Piston Engines, Propellers and Engine Control
The four-stroke cycle
A piston engine operates on induction, compression, power and exhaust strokes. Power depends on volumetric efficiency, mixture and ignition timing. The mixture is controlled by the carburettor or fuel injection; the mixture ratio affects exhaust gas temperature (EGT) and cylinder head temperature (CHT). Best power mixture is rich of peak EGT; best economy is lean of peak, but too lean causes detonation and high CHT. Detonation is uncontrolled explosive combustion; pre-ignition is ignition before the spark. Both damage the engine and are often confused in questions.
Ignition and cooling
Dual magnetos provide redundancy; the engine runs on both, and a magneto check shows the drop on each. Air cooling uses cowl flaps and baffles; high CHT needs richer mixture, lower power or more cooling.
Propellers
A propeller converts engine power into thrust; thrust depends on blade angle, RPM and forward speed. A constant-speed propeller uses a governor to vary blade angle to hold RPM; the propeller control sets RPM, the throttle sets manifold pressure. Fixed-pitch propellers are simpler but less efficient over a range of speeds. Feathering reduces drag after engine failure; reverse thrust is achieved by reversing blade angle on turboprops.
Worked example
If manifold pressure is 24 inHg and RPM 2400, a rule of thumb is to avoid high manifold pressure with low RPM (over-square), which increases detonation risk. Keep manifold pressure within the manufacturer's limits for the selected RPM.
Traps
Assuming the propeller governor controls manifold pressure (it does not; the throttle does), and forgetting that carburettor ice can occur at outside air temperatures well above freezing.