Design of continuously variable power split transmission systems for automotive applications

Abstract

The continuously variable power split transmission (CVPST) concept is presented in the context of an automotive application. The system consists of a variable pulley set (variator) coupling two of the three rotating elements of a planetary gear train, namely the sun and the ring gears. The arrangement is such that a ‘power split’ feature is attained in which the input shaft delivers power to the sun gear as well as to the driving pulley, which in turn drives the ring gear of the planetary through the variator. Because the input shaft delivers the input power at two locations (at the sun gear and at the driving pulley), the variator carries only a fraction of the total power flowing through the input shaft. The third rotating element of the planetary gear set (the planet carrier) collects the power flows from the ring and the sun gears, delivering the total power output to the vehicle. This feature allows an increased engine power envelope for potential automotive applications. This feature also reduces the power losses associated with power transmission, especially at the low speed-high torque modes, while providing continuously variable transmission ratio capability. A term dubbed the ‘cross-over coefficient’ is established which can be used to prescribe both the fraction of total power flowing through the variable element and the velocity ratio span for the transmission. An extension of this can be obtained by means of an external gearbox, allowing a stepless span in the optimum range of both engine and transmission. Design procedures are presented for an automotive application in which a stepless extension (synchronized variator) of the useful transmission ratio span is achieved by means of an external step-up gearbox. The two-stage system allows operation in the optimum range of both engine and transmission.