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Control of the geared neutral point in a traction drive CVT

The split-torque, geared-neutral transmission concept has potential for simplifying transmission design by eliminating the need for launching devices such as torque converters, fluid couplings, or slipping clutches. In addition, it can provide forward and reverse ratios without the need for additional clutches or gearing. Because of the theoretical infinite ratio range of the transmission, it has considerable potential for improving vehicle fuel economy and performance. However, the difficulty associated with the control of such transmissions has prevented the concept from being successfully implemented in automobiles. A dual-cavity, half-toroidal, traction-drive transmission design using the geared-neutral concept was developed. Several transmissions were built for control development and vehicle implementation. This paper describes the control system developed for the geared neutral part of the transmission operation. The control strategies and algorithms were developed based on a detailed dynamic mathematical model and computer simulation of the complete driveline of the vehicle. Both engine and transmission controls were coordinated to ensure smooth and stable operation of the geared neutral point. Specific attention was given to providing characteristics similar to those of a conventional automatic transmission, such as forward and reverse creep. Both simulation and vehicle test results are presented to show that stable geared neutral operation is possible under all conditions.