Automotive engines are designed to convert chemical energy to mechanical energy. The efficiency of this conversion is governed by thermodynamics. The two most common engines utilize gas oil and gasoline fuels for this purpose. However, the combustion processes are radically different. The combustion sequence and relative characteristics for both engine types will be discussed. Due to different combustion requirements, the fundamental properties of both fuels will also be examined as these are significantly different for the two fuel types. The main fuel properties discussed are energy density, stability, fluidity, corrosion, contaminants, safety, wear and environmental aspects. Also, with the advent of various renewable components in both fuels, new trends are emerging for both fuel quality assessments as these are molecularly distinct from their crude oil counterparts.