To the un-initiated, a manifold pressure (MAP), refers to the amount of air in Pounds per square inch entering the aircraft engine i.e. the cylinders. This is a critical parameter for a variety of reasons.
The point of manifold pressure is the throttle valve and the intake manifold of the engine. If we assume that the aircraft is on the tarmac with engines switched off, the MAP would naturally be pressure at the airport. If the airport is at sea level, the MAP pressure should be 29.92. If the altitude were to increase, the atmospheric Slimline Pressure Gauge would decrease.
The moment the pilot starts the aircraft engines (but not touch the throttle), the manifold pressure will decrease because the air suction created by the cylinders would create a low pressure in the manifold.
The rule of thumb is that in a normally aspirated piston engine aircraft, the MAP will be slightly below atmospheric whereas, in a turbocharged aircraft, the presence of the compressor and turbine, enable the engines to supply power at a much higher pressure.
In a turbocharged aircraft, the MAP does not vary with the altitude because the presence of the compressor means the aircraft engine does not have to rely on free-flowing air. The compressor creates a continuous stream of high-pressure air that enables the aircraft to climb quickly.
Of course, the efficiency of the compressor depends on quantum of external air that is available to it. As the aircraft flies higher and higher, the quantum of external air available becomes less and less and the engines will ultimately lose power. However, the loss of power happens more gradually as compared to normally aspirated piston engine aircraft.
The role played by the MAP gauge
When power needs to be reduced, the MAP before the propeller RPM needs to be reduced first. If the pilot reduces the RPM Sensor before the manifold pressure, then there is every possibility that the aircraft engine components will be damaged. This is because air at higher pressure would be entering into the cylinders even though the coarsening of the propeller reduced the engine speed.
To increase the engine power, the props should be set first and then the manifold should be altered. The role played by the manifold gauge is to indicate the health of the fuel-air mixture entering the aircraft engine. It is vital that the fuel-air mixture ratio stays the same and this can be confirmed via the MAP gauge. Any changes will indicate problems with the air intake.
If the air intake decreases, the fuel portion increase making the mixture ‘richer’. But this does not help because lack of oxygen in the combustion process, will reduce the power generated in cylinder. The aircraft would end up burning more fuel but without any increase in the power generated. Now you can understand the role played by MAP gauge in aircraft.
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