The main components on a diagram of an internal combustion engine include the cylinder, which is the body of the engine, and the crankshaft, connecting rod, intake and exhaust valves, spark plug and piston. Fuel intake, burning and compression occur in the cylinder. The number and arrangement of cylinders in the engine determine the size and shape of the crankshaft, which rotates freely. The connecting rod, which connects the piston to the crankshaft is the primary mover in the engine.
The number of intake and exhaust valves in a combustion engine vary based on the number of cylinders. The intake valve brings the air-fuel mixture into the cylinder, and the exhaust valve releases the combustion gas exhaust. The spark plug is a metal shell with two electrodes separated by an air gap. When current from the ignition system jumps across the electrode, it produces a spark that ignites the compressed air-fuel mixture in the combustion chamber. Pressure increases as the burning mixture expands and forces the piston down the cylinder, which drives the connecting rod to turn the crankshaft.
Prior to the 20th century, vehicle engines were external combustion or steam engines. In this type of engine, combustion occurs outside the engine, and the heat from the burning fuel, usually coal, boils water that creates steam. Once pressurized, the steam flows into the engine and forces the piston down in the cylinder.