I know that any burner that short cycles such as a unit used to heat a home prevents good efficiency, and therefore a steam generator for a car would be challenged to modulate enough to prevent a lot of short cycling . I would think that with the use of a computer the water pump that controls steam temperature, and the burner that controls boiler pressure the system could be much better controlled than the Doble which J leno say will give 20 m/g pushing a 5000 lb. square car. He says the burner is on and off two or three times a minute , if this burner cycling was present in a home heating unit the cost to operate would be very high . The biggest challenge is reclaiming the latent heat from the spent steam , maybe the condenser used in the Chrysler jet engine car would do the job .Another place to save some fuel is to allow the car to coast .