Intake Stroke: A mixture of fuel and air is drawn into the cylinder as it moves down the piston.
Compression Stroke: The mixture is squeezed, or compressed, into a smaller space as the piston moves back up.
Ignition: When the piston is almost at the top of the cylinder, a spark plug ignites the mixture. Stored chemical energy is converted to thermal energy, which heats the gas.
Power Stroke: As the heated gas expands, it pushes the piston down. The piston, in turn, moves the crankshaft. Thus thermal energy is converted to mechanical energy.
Exhaust Stroke: The piston moves back up, pushing the heated gas out. This makes room for new fuel and air, so that the cycle can be repeated.