The Four-Stroke Engine Cycle: Why Combustion Makes Your Car Move

Stroke 1 — Intake: Drawing in the Fuel-Air Mix

The cycle begins as the piston moves downward from its highest point — called top dead center (TDC) — toward the bottom of the cylinder. This downward movement creates a drop in pressure, drawing a mixture of air and fuel (or air alone, in direct-injection systems) into the cylinder through the open intake valve.

In a gasoline engine, the fuel-air ratio is carefully metered by the fuel injection system to ensure efficient combustion. The intake valve closes once the piston reaches the bottom of its travel, sealing the mixture inside. A restricted air intake — caused by a clogged air filter, for example — reduces the volume of air entering the cylinder, which directly limits power output.

Stroke 2 — Compression: Squeezing for Power

With both valves now closed, the piston travels back upward, compressing the fuel-air mixture into a much smaller space. This is measured as the engine's compression ratio — a typical modern gasoline engine compresses the mixture to about one-tenth of its original volume (a 10:1 ratio). Diesel engines use ratios of roughly 14:1 to 25:1, which is high enough to raise air temperature beyond the fuel's ignition point without needing a spark.

Compression is critical to efficiency: the tighter the squeeze, the more energy is released during combustion. Worn piston rings or leaking valves reduce compression, causing noticeable drops in power and fuel economy — a symptom that a mechanic can diagnose with a compression test.

Stroke 3 — Combustion (Power): The Stroke That Does the Work

At the top of the compression stroke, a spark plug fires (in gasoline engines), igniting the compressed mixture. The resulting rapid expansion of gases drives the piston forcefully downward — this is the power stroke, and the only one of the four that generates energy. The force of the piston's downward movement is transferred through the connecting rod to the crankshaft, converting linear motion into the rotational force that ultimately reaches your wheels.

This is also where torque and horsepower are generated. The violence and precision of this stroke explain why engine oil film thickness matters so much — metal components moving under extreme heat and pressure depend on a continuous lubricant barrier. For a deeper look at why oil degradation is a genuine risk, see our guide on engine oil and long-term engine health.

Stroke 4 — Exhaust: Clearing the Chamber

After the power stroke, the exhaust valve opens and the piston travels upward again, pushing the spent combustion gases out of the cylinder and into the exhaust manifold. Clean, complete evacuation of exhaust gases matters because any residual gas dilutes the incoming fresh charge in the next intake stroke, reducing efficiency.

Those exhaust gases travel downstream through the exhaust system, where the catalytic converter chemically processes harmful byproducts — carbon monoxide, hydrocarbons, and nitrogen oxides — into less harmful compounds. Our explainer on what happens inside a catalytic converter covers that process in detail. Once the piston reaches TDC again, the exhaust valve closes, the intake valve opens, and the whole cycle repeats.

Why Multi-Cylinder Engines Feel Smooth

A single-cylinder engine firing once every two crankshaft rotations would feel rough and uneven — like a heartbeat with long pauses. Modern passenger vehicles use four, six, or eight cylinders timed to fire in an overlapping sequence, so power strokes follow one another in rapid, staggered succession. A four-cylinder engine, for example, spaces its power strokes evenly across 180° of crankshaft rotation, delivering a reasonably smooth output.

This mechanical choreography is why cylinder count and engine configuration (inline, V-shaped, flat) influence refinement as much as raw output. It also explains why a misfiring cylinder — caused by a failed spark plug, faulty injector, or loss of compression — produces a noticeable vibration: the engine's firing rhythm is broken. To understand how these principles apply across different fuel types, see our comparison of petrol, diesel, hybrid, and electric powertrains.