Engine types, cycles & ignition
Typical training aeroplane engine (TP 1102 ch.3):
- Horizontally opposed cylinders
- Unsupercharged
- Fixed-pitch propeller: wood, composite, or metal
Horizontally opposed layout (PHAK ch.7), most common piston layout on small aircraft:
- Always an even number of cylinders: one on each side of the crankcase opposes the other.
- Small, light crankcase: high power-to-weight ratio.
- Compact arrangement: reduces frontal area and drag.
Four-stroke cycle, one piston sweep per stroke, in order:
| # | Stroke | Piston | Valves | Event |
|---|---|---|---|---|
| 1 | Intake | Down | Intake opens | Fuel-air mixture drawn in |
| 2 | Compression | Up | Intake closes | Mixture compressed |
| 3 | Power | Down | - | Mixture ignites, pressure forces piston down, turns crankshaft |
| 4 | Exhaust | Up | Exhaust opens | Burned gases purged |
Two-stroke cycle (PHAK ch.7):
- Intake, compression, power, exhaust in two piston strokes.
- Power stroke every crankshaft revolution.
- Typically higher power-to-weight than comparable four-stroke.
- Aviation use limited.
Ignition method splits piston aero engines (PHAK ch.7):
| Type | Ignition |
|---|---|
| Spark ignition | Spark plug ignites pre-mixed fuel-air mixture. Almost every GA trainer uses it; the four-stroke is the most common GA design |
| Compression ignition | No spark plug. Air compressed first, heating it enough that fuel injected into cylinder ignites on its own. Called a jet fuel piston engine. Runs on diesel or jet fuel (Jet A), not avgas |
- Both share same main mechanical components (cylinders, pistons, crankshaft). Only difference: how fuel is ignited.
- Separate classification axes, apply to either design:
- cylinder arrangement
- operating cycle (two- or four-stroke)
- cooling method (liquid or air)
Trap: compression FIRST, then ignition. Diesel aero engine has no spark plugs or magnetos; ignites from compression heat alone.
References
- TP 1102 ch.3 Engine Handling
- PHAK ch.7