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Diesel combustion diagnosis is frequently performed through the calculation of apparent heat release rate, using single zone combustion models. This computation lies on the solution of energy balance equation and utilizes experimentally determined in-cylinder pressure data. The information obtained by means of such an approach is considerably exempt of empiric adjustment parameters and can be used for the development and testing of multizone or multidimensional simulation tools. Due to its simplicity as well as to the strong experimental character conferred to the results, the application of this sort of experimental study in the exergetic analysis of the processes occurring in diesel engines is also an interesting approach. In this work, a single zone combustion model is used in the exergy diagnosis of a high speed direct injection diesel engine and the obtained results are discussed in detail.

Equations governing the flow through admission and exhaust ducts in diesel engines are presented taking into consideration the fluid compressibility, the variation of the duct cross section, and heat transfer and friction between the gases and the duct walls. The hyperbolic nature of the equations is shown and a solution is obtained using the method of the characteristics. The algorithm of this solution is included in a simulation model for a single cylinder diesel engine, based on the single zone combustion formulation. This model is used in a study to evaluate the engine performance indexes when different admission duct shapes are used.

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.

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. 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.