Hydraulic retarders are extensively used in heavy-duty vehicles because of their advantages, such as their large braking torque and long continuous operating hours. They can reduce the vehicle velocity by converting the kinetic energy of a traveling vehicle to the thermal energy of the working fluid. The water medium retarder is a new type of hydraulic retarder with the characteristics of high power density and simple structure. It uses the engine's coolant as the working medium, and the heat is directly taken away by the vehicle cooling system. Therefore, the heavy-duty vehicle can achieve long-term continuous braking during the downhill process. One of the main functions of water medium retarder is driving downhill at a constant speed which determines whether the vehicle drives stably and safely. Therefore, studying the constant-speed control strategy during downhill driving is particularly important.
In this paper, the structure and working principle of water medium retarder and the dynamic characteristic are analyzed. The dynamic models of vehicle and water medium retarder are established based on dynamic analysis during downhill process. The braking process that involves the water medium retarder is divided into three stages. Then the constant speed controller of water medium retarder which include three control algorithms is designed, respectively, PID algorithm, fuzzy algorithm and fuzzy-PID algorithm. The vehicle dynamic model and the constant speed control model of water medium retarder are established using MATLAB/SIMULINK. The simulation has been carried out and the comparative analysis of three algorithms mentioned above is conducted. The simulation results show that three controllers designed in this paper can quickly and accurately calculate the target filling ratio, fuzzy controller has better constant torque control performance, and the vehicle speed error is significantly reduced, which improves the stability of the vehicle during downhill process.