A good test of your instructors knowledge, ask them.
I am talking about an engine that is not boosted (turbo charged).
Call fuel/air mixture "Stuff".
MAP is the absolute pressure of Stuff measured in the intake manifold downstream of the throttle. The more the throttle restricts the airflow into the intake manifold, the more the manifold pressure will drop (as the engine 'sucks' harder) and vice versa. When the throttle is wide open, the MAP can only get as close to the ambient outside air pressure as possible. When the engine isn’t running, the MAP will be the same as the ambient air pressure.
It is common to have a ‘de-rated’ engine that is capable of producing more horsepower than the drive train can accept at lower altitudes. Horsepower is purely a function of the pressure differential inside the cylinder when it goes bang, compared to the ambient pressure outside the cylinder in the crankcase. In order to control this pressure differential, we must introduce the correct mass of Stuff into the cylinder to burn. We can only do this by varying the pressure of Stuff going into the cylinder, as we have no control over the density or the cylinder volume.
In order to limit horsepower to the maximum that the drive train can accept, the manufacturer (and the pilot) must therefore limit the intake manifold pressure to deliver a pre-determined maximum mass of Stuff to the cylinder under the given conditions.
Say full throttle will let 90% of the ambient pressure into the intake manifold (I don't know the real data). It is physically impossible to get 100%, as there is always some restriction of the air. As the pressure altitude increases, the ambient air pressure decreases and 90% of less ambient pressure is less intake manifold pressure. A lower pressure is required in the cylinder after combustion to push against a lower ambient crankcase pressure to deliver the same horsepower. This means less mass of Stuff to start with, and is why the MAP limit on the chart reduces as you climb.
As the OAT rises the density decreases, but the ambient pressure stays the same. This means that less mass of Stuff will go into the cylinder at a given manifold pressure, and less pressure differential (horsepower) will be produced after combustion. So in order to get back to the limit you must get more Stuff into the cylinder, and the only way to do that is to increase the pressure of the Stuff going in. This is why the MAP limit on the chart increases as the OAT rises, or more correctly the air density reduces.
This is also why the chart allows full throttle at altitude, because up there the engine can’t get enough Stuff to produce enough horsepower at full throttle to exceed the drive train limit. Shovel less coal, get less power.
It is all to do with finding the correct amount of Stuff to burn to create the required pressure differential between the inside and outside of the cylinder, given the ambient pressure outside the cylinder to start with.
By the way: Pilots that exceed the limit “because the engine is de-rated anyway” don’t realise the damage they may be doing by overstressing the drive train.
Hope this helps.
Last edited by the coyote; 9th Jan 2005 at 07:57.