Calculation and simulation of air mass flow forturbocharged diesel engines

Abstract: Air mass flow determination is a major issue for matching of turbocharger and dynamic simulation of diesel engine system. Generally speaking, there are two kinds of mean value models of air mass flow. One is based on the amount of air for fuel combustion, which involves engine power, fuel consumption rate, instantaneous excess air ratio and coefficient of scavenging; and the other is on the basis of the definition of volumetric efficiency, which relates to piston swept volume, air density and coefficient of scavenging. However, different purpose and conditions of research form various mathematical models of air mass flow, and the results of calculation varied wildly. In order to get the optimum simulation model, different mean value models of air mass flow were comprehensively compared and further analyzed in this paper. Furthermore, the influence of turbocharger, intercooler, exhaust gas recirculation rate, volumetric efficiency and operation condition were particularly taken fully into account. Firstly, relative condition correction was carried out for the first type of original models to make them can be applied to any operating condition beyond the rated condition. The precision of models were remarkably improved through multiplying the original models by the ratio of engine speed under any operating condition to rated speed. Secondly, by considering the effect of exhaust gas, the mass conservation equation was rebuilt, and then the correctional formula of air mass flow for turbocharged diesel engines which were equipped with exhaust gas recirculation system were derived. Finally, part of the above models were modified via taking account of the influence of combustion chamber geometry, compression ratio, cooling efficiency and scavenging efficiency on volumetric efficiency. Meanwhile, all the mathematical models were simulated with Matlab/Simulink and verified through comparing the simulation results with experimental values. Eventually, the simulation and experimental results show that the air mass flow model after revising fit well with the actual situation. In addition, the average relative error of the optimal model is 2.96% and therefore the design of model is reasonable and accurate. Consequently, the results of this study will be helpful to accurately calculate the air mass flow of turbocharged diesel engine, and provide useful reference for its model selection.