Simulation and Optimization of Commercial Truck Powertrain System
|School||China Agricultural University|
|Keywords||Commercial Truck Powertrain System Transient Performance Driving Cycle Simulation Optimization|
For a long time, commercial trucks have greatly promoted economic and social development, but meanwhile, they have caused serious environmental pollution and exacerbated energy crises. Powertrain system is the lifeline for the vehicles normal traveling. Therefore, the performance of powertrain has a direct impact on the power output, fuel economy and pollutant emissions of vehicles. In recent years, the simulation, matching and optimization of powertrain system has become the researching focus since it’s an essential solution to improve the global efficiency of powertrain and reduce the fuel consumption and pollutants.To predict the performance and pollutant emissions of vehicular turbocharged DI diesel engines during both steady-state and transient process, firstly, a zero-dimensional thermodynamic real time simulation model which based on the filling and emptying method was developed to describe the working process in cylinder. The thermodynamic model takes into account all the engine subsystems viz. turbocharger, intercooler, fuel pump and speed governor. In order to estimate the transient performance 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 heat-transfer coefficient with transient correction. 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. What’s more, the optimal mean value model of air mass flow calculation was determined via comparison various mathematical models of air mass flow and comprehensive optimization.Secondly, the torque balance equation that takes into account all the drag force was established based on the detailed analysis of dynamic relationship between the components of powertrain system. Then gear shifting strategies was optimized for fuel economy and maximum power, and the automatic transmission shift logic was controlled by using finite state machine of Stateflow. Study focused on the evaluation of vehicle performance, and particular attention was given to the engine behaviors under transient conditions, dynamic response characteristics of vehicle acceleration and braking process.Thirdly, according to crankshaft torque balance based on the conservation of angular momentum principle, the instantaneous values of engine speed and angular acceleration were simulated. 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 calculated through a detailed analytical model, which includes the contribution of piston rings assembly, loaded bearings, valve train and auxiliaries, and describes the non-steady profile of friction torque during each cycle. Moreover, the load torque was determined via the torque balance of powertrain system.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 then run the model to mimic vehicle real transient process under various (vehicle) speeds and gear. The validation of the model was done through comparison of the simulation results with measured values. Fortunately, the simulation results are quite in line with the actual situation, it’s showed that the design of model is reasonable and accurate.To estimate the engine fuel consumption and pollutant emission under real driving conditions, the vhehicle model was simulated based on the standard driving cycle. The simulation results show that the acceleration transient events and the related turbocharger lag phenomena significantly contribute to the cycle cumulative emissions and fuel consumption. Through the sensitivity analysis of fuel consumption and pollutant emissions, estimating the overall impact of design parameters on the fuel consumption and emission performance. The main factors include the vehicle weight, tire rolling resistance, aerodynamic drag and driveline configuration. Studies also indicate that it is difficult to change each of these parameters to reduce the fuel consumption, NO and PM emission simultaneously. Eventually, optimizing the transmission ratio for fuel economy and emissions based on the the sensitivity coefficients. Consequently, the results of this study will be greatly helpful to predict and analysis the economy performance and pollutant emissions of commercial trucks and provide a useful reference basis for the design and optimization of powertrain system.