Improvement in fuel economy for a parallel hybrid electric vehicle by continuously variable transmission ratio control

A continuously variable transmission (CVT) ratio control algorithm is proposed to achieve the improved fuel economy of the parallel hybrid electric vehicle (HEV) by considering the HEV powertrain response lag. In this algorithm, the target CVT ratio is modified by the vehicle velocity that is estimated after the powertrain response lag. To estimate the vehicle velocity after the response lag, an acceleration map is suggested. The CVT ratio control algorithm is validated by the HEV bench tester. From experimental results, it is found that the engine operation trajectory by the modified ratio control algorithm is shifted to a more efficient region compared with those by the conventional CVT ratio control for the mild acceleration mode, which gives better fuel economy. In addition, performance simulations for the federal urban driving schedule are carried out to evaluate the effect of the response lag on the HEV fuel economy using the HEV powertrain model. The simulation results show that a better fuel economy can be achieved as the response lag used in the modified CVT ratio increases. However, in actual applications, a compromise between the fuel economy and the acceleration performance would be required.