Fundamentals of Powerplants and Engine Theory
Domain 1 tests how powerplants convert chemical energy into mechanical energy and how cycle, geometry, and rating definitions are applied.
Cycles and events
- Reciprocating engines use the Otto cycle (spark ignition) or Diesel cycle (compression ignition); turbines use the Brayton cycle.
- A four-stroke cycle has intake, compression, power, and exhaust strokes, with valve overlap near TDC.
- Compression ratio (CR) = (swept volume + clearance volume) ÷ clearance volume.
Geometry and ratings
- Bore is cylinder diameter; stroke is piston travel; displacement per cylinder = π/4 × bore² × stroke.
- Indicated horsepower is developed in the cylinders; brake horsepower is measured at the shaft; friction horsepower is the difference.
- Volumetric efficiency compares actual intake charge with theoretical displacement; thermal efficiency compares work output with heat input.
Worked example
A cylinder has a 5.125 in bore, 5.5 in stroke, and 12.6 in³ clearance volume. Swept volume = 0.7854 × 5.125² × 5.5 = 113.5 in³. CR = (113.5 + 12.6) ÷ 12.6 = 10.0:1. The reasoning uses one cylinder's swept volume, not total engine displacement.
Traps
Confusing compression ratio with compression pressure; using total displacement in the CR formula; treating indicated and brake horsepower as interchangeable; assuming volumetric efficiency is 100% at all speeds.