Comparative studies regarding the optimization of internal combustion engines sometimes are carried out according to the perspective of an exergetic analysis. Time or space-time resolved irreversibilities and irreversibility production rates are determined, depending on the informations that the simulation model permits. However, the irreversibility production rates not always provide an appropriate criterion for comparisons to be made. For these cases Second Law efficiencies are used to evaluate the degree of reversibility of each process or of the complete cycle. Nevertheless this procedure does not allow to explore the advantages that a time or space-time resolved model can offer. In this work an instantaneous Second Law efficiency is proposed from the differential equation of the exergy balance. It is shown that this parameter provides a consistent comparison criterion for situations when comparisons based on the irreversibility production rates may lead to inappropriate conclusions.