| High-power tractors often work in the complex environment,and the working conditions are complex and changeable.In order to adapt to the various conditions of load and speed requirements,the transmission of tractor needs to set more working gears,so that the transmission have complex structure,difficult operation and the economy can not be guaranteed.Hydro-mechanical CVT(HMCVT)is a new type transmission device with the parallel form of hydraulicspeed regulation and mechanical speed,combined with the characteristics of hydraulic stepless speed and mechanical transmission efficiency,full speed can be adjusted continuously,it has good maneuverability,especially suitable for high-power tractor in complex operation.The transmission efficiency is an important parameter to measure the HMCVT.Its level is affected by the complex mechanism of the gearbox.To design an energy-efficient HMCVT,it is necessary to clarify the role of different parameters in the transmission efficiency and establish the system's efficiency characteristic map.At present,there are relatively few related specific researches conducted at home and abroad,therefore,it is significant to conduct in-depth research on the efficiency characteristics of High-power Tractor HMCVT.Under the support of the Jiangsu Natural Science Foundation(BK20140727)and the Jiangsu science and technology support program(BE2014134),this paper designed the transmission scheme of HMCVT aiming at the working characteristics of the high-power tractors,and carried out simulation study on its efficiency characteristics.The main research contents are as follows:1.According to the tractor's driving speed and working environment requirements,the working position of the transmission is determined,and designed the transmission scheme of HMCVT.And the total transmission ratio and each gear ratio were calculated based on the transmission scheme.The hydraulic circuit was designed,and the hydraulic components were selected according to the transmission output power.The speed and torque characteristics of the designed scheme were analyzed,the power cycle mechanism was analyzed emphatically,and the theoretical model of hydraulic power split ratio was established.2.It clarified the influence mechanism of different parameters on the transmission efficiency of the HMCVT,and under the load and the non load,analyzed the power loss of friction and oil stirring of the gear and bearing.On the hydraulic part,based on the mathematical model,the influence of the hydraulic system on the eficiency of the gearbox was emphatically analyzed.The results show that:the transmission efficiency of the hydraulic system increased with the increase of the displacement ratio,;the greater the pressure difference of the hydraulic pump motor inlet and outlet,the higher the transmission efficiency of the hydraulic system;the lower the transmission efficiency of the hydraulic system with the increase of the hydraulic viscosity;the increase of the input speed of the pump shaft led to the decrease of the transmission efficiency of the hydraulic system.3.Based on the SimulationX platform,the efficiency simulation model of the HMCVT was established by modifying the simple mathematical model of the hydraulic system,the planetary gear train and the fixed axis wheel system.The test bench was built,the bench hardware and the measurement and control system were designed.The viscous damping coefficient of the motor shaft fm was calibrated to 0.16 by the pump motor inlet and outlet pressure difference test.The combined coefficient Cs(Vpmax+Vm)/? is 3.5 × 10-12 through the output speed test of the transmission.The viscous damping coefficient of the pump shaft fp calibrated by the efficiency test result is 0.12.4.The pump control motor oil compensation loop model and the transmission model were established.Then,the simulation of the speed of the hydraulic motor and the running speed of the whole machine were carried out respectively,and the simulation results were basically consistent with the theoretical calculation.The transmission efficiency and control signal were interpolated to the transmission output end as components,then,the efficiency model of the "engine-gearbox" system is established based with the engine model.5.Based on the efficiency model of the "engine-gearbox" system,the efficiency characteristic of the full load condition was researched under the three operating conditions established by the engine characteristic data.The results show that:the transmission efficiency of the transmission is proportional to the load torque,and is inversely proportional to the rotational speed;the transmission efficiency is periodically changed with the adjustment of the gear position,the greater the absolute value of the displacement ratio,the lower the transmission efficiency;Under the influence of the circulating power,the HM1 and HM3 section of the three operating conditions,the transmission efficiency of the displacement ratio at the positive time is basically lower than that of the negative displacement ratio,while in the HM2,it is the opposite,which is in accordant with the theoretical calculation of the circulating power.6.The characteristic of the partial load condition was researched.The research shows that:under constant speed ratio,the transmission efficiency of gearbox is directly proportional to the load torque,and is inversely proportional to the speed;at constant speed,the transmission efficiency of the gearbox is different with the different combination of the engine speed and the speed of the transmission;the isoline of transmission efficiency shows the trend of U in the same period with the change of engine speed,the higher the position of the gearbox,the higher the transmission efficiency.7.Through the analysis of the effect of the transmission ratio of the front gear transmission to the pump,it was adjusted to 1,and the efficiency of the transmission scheme was optimized.The research shows that:under the full load condition,the new design scheme is better than the original design,and the maximum transmission efficiency is increased to 85.3%from 80.1%of the original design,the influence of engine speed has been greatly reduced.The increase of ip reduced the speed of pump motor shaft,and reduced the loss of hydraulic system caused by viscous damping.In conclusion,this paper designs a scheme for the transmission system of the tractor HMCVT,studies the efficiency characteristics of the HMCVT,and provides a theoretical basis for the optimal design of the transmission scheme and the practical application of the transmission on the tractor. |
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