Fundamentals of piston engine operation (thermodynamics, cycles, power and efficiency)
Fundamentals of piston engine operation
This section examines the thermodynamic cycle, power definitions and efficiency limits that the CAA NZ AME Piston Engines exam uses to test understanding of engine output. Questions often combine a formula with a judgement about detonation or mixture.
Core rules and concepts
- The four-stroke Otto cycle has induction, compression, power and exhaust strokes; one complete cycle needs two crankshaft revolutions.
- Compression ratio r = (swept volume + clearance volume) ÷ clearance volume. Ideal Otto efficiency = 1 − 1/r^(γ−1); higher r improves efficiency until fuel octane or cooling limits detonation.
- Indicated power is the rate of work done by gas on the piston; brake power is measured at the crankshaft. Friction power is the difference, and mechanical efficiency = brake ÷ indicated.
- Volumetric efficiency compares delivered charge with theoretical displacement; throttle loss, intake heat and poor exhaust scavenging reduce it.
- BMEP for a four-stroke, in psi, = 150.8 × torque (lb-ft) ÷ displacement (in³).
Worked example
A four-cylinder four-stroke displaces 360 in³ and produces 300 lb-ft. BMEP = 150.8 × 300 ÷ 360 = 125.7 psi. The constant 150.8 already accounts for two revolutions per cycle; using 75.4 would halve the result.
Traps
Do not label brake power as indicated; do not use the two-stroke constant; do not treat compression ratio as a pressure ratio; and distinguish detonation (spontaneous ignition after the spark) from pre-ignition (ignition before the spark).