Information to be gained using an air-standard model of an engine is discussed as well as the thermodynamic inconsistencies of the model. A number of useful trends arrived at by use of a time-independent fuel-air model are presented. The thermodynamic rate model, made possible by the advent of high speed computers, provides a great deal more insight on engine behavior than the fuel-air model but still has serious deficiencies. Progress towards expanding the thermodynamic rate model to include transient operation, multidimensions and Second Law analysis is discussed. Lack of experimental data on which to base sound models is progress limiting for the first two; obtaining these data should be given high priority. Second Law analysis is of potential help in understanding losses and improving fuel economy.