How a Four-Stroke Petrol Engine Works

Almost every job you will ever do on a car makes more sense once you can picture what is happening inside the engine. You do not need to be an engineer. You need a clear mental model of four repeating events — and once you have it, oil changes, misfires and warning lights stop being mysterious.

This article walks through the four-stroke petrol cycle the way it actually behaves, names the parts you will meet again and again, and points out where things go wrong. It is the foundation for everything else on this site.

The job of an engine, in one sentence

A petrol engine converts the chemical energy in fuel into rotational force at the crankshaft by burning a precisely measured mix of fuel and air inside a sealed cylinder. Everything else — the valves, the camshaft, the spark plugs, the cooling system — exists to make that burn happen at the right moment, repeatedly, thousands of times a minute.

The four strokes

"Four-stroke" means the piston makes four passes — two down, two up — to complete one full cycle. The crankshaft turns twice for every single power event. Here is what happens, in order.

1. Intake

The piston travels down the cylinder. The intake valve opens, and the downward motion creates low pressure that pulls in a mixture of air and atomised petrol. Think of a syringe drawing in liquid. On a modern car the engine control unit decides exactly how much fuel to inject for the air it expects.

2. Compression

Both valves close and the piston rises, squeezing the mixture into a small space at the top of the cylinder. Compressing the charge makes the coming burn far more powerful. Typical petrol engines compress the mixture to roughly a tenth of its original volume.

3. Combustion (the power stroke)

Near the top of compression, the spark plug fires. The mixture burns rapidly, pressure spikes, and the expanding gas drives the piston back down with force. This is the only stroke that produces power; the other three are paid for by the flywheel's momentum and the other cylinders.

4. Exhaust

The exhaust valve opens and the rising piston pushes the spent gases out into the exhaust system, where they pass through the catalytic converter. The valve closes, the intake valve opens, and the cycle begins again.

Why more cylinders?

One cylinder produces power only a quarter of the time, so a single-cylinder engine runs roughly and unevenly. Spreading the work across four cylinders that fire at staggered intervals means there is almost always a power stroke happening somewhere, which is why a four-cylinder engine feels smooth. More cylinders generally mean more smoothness and more power, at the cost of size, weight and fuel.

Where this connects to real jobs

Hold this cycle in your head and the maintenance schedule starts to explain itself:

• Engine oil keeps the piston, rings and bearings sliding instead of grinding. That is why a clean oil supply matters so much — see our guide to changing your oil and filter.
• Spark plugs and ignition coils light the mixture. When one fails, that cylinder stops contributing — a misfire — and the engine logs a fault you can read with a scanner. Our piece on OBD-II fault codes explains how a code like P0301 points to cylinder one.
• The cooling system stops the controlled burn from melting the engine. A temperature warning is one of the lights you must never ignore, as covered in reading dashboard warning lights.

Diesel and hybrid, briefly

A diesel engine follows the same four strokes but has no spark plug — it compresses air so hard that it gets hot enough to ignite the fuel on its own when injected. A petrol-electric hybrid still has a four-stroke engine; it simply hands some of the work to an electric motor and switches the engine off when it is not needed. The cycle you have just learnt sits at the heart of all of them.

Conclusion

Intake, compression, combustion, exhaust — said out loud, it is almost too simple. But that loop, repeated thousands of times a minute and managed by sensors and a computer, is the entire reason a car moves. Keep this picture in mind as you read the rest of the site. When something on a car misbehaves, the useful first question is always: which part of this cycle is being disrupted, and why?