Study On The Optimal Control Of Fuel Economy For Medium Hybrid Electric Vehicles

Compared to the conventional vehicles, the advantage of Hybrid Electric Vehicles (HEVs) is that the increase of kinds and combination ways of departments in powertrain.The driving mode of engine and motor can be optimized according to the vehicle driving conditions, which ensures that the engine and motor work in the high fuel economy area and the vehicle has low energy consumption, low emission and good performance. When the basic configuration and parameters of HEVs are determined, the key technology is how to control the engine, battery and motor work harmoniously.The predictive control of HEVs with global positioning system and geographic information system on board is an effective method for decreasing fuel consumption, recovering regenerative braking energy and prolonging battery life. Navigation system's composition and working principle are introduced firstly. Based on the road and traffic information, such as average speed, load layer and limited speed, provided by GPS/GIS in the future predictive route, past and current driving state models of vehicles, the driving state model for HEVs in the future predictive route is set up.Using a medium HEV as the research object in this paper, Based on the experiment data of engine, motor and NiMH battery, the numerical model of powertrain for hybrid system is established, including the numerical models of engine fuel consumption, motor/ generator efficiency, battery state of charge and vehicle dynamics. According to the analysis of discharging mode and charging mode, the relationship of engine torque with motor / generator torque, and the relationship between engine speed and motor / generator speed are gained. Base on the load characteristic of engine, a non-linear fuel consumption model of engine is simplified to linear quasi-static model. The predictive horizon in the future route is discretely divided into N stages, the minimum fuel consumption of a HEV at a driving cycle is chosen as the optimal control target function, and the motor torque is chosen as the optimal control variable. An optimal control of fuel consumption model of a medium HEV is established by applying dynamic programming method. The optimal control method for the torque distribution of HEVs is studied; some methods are offered to reduce the dynamic programming computation. Finally, some simulation examples of a medium HEV for the optimal control are given out, and the calculation results show that the global optimal control strategy applying dynamic programming algorithm can achieve the purpose of reducing fuel consumption.