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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.

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.