A conceptual design was prepared of a Rankine-cycle power system with organic working fluid and reciprocating engine for a low-emission automotive propulsion powerplant. The goal of the study was development of a system competitive in cost, performance, and driver convenience with the internal combustion engine system using current technology wherever possible. The component designs and characteristics are presented. The complete 100 net shaft horsepower system is packaged in the engine compartment of a late model sedan; the predicted performance characteristics are presented. The system is closely competitive, in 0 - 60 mph acceleration time and in level-grade top speed, with a 302 cubic inch displacement internal combustion engine with three-speed transmission. The fuel economy in customer-average mpg is approximately 20% less than the 302 cubic inch internal combustion engine. The use of thiophene as a working fluid, with a moderate maximum cycle temperature of 550F, permits a significant cost reduction relative to the equivalent steam system. Projection of current burner data indicates a strong potential for emission levels significantly less than the projected 1980 federal standards for all three of the major pollutants: unburned hydrocarbons, carbon monoxide, and nitric oxide. (Author)