Research On Lock-up Clutch Slipping Control Of Torque Converter At Starting Conditions

A CVT(Continuously Variable Transmission)car equipped with a TC(Torque Converter)as a starting device has better ride comfort than that equipped with a wet clutch.But there are still the following problems existing about TC.TC has low transmission efficiency.The power train is connected by TC flexibly,so the engine power cannot be transmitted to the gearbox in time resulting in slow acceleration response,and the load torque cannot be directly applied to the engine,which results in high engine speed,engine speed fluctuations,loud work noise,and additional loss of stirring oil,etc.All modern TCs are equipped with lock-up clutches.As for a CVT car,the lock-up clutch will not be engaged until the velocity get to the target one under certain acceleration pedal opening to improve transmission efficiency.But the problems above cannot be effectively resolved because of the lack of intervention in the starting process before engaging the lock-up clutch.In this paper,a CVT car equipped with a lock-up clutch is used as the study carrier,and the lock-up clutch slipping control strategy of torque converter based on the engine constant speed control is proposed.The control area of the lock-up clutch under the starting conditions is selected and the control algorithm satisfying the starting control target is developed.The lock-up clutch slipping control is used to intervene the starting process to achieve the doubleflow power transmission of hybrid and mechanical,apply partial load to the engine directly to improve the starting response and the transmission efficiency,moreover reduce the fuel consumption at starting conditions.The main research contents are as follows:(1)The torque transmission characteristics of TC and the dynamic characteristics of lockup clutch and CVT are analyzed theoretically.A CVT car dynamic model included a coupling power train model consist of TC and lock-up clutch is established.(2)The mathematical models of the hydraulic control system of the gearshift mechanism and lock-up clutch including each control valve are established based on the structure of the lock-up clutch control system.Moreover,a hydraulic control system simulation model is established,which lays the achieving foundation for the slipping control algorithm of the lockup clutch at starting conditions.(3)An average surface temperature calculation model of friction plates considering the heat convective transfer between solid and fluid surfaces and solid heat conduction is proposed based on the basic ways of friction heat transfer under certain assumptions.A temperature protection control logic and its control module are established based on the average temperature calculation model.(4)The speed range of the engine during the slipping control is determined according to the characteristics of engine torsional vibration.And the control range of slipping rate and power distribution ratio during the slipping control is determined based on the relationship not only between the two and the power train efficiency but also between the two and the torque fluctuation level.And the slipping control strategy at starting conditions is proposed based on the above.Moreover,the cooperative control of constant engine speed control and engaging pressure control of lock-up clutch is adapted and the target engine speeds are determined based on the characteristics of TC,which are used to make TC get to the coupling condition earlier and improve transmission efficiency.Different fuzzy control strategies are developed for the duty ratio control of the oil filling stage and the engine constant speed control stage.A fuzzy adaptive PID control strategy of lock-up clutch engagement pressure based on the target slipping rate is proposed for the lock-up clutch slipping control at starting conditions.The target slipping rate curves are determined based on the range of target jerks.And the lock-up velocities are determined according to the CVT variogram and the velocities of coupling point at pure hydraulic conditions.The slipping control algorithm is developed on the basis of the above.(5)The united simulation model of lock-up clutch slipping control at starting conditions is established.Comparative analysis of different accelerator pedal openings,different accelerator pedal opening rates and starting conditions of different vehicle mass are carried out based on the simulation model,moreover the friction plate temperature protection control logic is verified.The simulation results show that the proposed slipping control algorithm can accurately follow the target slipping rates,and ensure the smoothness of starting on the basis of improving the starting response and transmission efficiency.