| At present, the trucks and bus are operating in situation of high-power and high-load, which made the vehicle safe driving issues seriously, there should be strict even harsh demands to the performance of the vehicle.The traditional heavy vehicles have two serious drawbacks:Firstly, the traditional heavy-duty automotive power transmission line is generally the engine-dry clutch transmission. This traditional transmission mode has its inherent shortcomings, which are not smooth starting, load's large impact, which greatly reduces the comfort of the cockpit; Secondly, in mountainous and hilly areas, when the car in the moving process inevitably encounters the case of a long downhill, the driver uses the brake continuously, which will increase the time of the slipping between brake shoes and brake drum and causes the brake drum temperature's increasing that result in heat recession, eventually lead to brake failure and cause a serious traffic accident.Vehicle hydraulic auxiliary starting and braking system can improve the above two disadvantages. The system mainly consists of dry clutch assembly, hydraulic coupling assembly, wet clutch assembly and other components. As hydraulic fluid coupling is used as a transmission medium, means rotation speed between the pump wheel and turbine allows a big difference, so loading a hydraulic coupling can ensure smooth starting; the same time be able to prevent the transmission overload, thereby prolonging the transmission and engine parts'life. In addition, vehicle hydraulic auxiliary starting and braking system assisted braking system and can be used as auxiliary brakes of heavy vehicles. When the car encountered a long slope it should take a long time under braking situation and the system can lock the turbine coupling and keep it stationary, then the hydraulic oil will impact the stationary turbine causing force to resist pump wheel torque and wheel speed is reduced, at the same time pump wheel is combined with the dry clutch and transmission, finally achieved the brake function.Vehicle hydraulic auxiliary starting and braking system includes four sub-assembly:dry clutch assembly, hydraulic coupling assembly, wet clutch and band brake assembly. Hydraulic auxiliary starting and braking system through the four subsystems cooperate to complete the role of auxiliary starting and braking. Among them, in order to increase starting torque and to ensure reliable starting, coupling assembly used the full amount of charging oil to support starting, changed the charging oil rate for different stalls in assisted braking conditions.Using three-dimensional mapping software CATIA create three-dimensional model of hydraulic coupling assembly. According to the model structure characteristics, selected full-flow as the computational domain, the coupling interface parameters between the pump wheel and turbine used sliding grid method, pointing to the starting condition the full charging oil rate is used in numerical simulation,set the flow channel of the blade surface, outer surface as no slipping wall conditions. When numerically calculated the coupling assembly in the partially filled braking condition, the three multiphase flow model in current CFD methods are compared, considering the flow characteristics of fluid coupling assembly selected the hybrid model for gas-Liquid Flow in a non-steady-state calculations.The analysis of starting and braking condition of the flow field reveals the flow field distribution of the pump wheel and turbine. Through the string surface, the wing surface and the typical cross-surface analysis, the result is same to the beam theory. Turbine flow rate in the direction along the entrance to the exit increased, the maximum velocity appeared near the outer ring. The pump rotation speed in channel is just opposite to the pump wheel, the relative velocity decreases from the entrance to the exit; static pressure distribution showed the increasing trends from the circle loop inside to outside. Flow characteristics analysis provided a more reliable basis of structural parameters for the optimization of coupling assembly:firstly, for the direct impact of the flow, static pressure maximum of the pump wheel and turbine appeared in the outer ring and blade roots. Therefore, the transition fillet should be increased in the structural design to mitigate the impact; secondly, because the speed of pump and turbine is tremendously different and the high-speed flow of the pump wheel grate impact on the turbine blades at the entrance causing greater static pressure, which may cause unrecoverable deformation on the margin of leaf, so modifying the wedge angle on blade edge can reduce the phenomenon and improve the character; thirdly, at high speed ratio the appearance of higher relative speed at exit of turbo is due to the high impeller speed difference that resulted from the increasing of pumps, turbines differences in flow area. Choosing reasonable leaves number and angle of the flow field within the coupling assembly can improve the cycle flow of the torque transmission.Within the numerical simulation results of the coupling assembly, this paper drew the original curve of the coupling assembly and predicted the breaking torque of the coupling assembly. The original characteristics of the coupling assembly basically consist with the original characteristics of traditional hydraulic coupling that is the relation of the torque coefficient and the variation of speed ratio, which indicated the vehicle hydraulic auxiliary starting and braking system can reliable complete the work of torque transmission in the starting process. Braking torque prediction indicated the size of the braking torque coupling assembly is affected by the changing rate and speed, as the increase of rotational speed and changing rate, brake torque also increased rapidly, which is be propitious to invariable breaking torque controlling of coupling assembly system.The characteristics research in this paper has some significance in the further research work of the hydraulic coupling and development of the vehicle hydraulic auxiliary starting and braking system.
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