| As a large agricultural country,China has a huge number of agricultural vehicles.Traditional stepped shift tractors cause serious energy waste and exhaust emissions.Tractors equipped with CVT can improve traction characteristics,power performance and fuel economy.The electromechanical composite CVT has the advantages of high flexibility and transmission efficiency,and has a wide application prospect.Therefore,the application of hybrid power technology to tractors has great engineering application value.Based on the analysis of structure,working principle,working mode and advantages and disadvantages of the hybrid power system,and combined with the actual working conditions of tractors,an electromechanical hybrid stepless transmission scheme is proposed for the purpose of continuously variable transmission.The working mode of transmission scheme is analyzed.Based on the selection of transmission scheme and the requirement of tractor working mode,the engine,electric/generator and battery are selected and designed,and the parameters of CVT system are optimized by genetic algorithm.The transmission principle of transmission scheme is analyzed.The speed ratio,torque characteristic,power dividing ratio characteristic and efficiency characteristic are calculated theoretically.The relationship curves of speed ratio,torque,shunt ratio of electric power and efficiency characteristics are drawn by using MATLAB software.The efficiency of electromechanical composite CVT is compared with that of hydraulic mechanical CVT.The research shows that the transmission scheme designed in this paper has a wide range of variable speed.The transmission efficiency is higher than 80% under different mixing modes.The maximum efficiency can reach 95%.Compared with HMCVT transmission,the efficiency of the transmission has been improved more than 10%.There is little difference between the efficiency before and after the change of the CVT section,and it has more advantages in solving the stability of the tractor’s change section.The characteristics of engine and electric/generator are analyzed,and the models of engine speed regulation,fuel consumption and effective power characteristics are established.According to the above model,the universal characteristic curve of the engine is drawn.The external characteristic and efficiency model of the motor isestablished,and the efficiency map of the electric/generator is drawn.Aiming at the different working modes of the designed hybrid power system,the matching of engine,electric/generator and transmission system is studied,and the matching methods and ways are put forward.Tractive force characteristic of tractor with stepped transmission and equipped with electric continuously variable transmission are compared.According to the optimal power matching of ploughing operation,it is concluded that the power performance of the tractor equipped with electric continuously variable transmission is better than that of the traditional tractor,and the good matching among engine,electric/generator and transmission system can be achieved. |
