Manifold Air Pressure - What is it??

[Flight Safety]

Engine as an air pump

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)