Abstract: The influence rules about fluid air content affecting dynamic behavior of the hydro-mechanical variable transmission during power shift is researched. The theoretical analysis about oil bulk modulus is carried out, and the experimental research of oil bulk modulus with different air content is done. The theoretical curves of the fluid secant bulk modulus are in accord with the experimental results, which shows its mathematical model can be used in power shift simulation analysis of the hydro-mechanical variable transmission closed hydraulic circuit. The power shift mathematical model of the closed hydraulic circuit in hydro- mechanical variable transmission is built with the method that the closed hydraulic circuit is equivalent to a loop consisting of two double-acting hydraulic cylinders, and it is simulated in Matlab/Simulink. The results show that the more air in fluid, the lower bulk modulus and the greater speed fluctuation of the fixed displacement hydraulic component during the power shift, so the power shift quality is worse; control of the hydraulic component’s displacement and extension of load torque reverse time can effectively reduce the fixed displacement component speed and pressure fluctuation of the closed hydraulic circuit.

Key words: bulk modulus, fluid air content, hydro-mechanical, infinitely variable transmission