The Clean Air Act Amendments of 1970 require that new model automobiles of 1976 and later meet stringent exhaust emission standards. The technical difficulties appearing in ‘Clean-up’ of the conventional automobile engine require that alternate engines be developed in the event that the conventional engine cannot be made to meet the standards. Although the alternates cannot be mass produced by 1976, prototype engine development and demonstration by that date may lead to decisions to mass produce one or more in the early eighties. Features of the major alternate engine systems are reviewed, projections are made of the prospects of each as a replacement for the conventional engine. The introduction to production of rotary Otto cycle systems by the mid-1970's, gas turbines by the early 1980's and high performance all-electric systems by the late 1980's is projected in this paper.