A car engine contains cylinders which contain pistons that move up and down, with the bottom connected to a crankshaft which converts the cyclical vertical motion of the piston into a constant rotating motion of the shaft. What forces the piston to move is a mixture of vaporized gasoline and air, which is then ignited by an electrical spark from the cylinder�s spark plug. That ignition forces the mixture to rapidly expand in volume, with enough energy to force the piston down, rotating the crankshaft, which both propels the car forward and provides enough energy to other mechanisms in the engine (connected to the crankshaft) to operate the valves at the top of the cylinder which control the intake of the fuel mixture and expelling the burned mixture out to the exhaust pipe. This cycle repeats several thousand times per second.