Fuel Consumption Tuning for Electric All Wheel Drive System

Sammanfattning: This report investigates methods to reduce fuel consumption in SAAB's prototype hybrid car. It is primarily concerned with changes in the final drive ratio (FDR) of the gearbox. Reducing the FDR lowers the engine speed, allowing the engine to run at a more efficient operating point. However, this has a negative impact on engine power and vehicle performance. It was thought that the hybrid's eXWD (electric cross wheel drive) system could make up for this loss of performance. Computer simulations showed significant improvements in fuel economy when the FDR was reduced. This was confirmed by real world tests on a chassis dynamometer, although the number of tests were too low to establish the precise fuel consumption reduction. The effects on performance were evaluated by creating a quasi static model of the drivetrain and calculating which combinations of speed and acceleration corresponded to the engine's torque limits. The loss of acceleration was found to be lower than the increase in acceleration made possible by eXWD for most choices of FDR. It is therefore possible to reduce the FDR in the hybrid version of the car and still maintain a performance advantage compared to the front wheel drive version. No other major disadvantageous effects were found. The conclusion was therefore that a reduction of the final drive ratio is a feasible method to improve the fuel economy of the prototype car, given that maximizing performance is deemed less important than increasing fuel economy.