intake manifold. You cannot burn fuel without the oxygen from the air.

[Charlie Foxtrot India]

Most manifold pressure gauges I use start at 10"Hg.

One day you might have a physics type person as a student, who wouldn't believe you about there being a vacuum, because strictly speaking there isn't. Even the words partial vacuum are a bit misleading, in my view you either have a vacuum (and are probably in outer space) or you don't, even though my dictionary has a definition of partial vacuum....(like my hoover when there is too much dog hair in the filter.)

Similarly I think it's important to talk of lower than ambient pressure when teaching about lift. I have heard people describing a vacuum above the wing. :eek:

*Visions of a Warrior shooting upwards at escape velocity......*

[Flight Safety]

From the dictionary(dot)com website...

Some definitions of the word vacuum

a. Absence of matter.
b. A space empty of matter.
c. A space relatively empty of matter.
d. A space in which the pressure is significantly lower than atmospheric pressure.

Definitions "c" and "d" are the ones I used. Anyone can tell you that an absolutely pure vacuum as defined by "a" and "b" does not really exist, nor can it be created on earth.

BTW, I happen to BE a physics type person.

[Charlie Foxtrot India]

Fair enough then! My dictionary has similar definitions.:)