INCREASING realization by automotive engineers and by oil refiners that the nature of the fuel available largely determines engine-performance possibilities has resulted in valuable cooperative research, important progress and the preparation of a firm groundwork for future progress. The author outlines the more important characteristics that automotive-engine fuel should possess and indicates some of the relations existing between possible engine efficiency and fuel characteristics.Heat of combustion of a fuel, volatility, tendency to detonate and freedom from impurities are discussed. Actual data from two commercial cars for which optional cylinder-heads were available are presented. These data are analyzed and the conclusions reached are that, altogether, the trend of the last few years in automotive-engine design apparently will be continued; that the fuel of the future will permit using engines of greater efficiency than those of today and that the advantages of engines of high brake mean effective pressure will be still more appreciated by the owner who drives for pleasure and by the commercial-car owner.INCREASING realization by automotive engineers and by oil refiners that the nature of the fuel available largely determines engine-performance possibilities has resulted in valuable cooperative research, important progress and the preparation of a firm groundwork for future progress. The author outlines the more important characteristics that automotive-engine fuel should possess and indicates some of the relations existing between possible engine efficiency and fuel characteristics.Heat of combustion of a fuel, volatility, tendency to detonate and freedom from impurities are discussed. Actual data from two commercial cars for which optional cylinder-heads were available are presented. These data are analyzed and the conclusions reached are that, altogether, the trend of the last few years in automotive-engine design apparently will be continued; that the fuel of the future will permit using engines of greater efficiency than those of today and that the advantages of engines of high brake mean effective pressure will be still more appreciated by the owner who drives for pleasure and by the commercial-car owner.