Thermal And Valve Working Accuracy Analysis On A New Hydraulic Variable Valve Timing System

Many automobile producers place greater demands on the engine due to increasingly stringent emission regulations, they require some engines that with low energy, low consumption, low emission and high power. Variable valve technology is an effective way in many engine technologies being used that can save energy and reduct emissions, and the hydraulic valve system is one of these valve technologies. This paper regards the valve working precision as the main line based on the developed hydraulic variable valve system, the heat production and the oil temperature change of the system are analyzed, and then the effect of the oil temperature on the accuracy of the valve are studied. In addition, some relevant solutions are put forward based on these influences. These specific studies are as follows:To study the change of hydraulic oil temperature, a thermodynamic model for the system is built using the simulation software AMESim in this paper based on the hydraulic variable valve system. Then, the simulation result is compared with the bench test data and showes that the simulation error within 5% and the match degree is high, such as the tank oil temperature, lift curve, and the model can accurately reflect the oil temperature change during the system operation. In addition, the main produce heat and dissipate heat are slso studied and their thermal power are calculated. The influence of engine speed to the final equilibrium temperature of the hydraulic system is also analyzed, and then the oil temperature field is also determined.In order to study the temperature of hydraulic oil how to affect the accuracy of valve working. Firstly, the leakage of the system is calculated by the simulation model under different viscosity. Then, the influence of the hydraulic oil viscosity on the valve opening time, the valve closing time and the maximum lift are analyzed. Finally, the feasibility studies of improving the accuracy of the hydraulic valve timing and lift is studied, and two controllers of PID and sliding mode control are proposed.The results show that some delay caused by the lage viscosity of hydraulic oil, a PID controller is designed to control the valve timing. The error of the valve opening time is only 1 CA?, and the error of the valve closing time is only 2 CA? after the system is controlled, so the engine valve timing can be adopted precisely. To solve the effect on the lacking of valve maximum lift, a sliding mode controller is adopted to adjust the pressure of accumulator and pipe flow through needle stem according to the actual lift and set point, the result show that the valve lift control accuracy can be improved about 5.8%~11.2% when the engine speed during 500 ~ 1 500 r min. Anyway, the proposed controller can eventually reduce the impact of the oil temperature change and some external disturbance to the hydra ulic valve system.