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Hydraulic retarders are extensively used in commercial vehicles because of their advantages, such as their large braking torque and long continuous operating hours. In this article, the structure and working principles of hydraulic retarders are introduced, and their dynamic characteristics are analyzed. The theoretical model of a hydraulic retarder is then established based on the dynamic analysis of a vehicle driving downhill. The braking process that involves the hydraulic retarder is divided into three stages. Moreover, the filling ratio controller of the hydraulic retarder is designed by adopting fuzzy control theory to control the braking torque of the vehicle while driving downhill. The vehicle dynamic model and constant-speed control model were then established in the MATLAB/Simulink environment. The simulation results showed that the fuzzy logic controller designed in this study has good constant-torque control and anti-inference performances, which can accurately and immediately produce braking torque to satisfy the braking requirement, thereby enabling the vehicle to drive downhill at a constant speed. As a result, the control strategy designed in this article can lead to significant improvements toward a safe road transport.

Hydraulic retarders are auxiliary braking devices that reduce vehicle speed by converting its kinetic energy into thermal energy of a working fluid. This study analyzes the torque-output characteristics of a new type of hydraulic retarder and optimizes its control strategy under different braking conditions. The design of an observer is based on an analysis of the vehicle dynamic system to ensure that it sets the control target of the dynamic filling ratio for hydraulic control system. With the assistance of the observer, the hydraulic retarder can produce the required braking torque as quickly and as accurately as possible. This study simulates a feedback control system of the vehicle dynamic model with the observer. The results prove that even if the vehicle is driven under complex conditions wherein the slope of the road is changed, the observer effectively sets a control target of the filling ratio for the hydraulic control system. This mechanism strengthens the adaptability and accuracy of the hydraulic retarder control strategy.