--- title: "Two-stroke versus four-stroke engines" subject: "Automotive Technology" chapter: 1 key: "semester_5/automotive_technology:two-stroke-vs-four-stroke" source: https://www.bctengineer.com/q/semester_5/automotive_technology/two-stroke-vs-four-stroke site: BCT Engineering --- # Two-stroke versus four-stroke engines Automotive Technology · Chapter 1: Introduction - Asked in: year not given, 4+4 marks - Video: [4-Stroke & 2-Stroke Engine | Its Parts & Working Explained](https://www.youtube.com/watch?v=8dAbcbAJRw8) — The Engineers Post - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/automotive_technology/two-stroke-vs-four-stroke - Whole chapter: https://www.bctengineer.com/md/semester_5/automotive_technology/chapter-1 ## Question (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. ## 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. ## Answer #### (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. ```text 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) ``` 1. **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. 2. **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. 3. **Power (expansion) stroke:** the burnt gases expand and push the piston from TDC to BDC. Work is delivered to the crankshaft. 4. **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 | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How many crankshaft revolutions are needed for one power cycle in a four-stroke engine? A. One (360°) B. Two (720°) C. Four (1440°) D. Half (180°) Answer: B. Four piston strokes take two crankshaft revolutions, i.e. 720°. (Key point: Four-stroke cycle) 2. What happens during the compression stroke of a four-stroke petrol engine? A. Inlet valve open, piston goes TDC to BDC B. Both valves closed, piston goes BDC to TDC and the spark fires near TDC C. Exhaust valve open, burnt gas swept out D. Burnt gases push the piston down Answer: B. Both valves are shut while the piston rises and compresses the mixture to about 1/8 to 1/10 of its volume; the plug fires near TDC. (Key point: Compression stroke) 3. Why is a flywheel needed in a four-stroke engine? A. Only the power stroke produces work, so the flywheel carries the piston through the other three B. It opens the valves C. It cools the cylinder D. It meters the fuel Answer: A. Work is produced in one stroke of four; the flywheel stores energy to carry the piston through the other strokes. (Key point: Role of flywheel) 4. Which statement correctly compares two-stroke with four-stroke engines? A. Two-stroke has higher thermal efficiency and lower emissions B. Two-stroke has a power stroke every revolution but loses fresh charge through the exhaust port C. Two-stroke uses poppet valves with a camshaft D. Two-stroke gives exactly twice the power in practice Answer: B. A two-stroke fires every revolution but fresh charge escapes, lowering efficiency and raising HC emissions; practical power gain is only 1.3 to 1.5 times. 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