Wun, G., Chuang, C., Syu, Y., Wang, C. et al., "Development of Hydraulic- Controlled Variable Valve Lift System for Scooter Engine," SAE Int. J. Engines 9(4):2493-2497, 2016, doi:10.4271/2016-32-0095.
In this research, with proposed design of hydraulic-controlled variable valve lift system, a feed pump is used to supply the low-pressure oil into the hydraulic pump which will compress low-pressure oil as high-pressure oil and pumps the high-pressure oil into the hydraulic cylinder. In the meanwhile, the intake valve opened in accordance with the plunger is pushed by the hydraulic cylinder. To achieve the change of intake valve lift, a DC motor connected with the control rack that is used for controlling oil-drain time of the hydraulic pump. In addition, a solenoid valve is installed in order that the intake valve will be closed quickly without controlling the drain flow when the maximum valve lift is reached. Commonly, the throttle is not required with variable valve lift system, but the throttle is kept for engine start. Hence, except for engine cranking condition, the throttle maintains in wide open (WOT) under different test conditions. After realizing the proposed variable valve lift system, the valve profile is measured with the established bench at the beginning, and the engine experiment is executed. The experimental results show that the proposed design of hydraulic-controlled variable valve lift system can approximate the original valve operation. Furthermore, the improvement of pumping loss reaches up to 12.18%∼20.75%. Finally, the chassis dynamometer experiment results show that the improvement of fuel economy reaches up about 2.96% ∼ 8.35% running on the ECE-40 driving pattern.