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A statistical method is described which allows a very accurate determination of the correlation between pressure and volume. The basis of the method is the pressure measurement of an engine under motoring conditions and correcting the pressure diagram with respect to heat and blow-by losses. Computation of the polytropic exponent dependent on error of phase lag determines the correlation in the pressure volume diagram. Since the internal processes in an IC engine under motoring conditions differ significantly from the ones under firing conditions, some modifications in the heat and blow-by losses models are necessary in order to be successfully applied in the proposed method. A simple optimization approach has therefore been used to correct both models. Several pressure diagrams measured under engine motoring conditions and at the different engine speeds have been studied.

A computer submodel, predicting the twin-turbine discharge coefficients is presented. Gas flow through axial separated spiral volute casing into the interspace is solved with divided expansion in two convergent nozzles. Mixing taking place in the interspace is assumed to be isobaric and the gas expansion through the rotor is simulated with rotor characteristics derived from conventional turbine characteristics. The results of computation are turbine discharge coefficients and efficiency map, determining the twin-turbine boundary conditions under partial admission. The method has been validated using experimental data of 6- and 12-cylinder turbocharged diesel engines and the predictions are in good agreement.