Abstract: An energy conserving engine particularly for heavy power requirements in freight movers and power plants, consists of an Otto cycle Diesel engine modified to receive superheated water injection for steam operation. The waste energy resulting from heat due to fossil fuel combustion is utilized in a series of counterflow heat exchangers that are temperature and pressure monitored, and valved for computer control of the heat transfer rates, providing superheated water at the proper temperature and pressure for steam operation of the engine. The computer defaults to fossil fuel operation of the engine when superheated water and steam operation conditions do not exist in the counterflow heat exchangers.