<div class="htmlview paragraph">This work is based upon a simulation model for spark-ignition engines that considers the instantaneous heat transfer, the combustion process occurring at a finite rate as well as intake and exhaust processes. The model calculates the thermodynamic properties - including exergy - of each gas mixture composition of the working fluid. Besides the common features of thermodynamic simulation models, the determination of instantaneous irreversibilities, exergetic efficiencies of each process and an overall cycle exergetic analysis are also included.</div> <div class="htmlview paragraph">Based upon the simulation model, first and second law analysis are applied to a parametric study with emphasis in the combustion process and the valve timing effects. Exergy destructions taking place during the combustion of an ethanol fueled engine and a gasoline version of the same engine are compared.</div>