Most cars on the road today are powered by an internal combustion engine. This engine contains a number of hollow chambers, called cylinders, and each chamber has a large metal plug, a piston, that moves up and down in its cylinder.
Gasoline and air mix in the cylinder. As a piston moves upward in its cylinder, it compresses the air and gas mixture, and a spark from the spark plug ignites the mixture. The burning, or combustion, of the mixture increases the pressure on the top of the piston, forcing it down the cylinder again. Then the piston bounces back up into the cylinder, compressing the new supply of gas and air, and the process starts all over again.
Each time the piston moves down through the cylinder, it turns a shaft in the engine, which then turns another shaft attached to the gears. The gears turn the rear axle, and the axle turns the rear wheels. The movements of the pistons in an engine are timed so that the car rolls along smoothly, rather than a little bit at a time.
All car engines today have from four to eight cylinders, but one 1930s engine had 16 cylinders!