An External Combustion Engine (ECE) was designed, tested, and retrofitted to a 40 ft, 46-passenger city bus. The bus was tested and demonstrated comparable acceleration and lower external noise, odor, and air pollution emissions than a diesel bus. Presently, ECE bus fuel consumption is considerably higher than that of a diesel bus. It is shown in this paper how ECE fuel consumption can be improved with further development, although not to lower levels than the diesel. More time in service is needed to determine the ECE bus operating costs, which depend largely on the costs of powerplant maintenance.