- Practice · 3+3 marks
Two-stroke and four-stroke engine comparison
Short answer
The two-stroke engine completes a cycle in one revolution using ports; upward stroke compresses the charge and draws fresh mixture into the crankcase; downward stroke (power) expels exhaust and transfers fresh charge via ports. Compared to four-stroke: higher power-to-weight ratio but lower thermal efficiency, poor fuel economy, higher HC emissions, and oil mixed with fuel.
Explain the working of a two-stroke petrol engine with the help of a sketch. Differentiate between two-stroke and four-stroke engines.
Two-stroke engine working
A two-stroke engine completes the cycle in two piston strokes (one revolution). It has ports in the cylinder wall instead of valves: the inlet port, the transfer port and the exhaust port. The crankcase acts as a pump.
spark plug
|
+---+---+
| | exhaust port --> E
| [P] |
| | transfer port <-- T
|_______|
| crank |<-- inlet port (reed valve) <-- mixture
+-------+
- Upward stroke: the piston covers the transfer and exhaust ports and compresses the charge above it. At the same time the rising piston lowers the crankcase pressure and fresh mixture is drawn in through the inlet port. Near TDC the spark ignites the charge.
- Downward stroke: the burning gas expands and pushes the piston down (power). The piston first uncovers the exhaust port, so burnt gas leaves. As it moves further it compresses the mixture in the crankcase and uncovers the transfer port, so the fresh charge flows into the cylinder and pushes out the remaining exhaust gas (scavenging). A deflector on the piston directs the flow upward.
Comparison
| Basis | Two-stroke | Four-stroke |
|---|---|---|
| Power stroke | Every revolution | Every two revolutions |
| Valves | Ports, no valve mechanism | Poppet valves with cam |
| Power for same size | Theoretically double, practically 1.3-1.6 times | Lower |
| Turning moment | More uniform, smaller flywheel | Less uniform |
| Fuel economy | Poor, fresh charge escapes with exhaust | Better |
| Thermal efficiency | Lower | Higher |
| Lubrication | Oil mixed with fuel | Separate sump system |
| Pollution | More HC and smoke | Less |
| Cost and weight | Lighter, cheaper | Heavier, costlier |
| Use | Mopeds, chainsaws, outboards | Cars, trucks |
✓ Answer checked by Claude Opus 5.5 (high effort)
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