Jun 11, 2026Updated Aug 22, 20268 min read

How a Four-Stroke Engine Works (All 4 Strokes Explained)

A four-stroke engine turns burning fuel into smooth rotation in four steps: intake, compression, power, exhaust. What happens in each stroke, and what limits the engine.

Open the hood of almost any car built in the last hundred years and you find the same machine: a four-stroke piston engine. The fuel changed, the electronics changed, the materials changed. The cycle did not. Nikolaus Otto got it right in 1876.

The idea fits in one sentence: trap an air-fuel mixture in a cylinder, squeeze it, ignite it, and let the burn shove a piston that turns a crank. Most explanations go fuzzy in the detail, so this one takes the cycle stroke by stroke.

Quick Answer: What Are the Four Strokes?

One full cycle takes two crankshaft revolutions (720°) and four piston strokes:

Intake: piston moves down, intake valve open, cylinder fills with air and fuel.
Compression: both valves closed, piston moves up, mixture squeezed to a tenth of its volume.
Power: spark ignites the mixture, pressure drives the piston down. This is the only stroke that produces work.
Exhaust: piston moves up, exhaust valve open, burned gas pushed out.

Then the cycle repeats. At 6,000 RPM, each cylinder runs the full sequence 50 times per second.

Stroke 1: Intake

The piston descends from top dead center with the intake valve open. The growing cylinder volume drops the pressure, and atmospheric pressure pushes air in through the intake tract. The engine does not "suck" so much as the atmosphere shoves.

The amount of air that gets in is the single biggest factor in how much power the engine can make. The throttle plate is a restriction in this path: close it and the cylinder fills less, burns less fuel, makes less torque. At wide open throttle the restriction is gone and the engine breathes freely.

Stroke 2: Compression

Both valves close and the piston rises, squeezing the mixture into the small volume above the piston. Gasoline engines run a compression ratio of 10:1 to 13:1.

Compression earns its keep through thermodynamics: the more you squeeze the charge before burning it, the more work you can extract from the expansion afterwards. Higher compression ratio means higher efficiency, up to the point where the heat of compression ignites the fuel before the spark does. That uncontrolled ignition is engine knock, destructive enough that we wrote a separate post on what causes it and how to stop it.

Diesel engines exploit this on purpose: at 16:1 to 23:1 compression, the air alone gets hot enough (over 800 K) that fuel injected into it ignites spontaneously. No spark plug needed.

Stroke 3: Power

A few degrees before the piston reaches the top, the spark plug fires. The flame front takes a few milliseconds to sweep across the chamber, so ignition has to come early: the goal is peak pressure arriving a few degrees after top dead center, when the crank geometry can best use it.

Pressure spikes to 50–100 bar and drives the piston down. The connecting rod turns that linear shove into crankshaft rotation, the crank-slider mechanism at the heart of every piston engine built.

In a four-cylinder engine at idle, each cylinder fires once per 720°, so the crank gets four discrete kicks per two revolutions. The flywheel smooths those kicks into continuous rotation.

Stroke 4: Exhaust

The exhaust valve opens a few degrees before bottom dead center, while there is still useful pressure to start gas moving, and the rising piston pushes the burned charge out. At the top, for a few degrees, both valves are open. This valve overlap uses the momentum of outflowing exhaust to help pull in fresh charge. Race cams stretch the overlap for top-end power at the cost of a lumpy idle.

Then the intake valve opens fully, the piston descends, and the engine is back at stroke one.

The Parts Doing the Work

Eight components carry the whole cycle:

Piston: the moving wall that compresses the charge and receives the power stroke.
Piston rings: seal combustion pressure above the piston and scrape oil below it.
Connecting rod: links piston to crank, converting linear shove into rotation.
Crankshaft: the rotating output; its throws set the stroke length.
Camshaft: geared to spin at half crank speed (720° per cycle), opening each valve at the right stroke.
Valves: intake lets charge in, exhaust lets burned gas out; both seal shut for compression and power.
Spark plug: fires once per cycle, a few degrees before top dead center.
Flywheel: stores rotational energy to carry the crank through the three non-power strokes.

From Explosions to Horsepower

Two terms get conflated constantly:

Torque is twist: how hard each power stroke shoves the crank. The amount of mixture the cylinder burns per cycle sets it.
Power is torque times speed: how often those shoves happen. In imperial units, horsepower equals torque (lb-ft) times RPM divided by 5,252.

That formula explains real dyno curves. Torque peaks where the engine breathes best (its volumetric-efficiency sweet spot); power keeps climbing past it because rising RPM outweighs slowly falling torque, until breathing collapses and both fall. You can sweep a virtual engine on a dyno and watch this happen in our free engine simulator.

Why More Cylinders? And Why Does a V8 Sound Like That?

One cylinder gives one bang per 720°. Eight cylinders, fired at even intervals, give a bang every 90°: smoother torque, a smaller flywheel, and a higher exhaust pulse frequency.

The sound of an engine is mostly firing frequency. A four-cylinder at 6,000 RPM fires 200 times a second. A crossplane V8 produces its signature burble because its firing intervals are uneven per bank, layering pulse spacings the ear reads as a low, syncopated rumble. Same physics and a different firing order give a different character.

Watch the Whole Cycle Run

Diagrams freeze the engine; the real thing is a blur of valves and flame arriving 50 times a second. In our in-browser engine simulator you can crank an inline-four or a V8, watch the pistons and combustion events in 3D, pull a dyno curve, and read an event log that narrates each state change in plain language: starter engagement, first fire, rev limiter, even fuel starvation from vapor lock.

Four-Stroke Engine FAQs

Why is it called the Otto cycle?

After Nikolaus Otto, who built the first practical compressed-charge four-stroke in 1876. The thermodynamic idealization of his engine (isentropic compression, constant-volume heat addition, isentropic expansion) still carries his name. The full derivation of why compression ratio drives efficiency, and where knock sets the ceiling, is in The Otto Cycle Explained.

What's the difference between a four-stroke and a two-stroke?

A two-stroke fires every revolution instead of every other one, using ports and crankcase pressure instead of valves. It makes more power per displacement, burns dirtier, and drinks more fuel, which is why two-strokes survive mainly in chainsaws, dirt bikes, and giant ship engines.

What limits an engine's redline?

Mostly valvetrain control and piston acceleration. Past a certain speed the valves "float" (springs cannot close them fast enough) and connecting-rod stress grows with the square of RPM. The rev limiter cuts spark or fuel below that point. Hit it in the simulator and the event log names the cut.

Why does an engine need to idle? Why not just stop?

A four-stroke produces no power below self-sustaining speed; it needs stored rotational momentum to carry the crank through the three non-power strokes. Drop below idle speed without enough torque and the cycle collapses into a stall. Start-stop systems avoid idling by using a beefed-up starter to relaunch the cycle on demand.

Is a jet engine also a four-stroke?

No. A jet runs the same conceptual steps (intake, compression, combustion, exhaust) continuously and simultaneously in different parts of the machine, rather than sequentially in one cylinder. We compare the two approaches in Jet Engine vs Rocket Engine.