Integrated control strategy for CVT powertrains with consideration of vehicle drivability

Abstract

During shift, power flow is not interrupted in powertrains equipped with continuously variable transmission (CVT). When hard acceleration is commanded, engine speed will flare and corresponding torque will be consumed, which leads to a drop in vehicle drive torque and also the vehicle acceleration. This is the reason why CVT vehicles have poor drivability during hard acceleration maneuver. Conventional method such as torque compensation doesn’t always work due to the limited backup torque of engine. According to this, means to evaluate the drivability of CVT vehicles are studied, affect factors of drivability are analyzed in detail. Hard acceleration process of CVT vehicle is studied by theoretical analysis, based on which engine torque and ratio change rate of CVT are identified as two key control parameters that decide the drivability of CVT vehicles during hard acceleration maneuver. Therefore, a control strategy based on restricting the change rate of CVT ratio together with torque compensation is proposed, and two different algorithms to establish the limitation of ratio change rate are proposed. These two algorithms are simulated and compared with each other, results indicate that drop of vehicle acceleration is eliminated evidently by limit the change rate of CVT ratio, but small ratio change rate also results in a longer time to finish the accelerate process, an algorithm to decide a proper ratio change rate is needed in order to tune these different characteristics. In order to get better control effects, a new fuzzy logic based algorithm is proposed to decide a proper ratio change rate during kick down conditions, simulation and experiment results indicate that, the amount of vehicle acceleration decrease is reduced from about 1 m/s2 to almost 0, in the mean time the accelerate process only delayed for about 0.3 s. The proposed control strategy and algorithm can effectively tune the characteristics of CVT equipped vehicle during kick down conditions.