Section 16.2

Engine Performance

Power, mechanical efficiency, the factors affecting power, mixture strength, and detonation vs pre-ignition.

Notes

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Main ideas

Power = torque × rotational speed. Indicated power (IHP) is developed in the cylinders; brake power (BHP) is what reaches the propeller shaft; the difference is friction power, so mechanical efficiency = BHP/IHP.
Power depends on how much charge is burned: manifold absolute pressure (MAP), engine RPM, mixture strength, and the density of the induction air (falling with altitude, temperature and humidity).
The chemically correct (stoichiometric) mixture is about 15:1 air:fuel by mass. A RICH mixture is used for maximum power and for cooling at high power; a LEAN mixture for economy in the cruise — set with the mixture control and EGT.
On a constant-speed propeller aircraft, power is set by MAP (throttle) together with RPM (prop control); the engine limits and the POH define acceptable MAP/RPM combinations.
Two destructive abnormal-combustion events limit power: DETONATION (the end-gas explodes violently after the spark) and PRE-IGNITION (the charge is ignited before the spark by a hot spot). Both cause knocking, overheating and damage.
⚠ Exam trap: detonation happens AFTER the spark (promoted by over-lean mixture, low-grade fuel, high MAP/temperature); pre-ignition happens BEFORE the spark (a hot spot/glowing deposit) — they are different faults with similar damage.

Key formulas

Solved examples

A pilot leans aggressively at high power on a hot day and hears a metallic knocking. What is the likely cause and the fix?
An over-lean mixture at high power and temperature has caused detonation — the end-gas is exploding rather than burning smoothly, knocking and overheating the engine. The fix is to enrich the mixture (and/or reduce power/MAP), which cools the charge and restores smooth combustion; using the correct fuel grade is also essential.

Mind map

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