A six-stroke engine provides two power strokes in each cycle, improving efficiency over a traditional four-stroke engine. It works by using the heat from the exhaust stroke to generate a secondary expansion and power stroke. The first four strokes are identical to a four-stroke engine. In the fifth stroke, water is injected into the hot combustion chamber and turns to steam, expanding to drive the piston down for a second power stroke. This recovers waste heat from the exhaust to improve fuel efficiency over a four-stroke engine by up to 30-40%. However, additional systems are required to inject water and utilize the secondary expansion, increasing complexity over a traditional design.