A direct injection system forces fuel into the engine’s combustion chambers. Some gasoline and all diesel injection systems are direct systems. As the piston moves down, the air is drawn in. The piston then moves upward, and the air is compressed. At the top of the compression stroke, fuel is injected into the cylinder. In the power stroke, the injected fuel touches the hot air and burns rapidly as the piston moves down. In the exhaust stroke, the piston moves up, forcing the gases out.