The coolant in the cooling jacket of a dual cycle internal combustion steam engine is intentionally maintained at an elevated temperature that may typically range from about 225° F.-300° F. or more. A non-aqueous liquid coolant is used to cool the combustion chamber together with a provision for controlling the flow rate and residence time of the coolant within the cooling jacket to maintain the temperature of the coolant at a selected elevated temperature that is substantially above the boiling point of water but below the boiling point of the coolant. The coolant is passed from the jacket through a heat exchanger in a first circuit to transfer heat to a vaporizable working fluid such as water and is then returned. An optional second circuit is an intrajacket perturbation circuit within the engine can be used to disrupt and disperse pockets of vapor that may tend to form before damaging hot spots can develop around the combustion chamber. A cooling jacket design is tailored to extract heat at the highest possible temperature from each heat transfer zone as by having the coolant follow a circuitous helical pathway to achieve more efficient and improved heat transfer from the combustion chamber to the cooling medium.