CVT Ratio Scheduling Optimization with Consideration of Engine and Transmission Efficiency

This paper proposes a transmission ratio scheduling and control methodology for a vehicle with a Continuous Variable Transmission (CVT) and a downsized gasoline engine. The methodology is designed to deliver the optimal vehicle fuel economy within drivability and performance constraints. Traditionally, the Optimum Operating Line (OOL) generated from an engine brake specific fuel consumption map is considered to be the best option for ratio scheduling, as it defines the points at which engine efficiency is maximized. But the OOL does not consider transmission efficiency, which may be a source of significant losses. To develop a CVT ratio schedule that offers the best fuel economy for the complete powertrain, an empirical approach was used to minimize fuel consumption by considering engine efficiency, CVT efficiency, and requested vehicle power. A backward-looking model was used to simulate a standard driving cycle (FTP-75) and develop a new powertrain-optimal operating line (P-OOL). Simulation results using the backward-looking model show a significant improvement in overall fuel economy when using the P-OOL (considers engine and CVT efficiency) compared to the OOL (considers only engine efficiency). Next, a forward-looking, velocity-driven model was developed to simulate the real-time behavior of a vehicle. Fuel economy results were compared when implementing the P-OOL and the OOL with a hardware-based CVT shift rate constraint. Finally, a control algorithm that considers powertrain loss and inertia torque due to CVT ratio changes is proposed to minimize powertrain response lag when operating along the P-OOL. This combined ratio scheduling and response lag control methodology is shown to improve vehicle fuel economy with real-time simulated driving conditions.