Previous SPS vehicular steam expander development is reviewed. A counterflow expander installed in the 3000 lb (1364 kg) California Steam Car is shown to give 6.2 mpg (2.2 km/l) over the Urban Federal Driving Cycle, for BSFC = 1.2. Advantages of the uniflow expander with impulse valves are discussed, and the equation for theoretical cycle efficiency is derived. Laboratory tests on a 4 cylinder uniflow with impulse valves and a 2000 psia (136 atm), 1400° F (760° C) monotube boiler are described, and efficiency results for steam temperatures up to 1100° F (593° C) are presented. The measured brake thermal efficiency of the test uniflow increases with expansion ratio E from .146 to .225 for 5.7〈 E 〈 14.0. The counterflow efficiency decreases with E, from .18 to .11 for 7 〈 E 〈 15. Internal losses are discussed, and the higher efficiency of the uniflow is justified on the basis of lower internal leakage, heat transfer and friction.