Second law analyses of both spark-ignition and diesel engines are presented using two-zone models. The analyses include descriptions of the evaluation of the various terms in the availability balance. Chemical and thermomechanical availability are separated using a definition which allows the portion of the fuel availability that can be extracted by a combustion engine to be distinguished from that which requires interaction with the reference environment. The chemical availability must be calculated correctly in order to obtain an availability balance. The diesel model includes a parameter that allows the effect of fuel-air mixing rates to be simulated. The analyses for the spark-ignition and diesel models are applied in a parametric study of the effects of equivalence ratio, fuel-air mixing, residual fraction and combustion duration on the chemical and thermomechanical availability and the irreversibility.