Two successive surveys and round-robin interviews were conducted to determine if there might be in the offing for automobiles an alternative engine which would exhibit low emissions meeting the most stringent requirements. Comparison between engines were couched in terms of selected “acceptability factors” which went well beyond emissions, alone. The overall acceptability was evaluated considering emissions, customer requirements for an engine, manufacturers' requirements for an engine, and engine efficiency and fuel versatility. An attempt was made to establish a time scale as to R and D requirements and eventual production. Comparison of all engines was made with equivalent pre-control Otto cycle engines as the standard.Alternative engines were deemed to be any power plant that was not based on spark ignition Otto cycle engines, or diesel engines. The remaining heat engines largely used continuous combustion as the heat source. Electrical power plants considered were batteries and fuel cells. Hybrid engines and fuel-less stored energy power plants were given some consideration.The unsettled nature of the present socio-economic structure precluded singling out any particular engine or power plant most suitable for development. Evaluations of the characteristics, comparison with pre-control spark ignition engines, problems yet to be solved, and the most optimistic time schedule to be anticipated are attempted and reported. Best probable candidate power plants are then enumerated.Keeping in mind that the future will require lowered emissions in urban air and improved efficiency of the overall transportation system, comment is made on the use of engines and automobiles. Methods of accomplishing in part the goals of the clean air act with changes in structure of the transportation system, taxes, and urban areas are suggested only as means of illustrating that low-emission engines are only one part of the overall problem of maintaining clean air and viable urban centers.