| The continuous variability of maximum valve lift,opening duration and valvetiming can be attained by using fully hydraulic variable valve system(FHVVS)in gasoline engine,which can replace the traditional throttle and realize the adjustment of the air intake continuously.The FHVVS has the advantages of energy conservation and emissions reduction for the engine,which can reduce pumping losses and the fuel consumption by 10-15%at small and medium load.Fully variable valve technology has become one of the important development trends of the new technologies of internal combustion engine.The dynamic characteristics of the valve train have important influence on improving power and reliability of the engine,improving the fuel economy and reducing the vibration and noise of the valve.In this paper,the simulation model and dynamic characteristics of the fully variable hydraulic valve system have been carried on,and the main work can be concluded as four aspects as follows:(1)The structure and working principle of FHVVS system were introduced.FHVVS system adopted the driving mode of "cam-hydraulic tappet-hydraulic piston-valve".The hydraulic oil was compressed by the cam driving mechanism to generate high pressure to open the valve.The drain time of the system was controlled using a mechanical control valve.By taking a seat cushion institutions,the valve was seated smoothly.(2)Based on AMESim software,the simulation model of the FHVVS system was built.The simulation model of the FHVVS system could be divided into cam driven system,hydraulic transmission system,valve components,and control system.In this paper,the simulation model was used to simulate the working process of the FHVVS system.(3)The test bench was built to measure valve lift and pressure fluctuations within hydraulic tappet chamber of the FHVVS system.An electric motor which was able to simulate different speed of the engine was selected to drive the cam shaft,which could simulate the working status of the engine valve train.The appropriate temperature and pressure of the hydraulic oil were supplied by hydraulic pumps,to simulate lubrication system of the valve train.The laser displacement sensors and the piezoelectric pressure sensors were selected to measure the valve lift and hydraulic tappet cavity pressure signal.(4)Using test results,the dynamic characteristics of the FHVVS system were analyzed mainly from two aspects of the valve motion law and the pressure fluctuation of the hydraulic transmission system.Analysis results show that the FHVVS system can vary the maximum valve lift,valve opening duration continuously,achieving the goal of the abolition of the traditional throttle.The FHWS system can achieve the continuous adjustment of the intake.With the increase of valve lift,the hydraulic system presents a periodic fluctuation,and the frequency of the fluctuation is not affected by the speed of the engine and the phase angle of the oil discharge.The lowest pressure is not lower than that of the low pressure system,the system does not appear the phenomenon of "out of control" at high speed of engine 5000r/min.(5)The deviation between the simulation results and the test results were analyzed.The simulation results are proved to have guiding significance.Based on the simulation model and test results,the dynamic performance of the FHVVS system was analyzed.Results show that when the engine operates at high speed,the valves do not appear the phenomenon of "distortion",meanwhile the velocity and acceleration curves of the valve do not exceed the theory of maximum allowable values,which can meet the needs of high-speed running of the gasoline engine and good dynamic characteristics.Factors affecting the valve seating speed of the FHVVS system were analyzed.By reasonably reducing the throttle area of the seat cushion mechanism and increasing the one-way valve in the valve opening stage,the system can reduce the seated speed of valve and pressure fluctuations of the hydraulic piston chamber.The effects of two kinds of different valve drainage structure on the dynamic performance of the system are analyzed.Compared with the mechanical control valve of long hole,mechanical control valve with circular hole shows decreased seating speed,increased valve opening duration and the maximum lift under the same discharge angle.The factors affecting the internal pressure fluctuation of FHWS system were analyzed.By increasing the system stiffness,reducing the quality of moving parts,and with reasonable valve spring,the FHVVS system can reduce the hydraulic system pressure fluctuation,enhance engine allowable speed and optimize the dynamic performance of the system. |
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