Q12Basic Mechanical Engineering
Question
4 marks
Explain differentiate between 2 stroke & 4 stroke engine.
Answer
Four-stroke engines are heavy, highly efficient, and complete a cycle in two crankshaft revolutions using mechanical valves. Two-stroke engines are light, less efficient, and fire every revolution using cylinder ports.
Differentiating Two-Stroke and Four-Stroke IC Engines:
- 1. Thermodynamic Cycle Completion: A four-stroke engine requires four distinct strokes of the piston (Suction, Compression, Expansion, Exhaust) taking exactly two complete revolutions () of the crankshaft to complete one cycle. A two-stroke engine completes the entire cycle in just two strokes (up and down) and a single crankshaft revolution ().
- 2. Power Density: Because it fires twice as often, a two-stroke engine theoretically produces twice the power of a four-stroke engine of the identical size. This grants it an exceptionally high power-to-weight ratio, making it ideal for chainsaws, dirt bikes, and small outboard motors.
- 3. Gas Exchange Mechanism (Breathing): Four-stroke engines rely on complex, heavy, camshaft-driven poppet valves and timing chains to seal the cylinder. Two-stroke engines have absolutely no mechanical valves; instead, they have simple holes (ports) cut directly into the side of the cylinder wall, which are covered and uncovered by the sliding piston itself.
- 4. Thermal Efficiency & Emissions: Four-stroke engines dedicate distinct strokes to intake and exhaust, leading to complete scavenging and excellent fuel efficiency with very clean exhaust. Two-stroke engines suffer from 'short-circuiting' where fresh unburnt fuel blows straight out the exhaust port during the scavenging phase, resulting in terrible fuel economy and highly polluting exhaust (exacerbated by burning oil mixed in the fuel).