Study On Fully Variable Hydraulic Valve System Drived By Valve-Train Cam

Fully variable valve system (FVVS) can realize the continuous variability of the maximum valve lift, the valve opening duration and the valve timing. So there is an important significance to energy conservation and emission reduction of the engine. FVVS can control the number of the working medium in the cylinder by the way of EIVC or LIVC. So the throttle valve is cancelled and the the load of SI engine is controlled by FVVS. Owing to the totally elimination of the throttling action to the air inflow, this kind of unthrottled SI engine can greatly decrease the pumping loss, which makes the fuel consumption reduce 10-15% under small and medium-sized load. In addition, FVVS can increase the engine power by improving the volumetric efficiency and decrease the emission of the harmful gas by the internal EGR of the engine. FVVS has been one of the development directions of the new technology of the engine.The main formation types of FVVS are mechanical type, electromagnetic type and electro-hydraulic type. The electro-hydraulic type of FVVS has been taken into account because of the mobility of the liquid and the dirigibility of the valve movement rules. The electro-hydraulic types of FVVS which have a major influence are the EHFVVT system of Lotus and the UNIAIR system of Fiat. However, these systems also have the problems that the valve is seated strongly and the response speed of the solenoid valve is not so fast, which hinder the further development and the commodity production of the electro-hydraulic type of FVVS.In order to carry out the further research of FVVS, a kind of FVVS driven by valve-train cam is designed and developed. It is called SDFVVS for short in this paper. The intake valve is opened by the hydraulic valve actuator which is set between the camshaft and the intake valve, and it is closed by the reliever which could relieve the oil pressure of the hydraulic system in SDFVVS. The valve-seating velocity is controlled by the valve-seating buffer mechanism. Due to using the reliever as the switch controlled oil pressure in the hydraulic system, it not only reduces the cost of manufacture, but also overcomes the shortage that the frequency response characteristic of the solenoid valve is low, and it lays a good foundation of the further development of FVVS.According to the structural features of SDFVVS, the simulation model of the valve movement rules is established. The valve movement rules of SDFVVS are simulated. The valve lift and the hydraulic pressure of the SDFVVS are measured by using the displacement sensor and the pressure sensor. By the comparison of the results between the simulation and the measurement, the validity of the simulation is proved. The results of the simulation and the measurement show that SDFVVS can realize the continuous variable valve lift and EIVC by changing the time of relieving oil pressure, and the design goal is totally achieved.It is a difficult problem of FVVS that controlling the valve-seating velocity. In SDFVVS, valve-seating velocity is controlled by a variable throttle hole whose flow area is changed with valve lift. The effect of the shape, the minimal flow area and the flow area changing rate of throttle hole on valve-seating velocity is studied. Analysys on valve-seating velocity under different conditions shows that the effect of maximum valve lift on valve-seating velocity is significant, while the effect of engine speed is relatively small. Testing results also show that by reasonably matching the structural parameters, SDFVVS can give steadily seating.Improving the maximum speed of hydraulic valve train has the important meaning to the hydraulic valve actuation system. The maximum permissible speed of the hydraulic valve train is determined by the pressure fluctuation of hydraulic system. When the maximum permissible speed reaches or exceeds, the pressure fluctuation of hydraulic system will be intensified and minimum value of pressure decrease to below the pressure in low-pressure system. In the case, the hydraulic liquid can not drain in time, the characteristics of valve movement will be out of control, and the intake air volume can not be regulated by SDFVVS. The pressure fluctuation of fully variable valve train can be improved by decreasing the mass of move parts in valve train, increasing the stiffness of valve train, reducing the choking effect in the hydraulic system. Experimental results show that the pressure fluctuation of SDFVVS decreased in the newly designed valve-train, and SDFVVS can operate steadily and reliable up to 5600r/min.The valve-train cam which drives the movement of valve is the vibration source in SDFVVS. To improve the hydraulic pressure fluctuation and increase the maximum speed of the SDFVVS, the fundamental problem is to rationally design the valve-train cam profile. The simulation results show that the hydraulic pressure fluctuation can be decreased by using a polynomial cam that has the maximum acceleration at the beginning point and the maximum velocity which is relatively lower.The SDFVVS can realize variable maximum valve lift and the early intake valve closing by changing the time of relieving oil pressure, so that the SDFVVS can cancel the throttle valve and realize the load adjustment of SI engine. There is essential difference between unthrottled SI engine and TC engine. In order to simulate the intake performance, the one-dimension nonsteady air-flow model is established. The intake pressure in the intake pipe is measured by using intake air pressure sensor. Through comparing the simulation calculation results and the measured results, it is found that the simulation results are accurate to the measured results.Comparing the intake pressure measured in the intake pipe, it is shows that the intake pressure of the original engine is reducing quickly with the decrease of the intake air volume and the rise of the engine speed because of the throttle effect. The ratio of the pressure after the throttle valve to the pressure before the throttle valve is even less than the critical point. The gas flow in the throttle valve can be reached critical velocity. But in the unthrottled SI engine, the changes of the intake air volume even have no effect on the intake pressure. The intake pressure can be reduced by the rise of engine speed, but the reduced pressure range is small. So the intake pressure at the medium or small load in unthrottled SI engine is larger obviously than in TC engine. The results of the simulation for the intake flow show that unthrottled SI engine reduce the pumping loss at the medium or small load obviously, and which make the temperature of the working medium in the cylinder lower to some extent.In order to research the regulating action of the SDFVVS for the intake air inflow, the intake air volume of the unthrottled SI engine is measured by using an intake flow-meter. The research results show that the intake air inflow can be controlled by the meanings of early intake valve closing or late intake valve closing in SDFVVS. When using the meaning of early intake valve closing, the intake air volume rises with the rise of the engine speed in the same time of relieving oil pressure; for the meaning of late intake valve closing, the intake air volume reduces with the rise of the engine speed in the same time of relieving oil pressure. The SDFVVS can get the best valve timing in all speed range of engine. The volumetric efficiency of SDFVVS is bigger than that of the original engine in all speed range.