Speed changing law of hydro-mechanical CVT based on maximum efficiency of tractors

Abstract: To realize the optimal economic control of a tractor with the multi-range hydro-mechanical continuously variable transmission (HMCVT) under any steady running speed and allowable traction load, the speed changing law of the best economy for tractors was studied in this paper. In the conventional economy research of tractors equipped with multi-range hydro-mechanical CVT, to realize the best economy of the tractor and the optimal matching of the transmission with the engine, the main method is that the engine is kept on the optimal economic work range by adjusting the throttle of engine and the transmission ratio of multi-range hydro-mechanical CVT. This method lacks considering the efficiency influence of multi-range hydro-mechanical CVT. Really, the efficiency of multi-range hydro-mechanical CVT has large difference in different transmission ratio. This feature influences the economy and efficiency of tractors. It was considered in this paper that the economy of tractor was decreased due to the losing of power passing through the transmission when the power is transmitted from engine to load by transmission. If the total efficiency of tractors was largest, the power losing in the transmission train was least on finishing a unit of production task. The best economy of tractors could be denoted by the maximum efficiency of tractors. According to the energy transmission characteristics of tractors, considering the difference of efficiency of multi-range hydro-mechanical CVT under different transmission ratio, the study of the best economy for tractors was transformed into the research of maximum efficiency for tractors. It was presented that the speed change law of multi-range hydro-mechanical CVT based on the maximum efficiency of tractors. Based on the analysis of the efficiency characteristics of the whole transmission system, the total vehicle efficiency was optimized for the objective function of maximum efficiency and constraint conditions. Using the circularly optimizing method of 4 parameters which were target tractor speed, traction load, engine speed and engine torque, the optimal transmission ratio, optimal engine speed and torque were obtained on the working field of tractors. The result showed that the optimized tractor efficiency were between 35% and 40% under any target vehicle speed and traction load. The optimal values of transmission ratio of multi-range hydro-mechanical CVT, engine speed and torque were obtained when the vehicle efficiency was biggest. The speed changing law of the multi-range hydro-mechanical CVT was determined based on the maximum vehicle efficiency. The research showed that the best economy of tractors under any speed and traction load could be achieved based on the rule of maximizing the efficiency of whole vehicle. The speed changing law of the multi-range hydro-mechanical CVT for best economy of tractors, which was established based on the maximization of vehicle efficiency, laid a foundation for making the best economic control strategy of tractors with the multi-range hydro-mechanical CVT.