An engine works by converting force into mechanical work. Internal-combustion engines use fuel to create heat. The heat expands the gas in a piston, creating the force to move the piston, causing the crankshaft to turn and provide power to propel an automobile down the road.
Most automobile engines in use in 2014 are four-stroke engines. Inside the engine, the piston begins its cycle at the top of the cylinder. An intake valve opens, allowing fuel and air to enter the piston as it moves to the bottom of its cycle. The valve closes as the piston begins its compression stroke. As the piston nears the top of the cylinder, the spark plug fires to create an explosion that pushes the piston to the bottom again. As it starts to move back to the top, the exhaust valve opens to expel the exhaust gases. When the piston reaches the top of the cylinder, it starts the cycle again.
Internal-combustion engines are much more fuel-efficient than external-combustion engines. External-combustion engines use fuel, such as coal or oil, to convert water to steam to move an engine. While steam engines are powerful, they are also large and heavy, making them impractical for use in automobiles.