- Practice · 4+4 marks
Two-stroke versus four-stroke engines
Short answer
A four-stroke engine completes its cycle in four piston strokes (720°) with separate intake and exhaust valves. A two-stroke engine completes the cycle in two strokes (360°) using ports; it fires every revolution, is lighter, but has lower thermal efficiency and higher emissions, and requires oil-fuel mixing for lubrication.
(a) Explain the working of a four-stroke petrol engine with the help of a sketch of the four strokes. (b) Differentiate between two-stroke and four-stroke engines.
(a) Four-stroke petrol engine
In a four-stroke engine, one power cycle is completed in four piston strokes, i.e. two crankshaft revolutions (720 degrees). The engine works on the Otto cycle.
SUCTION COMPRESSION POWER EXHAUST
IV open IV, EV closed IV, EV closed EV open
EV closed spark near TDC gases expand IV closed
piston down piston up piston down piston up
TDC -> BDC BDC -> TDC TDC -> BDC BDC -> TDC
(draws in (compresses (work done on (pushes out
mixture) mixture) crankshaft) burnt gas)
- Suction stroke: the inlet valve is open and the exhaust valve is closed. The piston moves from TDC to BDC and draws in the air-fuel mixture.
- Compression stroke: both valves are closed. The piston moves from BDC to TDC and compresses the mixture to about 1/8 to 1/10 of its volume. Near TDC the spark plug fires.
- Power (expansion) stroke: the burnt gases expand and push the piston from TDC to BDC. Work is delivered to the crankshaft.
- Exhaust stroke: the exhaust valve is open. The piston moves from BDC to TDC and sweeps the burnt gases out.
Only the power stroke produces work, so a flywheel stores energy to carry the piston through the other three strokes.
(b) Two-stroke vs four-stroke
| Point | Two-stroke | Four-stroke |
|---|---|---|
| Strokes per cycle | 2 (one revolution) | 4 (two revolutions) |
| Power stroke | Every revolution | Every second revolution |
| Valves | Ports opened and closed by the piston | Poppet valves operated by cams on a camshaft |
| Power for same size | Theoretically twice; in practice 1.3 to 1.5 times | Lower |
| Lubrication | Oil mixed with petrol or injected | Separate sump, pressure lubrication |
| Thermal efficiency | Lower (fresh charge lost through the exhaust port) | Higher |
| Fuel consumption and emission | Higher, more unburnt HC | Lower |
| Weight and cost | Light, simple, cheap | Heavier, complex |
| Flywheel | Light | Heavier |
| Use | Small scooters, chain saws | Cars, buses, most motorcycles |
✓ Answer checked by Claude Opus 5.5 (high effort)
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