Abstract: The invention is an internal combustion steam engine comprising a conventional internal combustion engine, provided with a means for injecting water into power cylinders immediately after combustion of a fuel/air charge in the power cylinders such that the heat produced by the combustion vaporizes the water into steam and increases the force produced by the power stroke. The invention also discloses the use of compound cylinders which are powered by the exhaust gases and steam from the power cylinders. Also provided is an exhaust system wherein the exhaust gases and steam are utilized to heat the intake air and fuel and wherein the steam is cooled and condensed into water with absorbed pollutants and the remaining exhaust gases exit the heat exchanger to the atmosphere at substantially atmospheric temperature.