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To analyze the stage changing quality of small and medium-sized tractors equipped with hydro-mechanical continuously variable transmission (HMCVT) in the process of power changing, based on a HMCVT with continuously variable characteristic parameters, the traditional gear planetary transmission structure was changed into the traction planetary transmission structure, and a multi-stage HMCVT system was proposed. Considering the influence of changing time and displacement ratio of the hydraulic system on the stage changing quality, AMESim modeling and simulation analysis of the multi-stage HMCVT system were carried out by adding a variable concerning which the characteristic parameters of the traction planetary array were continuously changing. When the change period t0=0.8 s was set in the simulation, multi-parameter adjustment analysis was carried out on the three evaluation indexes of the change period quality, such as velocity drop, impact degree and sliding power, in the characteristic parameter k 1.5-2. The design of the three-stage HMCVT system can reduce the maximum sliding power by 25.6% or the maximum impact degree by 20.84% by changing the value of the characteristic parameter k in the L and H stages. By changing the value of the characteristic parameter k in the S and L sections, the sliding work can be reduced by 6.68%, but the impact of changing the characteristic parameter k on the impact degree changes within 1%. The overall velocity drop will increase with the increase of the characteristic parameter k. The research results can provide reference for further improving the stage changing quality.

Hydro- mechanical continuously variable transmission is a kind of power- split stepless transmission device,which realizes the stepless speed regulation by hydraulic regulating speed mechanism and the higher efficient by mechanical variable speed mechanism. The five simple and reliable stepless transmission device is designed autonomously,through the different combinations of hydraulic regulating speed mechanism and mechanical variable speed mechanism,the switch of hydraulic transmission,hydro- mechanical transmission and mechanical transmission is carried out to meet the requirements of smooth start,field operation and transportation,the power performance and fuel economy performance of diesel engine are given full play,and the transmission performance and efficiency of transmission are improved greatly. In order to reach the goal of energy conservation and emissions reduction,a simple introduction of energy storage device is given.

A hydro-mechanical continuously variable transmission（HMCVT） with a single planetary gear is introduced,and the power transfer of the transmission is analyzed.The efficiency characteristic of transmission is researched from different angles through the definition method and the meshing power method,and the main factors which influence the efficiency of transmission are acquired,the theoretical foundation for the optimization of design scheme and increasement of the transmission efficiency is established.A test bench of hydraulic system is designed,and the main factors which influence the efficiency of hydraulic system are analyzed.A test bench of HMCVT is established,the stepless regulation speed characteristic curves and efficiency characteristic curves are acquired,and the test results are analyzed.