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In Diesel engines equipped with jerk injection pumps there appear many effects derived from fuel compressibility and the short duration of injection. As a result of them, cavitation, secondary injection, dribble and other hazardous phenomena may occur. With the objective of studying these processes, a simulation model for an indirect Diesel injection system has been developed. The mathematical model takes into account cavitation and gas dissolved effects. A relationship is obtained theoretically to reproduce the dependence of density of fuel oil with the pressure. The deduction of this equation is based on some hypotheses: that the amount of air in the liquid is constant, the process isothermal, the liquid fuel and its vapour are in thermodynamic equilibrium, and finally, that the temperature, pressure and velocity of both phases are identical. The unidimensional two-phase flow equations are simplified and employed to resolve the high-pressure line.

From the pressure signal, in the cylinder of an spark-ignition engine, measured experimentally, we have obtained the variation of pressure due only to the process of combustion. We have proceeded to the study of this signal using Fourier's analysis and we have obtained several relations between the harmonic's amplitude of the transformed and the engine's operation parameters. These relations can be used as tools of detection of troubles or anomalous processes in the engine's combustion.