How does an internal combustion engine work? — RT, Kitchener, Ontario
An internal combustion engine burns a mixture of fuel and air in an enclosed space. This space is formed by a cylinder that’s sealed at one end and a piston that slides in and out of that cylinder. Two or more valves allow the fuel and air to enter the cylinder and for the gases that form when the fuel and air burn to leave the cylinder. As the piston slides in and out of the cylinder, the enclosed space within the cylinder changes its volume. The engine uses this changing volume to extract energy from the burning mixture.
The process begins when the engine pulls the piston out of the cylinder, expanding the enclosed space and allowing fuel and air to flow into that space through a valve. This motion is called the intake stroke. Next, the engine squeezes the fuel and air mixture tightly together by pushing the piston into the cylinder in what is called the compression stroke. At the end of the compression stroke, with the fuel and air mixture squeezed as tightly as possible, the spark plug at the sealed end of the cylinder fires and ignites the mixture. The hot burning fuel has an enormous pressure and it pushes the piston strongly out of the cylinder. This power stroke is what provides power to the car that’s attached to the engine. Finally, the engine squeezes the burned gas out of the cylinder through another valve in the exhaust stroke. These four strokes repeat over and over again to power the car. To provide more steady power, and to make sure that there is enough energy to carry the piston through the intake, compression, and exhaust strokes, most internal combustion engines have at least four cylinders (and pistons). That way, there is always at least one cylinder going through the power stroke and it can carry the other cylinders through the non-power strokes.