Research On Key Fluid And Control Technologies Of Water Medium Retarder For Commercial Vehicles

Braking conditions are complicated because of the increased load carried by vehicles. The output power of the enigne is increased by 100%, however the braking power output of is just increased by 40%. The inverstigation report of traffic accident in 2015 shows that 32% of the traffic accident is caused by braking failure. Strong braking forces and durable braking systems are required to regulate speed, especially for commercial and heavy-duty vehicles. Long-term use of service brakes leads to thermal failure and pollution, which cannot be completely solved by optimizing the service brakes. The natioal standard point out that retarder or other auxiliary braking devices must be installed in heavy duty vehicles.Hydraulic retarders are auxiliary braking devices that can reduce the vehicle speed by converting the mechanical energy of a driving vehicle to the total energy of the working fluid. Hydraulic retarders can be used instead of service brakes in non-emergency braking conditions. These retarders can reduce the frequency and time of use of service brakes, thereby minimizing the heat-related problems. Unlike other auxiliary braking devices, hydraulic retarders exhibit the advantages of high braking power, high efficiency, and zero pollution.The water medium retarder is the new type of hydraulic retarder. When the braking torque is required, the coolant flows to the working chamber forced by a controller. The high temperature is produced along with the braking torque and can be rejected directly by a radiator. Contrary to the oil medium retarder, the water medium retarder uses the radiating power to efficiently regulate the vehicle velocity because of the construction optimization.This research is conducted under the project of key technologies of computational fluid dynamic(CFD) analysis of commercial vehicles. The fluid and control problems of the water medium retarder are analyzed. The key research problems are the CFD analysis of the working chamber based on the fluid mechanics, the water medium retarder control strategies based on the braking requirements, and the control logic of the vehicle braking system. Several aspects are investigated as follows:1. The fluid flow of the working chamber is analyzed mathematically. The mechanism of the production of braking torque in the working chamber is analyzed, and the parameters that influence the braking torque are confirmed. The CFD analysis is conducted in the working chamber, and the key factors are verified using CFD and experimentation. The turbulent is confirmed for calculating the braking torque in the CFD simulation.2. The relation between the pressure downstream and the braking torque is determined, and the control strategies of the filling ratio are confirmed and verified. The braking factors that influence the vehicle dynamic parameters are analyzed. The control strategies and observers are analyzed from the perspective of the dynamics and thermodynamics. Accordingly, the target braking torque under the condition of safe coolant circulation is calculated. The braking processes are divided into three stages when the water medium retarder is applied. The stages are analyzed individually and the control strategies are derived. Two control strategies are proposed to satisfy the constant speed driving. A mass identification model is established for calculating the dynamic parameters.3. The three dynamic models are constructed in MATLAB/Simulink. These models are the combination of the braking system with the dynamic model of the vehicle. The auxiliary braking system only includes water medium retarder in the first model, which is the basic control model. The auxiliary braking system includes water medium retarder and engine brake in the second model, which includes the mass identification model. The auxiliary braking system includes water medium retarder, engine brake, and service brake in the third model. The service brake is controlled in a Stateflow system. The three models are compared and analyzed. The vehicle dynamic and thermodynamic parameters are analyzed. The first model is tested in the test bed to verify the simulation accuracy.