A substantially pollution-free heat engine employing steam as the working fluid and operable on a cycle approaching a Rankine cycle. The engine includes a lightweight, small-size, highly efficient, boiler expander and condenser system which utilizes advanced material technology in order to achieve maximum system operating temperatures of approximately 1,200*F. and operating pressures up to 3,500 psi. The engine is especially suitable for use in motor vehicles and is capable of fast start-ups so as to produce 65 percent of maximum power within 45 seconds or less. Moreover, the engine provides an idle mode of operation for driving vehicle accessories and the like and in addition is responsive to produce rapid power changes needed for acceleration and heavy pulls on steep grades.