Abstract: Reducing the pumping loss of gasoline engine at part load needs flexible variable valve mechanism. Cam-driven hydraulic variable valve has good engineering application prospects, but it also faces the problems of pressure fluctuation and difficult control of valve seating velocity (VSV). In this paper, the valve lift of 0~4 800 r·min^-1 is continuously changed in the range of 0~8.2 mm by adjusting the throttle valve opening (TVO). The influence of piston diameter on pressure fluctuation and throttle aperture on valve seating velocity is simulated, with the piston diameter and throttle aperture confirmed, and the effect of hydraulic oil temperature on valve motion characteristics and VSV are studied experimentally. The results show that an appropriate increase of piston diameter can reduce the system working pressure and the compression wave peak , which helps reduce pressure fluctuation, and finally the chosen diameter of tappet and valve piston is 17 and 14.5 mm respectively. The throttle aperture of less than 1.6 mm can make valve seating velocity of 4 000 r·min^-1 less than 0.5 m·s^-1. The maximum valve lift decreases gradually with the increase of throttle valve opening under the same speed; with the same TVO, the higher the motored speed, the larger the valve maximum lift and the greater the TVO, the larger the difference of maximum lift at different speeds. Hydraulic oil temperature has little effect on the lift with the throttle valve closed; under the same TVO, the valve chamber pressure and maximum valve lift decrease gradually with the increase of hydraulic oil temperature. The valve seating velocity is insensitive to hydraulic oil temperature, with a variance of only 4.9% at different temperatures

Key words: hydraulic variable valve mechanism, valve motion characteristics, pressure fluctuation, valve seating velocity, hydraulic oil temperature