Simulation and experiment on efficiency characteristics of hydraulic mechanical continuously variable transmission for tractor

Abstract: In order to estimate the efficiency level of a new kind of tractor hydraulic mechanical CVT, which would provide a theoretical reference for its structural optimization and power matching, the transmission efficiency of the CVT under partial and full load working condition were studied by the methods of both simulation and experimental calibration. First, a complete CVT transmission model was built, based on the software of SimulationX, and part of the model was modified by use of the components under a signal library so as to solve the direction problem of the power flow correctly. In order to ensure the reliability of the model, eight groups of experiments were made to calibrate some of key parameters of the model, such as viscous damping and leak coefficient of the hydraulic system. The results showed that the transmission efficiency level between the simulation model and the actual physical system were enough consistent to meet the research needs after parameter calibration, and that the maximum efficiency error is about 2.4%. Secondly, by a simulation analysis of the transmission efficiency of the CVT under full load working condition, the relationship among the transmission efficiency, the engine speed, and the load torque were studied. Furthermore, a map of transmission efficiency in partial load work condition was drawn also. Finally, this paper points out that it is important that the transmission ratio of gear pair g4 before pump shaft affects the transmission efficiency. The higher the transmission ratio of the gear pair, the higher the transmission efficiency will be. But a higher transmission ratio of gear pair g4 would affect both the transmission ratio distribution of each gear and the load level of motor shaft, making the hydraulic system design and component selection difficult. The conclusion in this paper shows that the transmission efficiency of the CVT is in inverse proportion to the rotational speed of the engine and in direct proportion to the torque of the engine, and the maximum transmission efficiency under the rated work condition is 81.5%; The transmission efficiency would be reduced under a negative pump displacement in the stage of HM2 and HM4 as well as positive pump displacement in stage of HM1, HM3 because of the power cycle. In summary, the transmission efficiency of the CVT would fluctuate widely because of the complex working conditions; therefore, the relationship between the speed ratio of CVT and the rotational speed of engine must be reasonably matched in order to raise the transmission efficiency.