A Simple Model for Predicting the Trapped Mass in a DI Diesel Engine

Although in combustion diagnosis models the uncertainty in the trapped mass is not critical, different authors have reported non negligible effects on the rate of heat release. Usually, an emptying-and-filling model is used to estimate the residual mass, whence the trapped mass is obtained. Generally, the instantaneous pressure at the intake and exhaust ports are not measured for combustion diagnosis applications and hence, it is difficult to estimate accurate values of the residual mass.

The objective of this work is to propose a simple physical model to estimate the residual mass in a DI Diesel engine for a combustion diagnosis model. The proposed model specially focuses on the exhaust port conditions, because they appear to be the most important factor affecting the residual mass estimation. The model considers the exhaust manifold as two volumes and two nozzles in series to simulate the effect of the gas accumulation; thus instantaneous pressure evolution in the exhaust port has been simulated so that an accurate in-cylinder conditions evolution can be obtained. The results have been validated by means of experimental measurements and one-dimensional unsteady gas-dynamic modeling (1D), both in motoring and combustion tests in two high speed Diesel engines. On one hand, the instantaneous simulated pressures in the cylinder and exhaust port are compared with experimental pressures. On the other hand, the trapped mass provided by the model is compared with the results of the 1D model. The results show that the proposed model is able to estimate the trapped mass with a suitable error for combustion diagnosis purposes.