Explainer #25 · Machines & Mechanisms
How does a four-stroke engine work?
A car engine is a row of pistons sliding up and down in tubes. Each one repeats four jobs: suck, squeeze, bang, blow. Only the bang pushes, but it pushes hard, about 3.5 tonnes on one piston, and a crank turns that up-and-down push into the round turning that drives the wheels.
The principle
The four-stroke engine. One piston, four jobs, over and over: suck, squeeze, bang, blow. A full cycle takes 2 turns of the crank.
Step 1: Intake (suck). The intake valve opens and the piston slides down, like pulling back a syringe. That pulls in air mixed with a little fuel.
Step 2: Compression (squeeze). Both valves close and the piston pushes back up, squeezing the mix to about 1/10 of its space (a compression ratio of about 10:1). Squeezed mix burns harder, like a tightly packed campfire.
Step 3: Power (bang). The spark plug lights the mix. It burns very fast and the pressure jumps to roughly 60 bar, about 60 times the air pressure around you. On a piston about 86 mm across that is about 35 kN, roughly 3.5 tonnes, shoving it down. This is the only step that makes power.
Step 4: Exhaust (blow). The exhaust valve opens and the piston pushes the burnt gas out, then it all starts again at Step 1.
Step 5: Crank and flywheel. The rod and crank turn the up-and-down into round, like your knees on bicycle pedals. A heavy flywheel keeps spinning between bangs. At 3000 rpm the crank turns 50 times a second, so each cylinder bangs 25 times a second, and a 4-cylinder engine about 100 times.
Common myth
The engine pushes all the time
Each piston only pushes in 1 of its 4 strokes. The other three are paid for by the flywheel and the other cylinders, which is why engines have several cylinders taking turns.How this video was made: Animated drawing, simplified: the parts are not to scale, the engine is shown much slower than real, and the valves are drawn opening and closing exactly at the top and bottom of the stroke. Real engines open them a little early and close them a little late. The 60 bar and 3.5 tonnes are rough values for a normal petrol engine at full throttle (my own calculation from pressure × piston area).
Where the facts come from
- Wikipedia: Four-stroke engine (intake, compression, power, exhaust; 2 crank turns per cycle)
- Wikipedia: Compression ratio (petrol engines about 8:1 to 12:1)
- Industrial Monitor Direct: Peak combustion pressure by engine type (petrol, about 60 to 105 bar)
- EPI: Forces on reciprocating engine parts (pressure on the piston to the rod and crank)
