This paper describes the characteristics of a low-pollution steam engine for the California Steam Bus Demonstration Project. The nature of combustion in an external combustion engine is such that lower levels of exhaust emissions should be expected. To demonstrate the lower exhaust emission level and to show that steam engines are feasible powerplants for automotive applications, a reciprocating-type steam powerplant was designed, built, tested, and installed in an urban bus.The system was designed to provide performance and handling characteristics similar to those initially found in the diesel-powered bus without significantly altering the appearance of the bus or reducing its seating capacity. To accomplish this limited objective for demonstration purposes, all available space, including that under the bus, was utilized for major components.A 3-cyl double-acting compound engine is used to extract work from steam delivered at 800 psia and 850 F. The exhaust steam is condensed in a vented system, where the exhaust is maintained at nearly atmospheric pressure. Condensate is recirculated to a monotube type vapor generator. The feedwater and burner control are similar to those used in the later Doble steam cars. Data on performance of the bus with this engine are presented.