Research on Optimal Gearshift Strategy for Stepped Automatic Transmission Based on Vehicle Power Demand

Selection of gearshift point plays an important role in the field of automatic transmission technology, which directly affects the vehicle dynamic and economic performance, etc. In order to designing optimal gearshift strategies for conventional passenger vehicles equipped with stepped automatic transmission, in this paper, the vehicle power demand was defined under different environment, different driving intention and different vehicle operating conditions. Dynamic programming (DP) method is used to solve the optimal static gearshift decision sequence based on the simplified model of powertrain system. The drivability is respected by imposing an inequality constraint on the power reserve limit and the fuel economy is the objective function. Considering the change of vehicle additional load and road slope, the gearshift strategy based on power reserve is proposed. NEDC, FTP75 and full throttle acceleration performance simulation were carried out respectively based on optimal dynamic shift schedule, optimal economic shift schedule and the proposed optimal gearshift strategy. And the gearshift strategy based on power reserve was subsequently implemented to the target vehicle and tested for dynamic and economic performance evaluation. The simulation and real vehicle results show that the proposed optimal gearshift strategy can achieve the optimal tradeoff between the fuel economy and the drivability, which is adaptive to the vehicle dynamic characteristics. Overall, the optimal shift schedule based on DP shows good fuel economy and the test drivers were quite satisfied with the timing of the shifts as well as the overall drivability the designed shift schedule based on DP provided. It is also observed that if applying the concept of vehicle power demand in the design of optimal gearshift strategy, the vehicle fuel economy can be improved without sacrificing the vehicle drivability.