Stirling engine
The experiments showed the Stirling engine could be better to power an extended-range electric vehicle rather than serving as the primary power for an automobile. The Spirit with the Stirling engine was better suited to run at a constant power setting, in contrast to the internal combustion engine with its easy throttle regulation over a wide range. Although successful in the MOD 1 and MOD 2 phases of the experiments, cutbacks in funding further research and lack of interest by automakers ended possible commercialization of the Automotive Stirling Engine Program.