Abstract
The combination of an Infinitely Variable Transmission with a crankshaft mounted integrated motor generator in a mild hybrid parallel configuration has shown potential for combined fuel economy benefits from both technologies. Modelling and simulation provided a good platform for the design study and the initial fuel economy prediction. The development of a Simulink model enabled the design of the hybrid controller functions: power assist, regenerative braking and active torque control. This paper shows how the simulation tools were used to develop a complete control strategy, to commission the prototype vehicle, to validate the models, and to assess fuel economy.
The vehicle under investigation is a North American 5.4L V8 SUV; the Infinitely Variable Transmission is a torque controlled, twin regime, split power system. The project aims at demonstrating the combined benefits of this transmission and the parallel hybrid layout. A design study has been completed initially to size the electrical machine for best fuel economy without driveability compromises. Then models for the motor, the power electronics and the battery have been established during the manufacturing of the synchronous electrical machine, and integrated in Torotrak´s existing System Development Tool to develop power assist, regenerative braking and active torque vibration control strategies.
The present paper describes how the plant model was then validated as a whole after the controller was uploaded in the vehicle. A number of typical driving manoeuvres were performed on the test track, on the vehicle dynamometer and in simulation environment to tune the models. The active vibration control is aimed at reducing combustion torque fluctuation in order to extend the engine operation range for best specific fuel consumption at low speed. The power assist relies on the torque control of the transmission and produces inertia and lag compensation as well as splitting the torque output during tip-ins. The regenerative braking optimises the energy recovered during braking events.
Once the three hybrid function strategies were validated, an energy management system was developed in simulation and established fuel economy figures. Over the US FTP combined cycle, the hybrid vehicle is 5.9% better in simulation than the baseline infinitely variable transmission vehicle, which had already shown 20% improvement over the standard automatic. In addition to the technology demonstrator vehicle, the project has produced a validated simulation environment for further investigations. With active torque vibration control, it is possible to quantify the benefits of a reduced idle speed and a more frequent use of the low speed, high torque operation at no NVH cost. The computer based platform also offers the possibility to look into alternative energy management controllers.
Keywords: Hybrid Electric Vehicle, Powertrain Control, Infinitely Variable Transmission, Driveability, Fuel Economy