Simulation of transient performance of vehicle turbocharged diesel engine during acceleration process

Currently, the transient response and emission performance of turbocharged direct injection (DI) diesel engines under transient conditions become the research focus. To predict the performance and nitrogen oxides (NO) emission of vehicle turbocharged DI diesel engines during acceleration process, firstly, a zero-dimensional thermodynamic real time simulation model was developed to describe the working process in cylinder, which based on energy and mass conservation within the engine cycle and the filling and emptying method. The thermodynamic model takes all the engine subsystems into account, namely turbocharger, intercooler, fuel pump and speed governor. In order to simulate the accelerating operation more accurately, the two-vibe curve model was adopted to simulate the actual heat release rate and fuel burning speed. In addition, the incomplete combustion was taken into consideration. Furthermore, the convective heat transfer rate to the combustion chamber walls was simulated through the Woschni's heat-transfer coefficient with transient correction. Secondly, According to crankshaft torque balance based on the conservation of angular momentum principle, the instantaneous values for engine speed and angular acceleration were calculated. Specifically, the engine indicated torque that includes the contribution of gas and reciprocating inertia forces of all cylinders was calculated according to the instantaneous cylinder pressure and engine dynamics. The friction torque of diesel engine was assumed to be a function of the mean friction pressure, working volume of cylinder and instantaneous engine speed. Moreover, the load torque was determined via the torque balance of power-train system. Subsequently, both thermal and prompt NO mechanism were applied to predict NO emissions, and the extended-Zeldovich mechanism and overall reaction rate theory were adopted to simulate the net formation rate of NO. Finally, all above models were integrated and a simulation platform of the entire vehicle system was established based on the Matlab/Simulink. Using a step input signal as the step throttle (fuel pump rack position) change, and the model can be run to mimic vehicle real acceleration process under various (vehicle) speeds and gear. The model was validated through comparison of the simulation results with measured values. Fortunately, the simulation results are quite in line with the actual situation, and showed that the design of model was reasonable and accurate. Particularly, the prompt NO mechanism contribute about 5.26~11.36 percent to the production of total NO under the steady-state operating conditions. During the early cycles of the acceleration transient conditions, the turbocharger lags behind about 0.14 seconds. Additionally, both thermal and prompt NO emissions increase considerably owing to the initially low air-fuel ratio. The results of this study can be greatly helpful to predict and analysis the transient performance and NO emission of turbocharged diesel engines.