Simulation And Experiment Of The Fluid Bulk Modulus Influence On Power Shift Of The Hydro-mechanical Variable Transmissions

Hydraulic mechanical variable transmission (HMT) is a kind of double power flowstransmission with characteristics of high efficiency and stepless speed variation, which iscompounded by hydraulic and mechanical transmissions. Power shift control is the key toimprove the HMT performance, there are several problems during the power shift,including: the hydraulic gear power flow reverses, the hydraulic components’ functionsinterconvert, original high and low pressures of the closed hydraulic circuit interchange,the original high pressure oil swells and the original low pressure oil is compressed. Theabove cause the fixed displacement hydraulic component’s speed fluctuation and powerinterruption, as well as along with vibration and noise.In order to solve problems of the power shift process, this paper carried out the studythat air content, load torque and volumetric efficiency affected the HMT power shiftdynamic characteristics. The main research contents included: First, it established modelsof oil density and bulk modulus; it designed respectively air content and bulk modulusmeasurement devices according to Heny’s low and bulk modulus definition, and carriedout hydraulic oil secant bulk modulus experiment research under different air contents.The results showed that secant bulk modulus model simulation results and experimentalresults were consistant, so the hydraulic oil secant bulk modulus model could be used forthe HMT power shift analysis. Second, the closed hydraulic circuit which consists ofvariable displacement and fixed displacement hydraulic components was equivalent to adouble-acting hydraulic cylinders system whose structure was simpler. This paperestablished the HMT closed hydraulic circuit power shift mathematical model, and builtits simulation model in MATLAB/Simulink, including variable displacement and fixeddisplacement hydraulic components equivalent models, vessel quality velocity andpressure models, load model. Third, it carried out the HMT closed hydraulic circuit powershift process simulation analysis, and studied influence laws that air content, load torquereverse time, volumetric efficiency and the hydraulic component’s displacement changeaffected the HMT power shift process. The simulation results showed: the more air in oil, the lower the fluid bulk modulus and the greater the fixed displacement hydrauliccomponent’s speed fluctuation during the power shift; controlling load torque reverse timeand the hydraulic component’s displacement change could effectively reduce the fixeddisplacement component speed and fluid pressure fluctuation; volumetric efficiency of theclosed hydraulic circuit could inhibit the fixed displacement hydraulic component speedand fluid pressure fluctuation.The above strategies all improved the power shift quality.