Engine Performance
Power, mechanical efficiency, the factors affecting power, mixture strength, and detonation vs pre-ignition.
Engine performance, summary notes
- 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.
- Power
- P = torque × angular speed
- Stoichiometric mixture
- ≈ 15 : 1 air : fuel by mass
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.
Performance concept map
Engine Performance
Engine performance quiz
Engine Performance, quiz
1. An engine develops 180 kW and turns a propeller at 2700 rev/min. The torque delivered is approximately:
637 N·m66.7 N·m4000 N·m2. An engine has a brake mean effective pressure of 900 kPa and a swept volume of 8 litres, firing every second revolution. The work done per cycle is:
7.2 kJ3.6 kJ112 kJ3. An engine produces 150 kW at the crankshaft and is supplied with 500 kW in the fuel. Its thermal efficiency is:
30%70%3.3%4. An engine develops a torque of 300 N·m at 2400 rev/min. The power output is approximately:
75 kW720 kW1.25 kW5. An engine draws in 0.8 of the air its swept volume could hold at ambient conditions. Its volumetric efficiency is:
80%125%20%6. Engine power is the product of:
Torque and rotational speedBore and strokeMAP and CHT7. Brake horsepower (BHP) is:
The power delivered to the propeller shaftThe power developed in the cylindersThe friction power8. The stoichiometric air:fuel ratio is about:
15:1 by mass1:15 by mass50:1 by mass