Study On Design Method Of Power-cycling Hydrodynamic Mechanical Transmission

Off-highway vehicles are generally equipped with hydrodynamic mechanical automatic transmissions to adapt to poor working environments and complex and variable operating conditions.The hydrodynamic mechanical(HM)transmission has the advantages of low-speed torque increase,stepless speed regulation and flexible transmission.In addition,the HM transmission could effectively enhance the vehicle’s operating capacity under low-speed and heavy-load conditions,and reduce the damage to each component caused by dynamic and impact load.However,its low efficiency restricts the improvement of the fuel economy and work efficiency of the vehicles.With the depletion of petroleum resources and the gradual transformation of our country’s economic development model from the labor-intensive to the high-tech,the performance disadvantage of HM transmissions has become more and more distinct.Therefore,it is urgent to find an alternative transmission solution with higher efficiency that retains the above advantages of the torque converter(TC).Because the power-cycling transmission has higher efficiency and larger stall torque ratio and its speed ratio can change continuously in the entire vehicle speed range,it can effectively improve the fuel economy and power performance of passenger cars.Combining the performance advantages of the torque converter and the power-cycling transmission,this thesis proposes a power-cycling transmission scheme based on the torque converter.In order to optimize both the fuel economy and power performance of vehicles equipped with the power-cycling hydrodynamic mechanical(PCHM)transmission,this thesis carries out research from five aspects: the configuration design,parameters optimization design,dynamic characteristics analysis,matching optimization and shift strategy design,and experimental verification.The main research contents are as follows:(1)In the PCHM transmission,the transmission ratio of the PCHM unit and the rear gearbox cooperate with each other to achieve the maximum speed ratio range required by the transmission in off-highway vehicles.Because the rear gearbox is used to expand the speed ratio range of the PCHM unit,the key to the design of the PCHM transmission configuration is the determination of the number of the front gearbox speeds.Furthermore,how the speed ratio of the PCHM unit changes when the front gearbox shifts is an important basis for determining the number of the front gearbox speeds.According to such an idea,the change in the speed ratio of the PCHM unit is analyzed during the shift of the front gearbox.It is concluded that the speed ratio range where the PCHM unit can effectively work is extremely narrow after the front gearbox is upshifted,and the speed ratio of the PCHM unit decreases sharply after the front gearbox is downshifted,inducing a rapid increase in engine speed.Based on this,the configuration of the PCHM transmission is successfully determined.Finally,the performances of the PCHM transmission and all the 24 basic configurations of the hydrodynamic mechanical power-split transmission are compared carefully.It is found that the former transmission has higher efficiency in part of the entire speed ratio range and a larger stall torque ratio than the latter transmissions,which proves the optimality of the designed PCHM transmission configuration.(2)For the parameter design problem of the PCHM transmission after its configuration is determined,the parameter optimization model of the transmission is established by considering the typical working behavior of an off-highway vehicle(taking the wheel loader as an example)and analyzing the transmission characteristics of the PCHM transmission.In the parameter optimization model,the objective functions are the average efficiency of the transmission over the entire vehicle speed range and the maximum tractive force of the vehicle.And the design variable vector contains 7components such as the number of transmission gears(the rear gearbox),the exit angle of the pump blades and the diameter of the torque converter.The reasons for the increase or decrease of the average efficiency and maximum tractive force in the Pareto optimal solutions are revealed under different transmission gears.The influence of the number of transmission gears on the average efficiency and maximum tractive force is analyzed.The efficiency and maximum tractive force for the PCHM transmission and those for the traditional HM transmission are compared in detail.It is concluded that by using the PCHM transmission scheme after optimization,the average efficiency and maximum torque multiplication capacity of the transmission in the wheel loader can be increased by 3% and 6.6%,respectively.(3)A dynamic model with respect to the efficiency and torque ratio of the PCHM transmission is established.In the model,the friction losses and shock losses of the flow field in the torque converter and the bearing friction losses of the planetary gear train(PGT/pgt)affected by the output speed and torque of the transmission are considered.Based on the typical operating conditions of the wheel loader,the orthogonal test method is used to identify two significant factors that influence the instantaneous efficiency and instantaneous torque ratio of the PCHM transmission.The dynamic characteristics of the transmission are studied for different torque converter diameters and speed ratios of the fixed ratio mechanism.Both change laws of the efficiency difference and torque ratio difference between the steady and dynamic conditions are obtained under typical operating conditions,which provides a theoretical guide for improving the efficiency and torque ratio of the transmission under unsteady conditions.(4)The differences in the power flow forms and in the arrangements of the torque converter between the PCHM transmission and traditional HM transmission make its capacity characteristic curve more divergent than the latter transmission;this will lead to a decline in both the efficiency and output power of the engine connected with the transmission.Based on the Pareto optimal solutions for a transmission gears number of5,the torque converter with low penetration is used as the speed regulating element,and the matching optimization design of the transmission and the engine is carried out under the full power matching and partial power matching modes.As a result,the fuel consumption rate of the engine is effectively reduced and the output power of the transmission is improved significantly.In view of the changes in the working state of the engine and in the efficiency of the transmission after shifting,the difference in vehicle driving power between before and after the shift is minimized and the the difference in instantaneous fuel consumption between before and after the shift is maximized,in order to design the dynamic shift strategy and economical shift strategy of the PCHM transmission.Compared with the traditional optimal shift strategy that does not consider the efficiency of the transmission,alghtough the optimal shift strategy that takes into account the transmission efficiency fails to ensure that the engine works at the maximum power point and the highest efficiency point after shifting,they reduce the vehicle’s maximum acceleration time and fuel consumption by 7.7% and 3.6%,respectively.The above research has enriched the design theory system of the PCHM transmission,and provided theoretical support for its engineering application and the development of automatic shifting strategies.(5)A test bench for the PCHM transmission is built.The input torque,input speed,output torque and output speed of the transmission are collected under both steady and dynamic conditions.Based on these data,the basic characteristics data of the transmission are calculated.The theoretical results of the design method of the PCHM transmission are verified through comparison with experimental results.