Most cars are powered by a piston engine that burns a mix- ture of petrol and air. The mix- ture is burned in a combustion chamber within a cylinder above a piston.

When it burns, the mixture expands rapidly and the pressure that this exerts on the top of the piston forces it down the cylinder.

The underside of the piston is connected by a rod (the connecting rod) to a cranked shaft (crankshaft) and this arrangement allows the down- ward path of the piston to be transformed into rotary move- ment of the shaft. From the crankshaft the power is trans- mitted to the wheels that drive the car through the clutch (or torque convertor), gearbox and final drive.

Inlet and exhaust valves at the top of the cylinder control the entry of the petrol/air mix- ture and the exit of burned gases into the exhaust system. The valves are operated by eccentric lobes on a camshaft, driven from the crankshaft.

The mixture is ignited in the combustion chamber by a spark plug. The high-voltage, or high-tension current neces- saryto produce a spark is gen- erated within a separate igni- tion system and usually fed to each cylinder as required by a distributor, normally driven by the camshaft.

To start the engine it must be rotated. This is done electri- cally by a starter motor which rotates the crankshaft, usually byengagingasmall pinion (or gear wheel) with the gear teeth round the outer edge of a flywheel, bolted to the end of the crankshaft. Besides provid- ing a means of starting the engine, the flywheel smooths out the power pulses from the pistons and allows the crank- shaft to turn relatively smoothly.

Once the starter is rotating the crankshaft, the up and down movement of the pistons sucks mixture into the cylin- ders, and when the ignition is switched on, combustion begins and the engine starts. In addition, the internal com- bustion engine will have a water- or air-cooling system, and its own lubrication system.