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The Four-Stroke Engine Worksheet
By the end you will: name the four strokes and what the valves and piston do in each, and relate compression ratio to efficiency.
A. Explore with the simulation
Slow the engine right down. For each stroke, record which way the piston moves and which valves are open.
Stroke Piston moves Valves Intake Compression Power Exhaust Change the compression ratio and record the ideal efficiency.
Compression ratio Ideal efficiency 4 : 1 8 : 1 12 : 1 How many turns does the crankshaft make in one full cycle? What keeps it turning between power strokes?
B. Calculate (then check in the simulation)
A cylinder holds 450 cm³ at the bottom of the stroke and 50 cm³ at the top. Find the compression ratio and the ideal efficiency (1 − 1 ÷ r^0.4; 9^0.4 ≈ 2.41).
A single-cylinder engine runs at 2,400 rpm. How many power strokes does it make each second?
C. Think it through
Why can't a petrol engine use a very high compression ratio, even though it would be more efficient?
Answer key (for teachers and parents)
- Intake: down, intake open · Compression: up, both shut · Power: down, both shut (spark at the start) · Exhaust: up, exhaust open.
- 4: 42.6% · 8: 56.5% · 12: 63.0%. A higher compression ratio gives a higher efficiency.
- Two turns. The heavy flywheel stores energy from the power stroke and carries the engine through the other three.
- r = 450 ÷ 50 = 9. Efficiency = 1 − 1 ÷ 2.41 ≈ 58.5%.
- 2,400 rpm = 40 turns a second; one power stroke every 2 turns = 20 a second.
- Squeezing the fuel–air mix too hard heats it so much that it explodes by itself before the spark (knocking), which damages the engine. Diesel engines use higher ratios because they have no spark.
Free to print and share for teaching. Made with the interactive simulation at jugnix.com/simulations/four-stroke-engine.