Academy · Car Engines · Lesson 2/11
Suck, squeeze, bang, blow
The four-stroke cycle — the heartbeat of nearly every car ever made. ~6 min
Nearly every car engine ever built runs the same four-beat rhythm, invented by Nikolaus Otto in 1876 and never bettered for the job: suck, squeeze, bang, blow. Four strokes of the piston — two full turns of the crank — for one push of power. Engineers call it the four-stroke cycle; everything in this lesson happens inside one cylinder in a few hundredths of a second.
The four strokes
Suck (intake). The piston slides down with the intake valve open, and the growing space above it fills with air and a fine mist of petrol — about one part fuel to 14.7 parts air by mass, the chemically perfect ratio.
Squeeze (compression). Valves shut, the piston rises and crushes the mixture to a tenth of its volume or less. Squeezing does two things: it makes the coming burn far more energetic (more oxygen and fuel packed close), and it heats the mixture towards ignition. The compression ratio is the single number that most defines an engine's character.
Bang (power). A few degrees before the top, the spark plug fires. The flame crosses the chamber, pressure spikes to 50–100 bar, and the piston is driven down with tonnes of force. This is the only stroke that produces work — the other three all cost work, paid back with interest here.
Blow (exhaust). The exhaust valve opens and the rising piston shoves the spent gas out towards the tailpipe, still hot enough to glow. Then the cycle begins again — up to fifty times a second per cylinder at speed.
Why a flywheel
Notice the imbalance: one power stroke out of four. A single-cylinder engine pushes, then coasts for a turn and a half. The fix is the flywheel — a heavy disc on the crank that stores the bang's energy in its spin and pays it back through the idle strokes, smoothing the jerks into rotation. (The other fix is more cylinders taking turns — that is lesson 4.)
Valves keep the time
The intake and exhaust valves are the cycle's gatekeepers, opened by lobes on a camshaft geared to spin at exactly half crank speed — because each valve must open once per two crank turns. Their timing is tuned in single degrees: open the intake a touch early and the engine breathes better at high revs; close the exhaust a touch late and it scavenges better. Get any of it wrong by a few degrees and the engine wheezes — timing is that sensitive, which is why "cam profiles" are spoken of with reverence in tuning circles.