This invention is a piston in cylinder engine using water injection into a relative vacuum heated to steam by expanding in the cylinder and by an electric arc or other heat source. The resulting steam explosion applies a work force on the piston. The piston has jet nozzles uncovered at the end of its work stroke to jet the piston to help propel it during the return stroke and to form a vacuum in place of the usual compression stroke. The piston has a cover plate with tapered pins depending into jet nozzles through the piston to block the jet nozzles during the main work stroke.