On the power stroke of an engine, the force that drives the piston down the cylinder is the pressure created by the combustion of the fuel-air mixture. As the piston moves up the cylinder on the compression stroke, the air-fuel mixture is compressed, increasing its temperature and pressure. This causes the mixture to ignite, creating an explosion that rapidly increases the pressure in the cylinder. The high pressure forces the piston down the cylinder, generating power.

This process is repeated for each stroke of the engine: the piston moves up on the intake stroke to draw in the air-fuel mixture, then moves up again on the compression stroke to compress the mixture, which then ignites and drives the piston down on the power stroke. The piston then moves up again on the exhaust stroke to push out the exhaust gases. This process repeats continuously while the engine is running.