| Performance,fuel economy and emissions of internal combustion engine(ICE)are required to be strict increasingly under the pressure of environmental degradation.Air hybrid,which is derived from the traditional ICE,is used to realize regenerative braking for road vehicles.Thus,braking energy is recycled effectively so as to achieve the goal of reducing energy consumption.Unfortunately,gas exchange process and their switches among different operating modes for air hybrid are difficult to be implemented if traditional valvetrain with cam is used alone.Hower,fully flexible variable valve actuation system with flexible control of valve timing,variable valve opening duration and continuous variable valve lift can solve these problems.Therefore,development of fully flexible variable valve actuation system has important and practical significance.Fully variable electric-hydraulic valve actuation system(FEVVA)with flexible adjustment to valve motion has much practical significance,but its development and application in reality appear large resistance as follows: 1)Solenoid valve commonly used in FEVVA system can not meet the requirement of high frequency operation of the engine because of low response rate,2)A big impact between valve face and valve seat insert reduces the service life of the valve and the related parts,3)Its high energy consumption can not been avoided,and 4)Precise and robust valve movement can not be guaranteed due to back pressure of valve,system leakage and instability of hydraulic oil bring about valve opening delay and valve motion disorder.Therefore,this paper proposed a kind of fully flexible electro-hydraulic valve actuation system and dedicated to solving several key problems as follows: 1)Development of flow control rotary valve synchronized with the engine crankshaft helps to effectively avoid reliance on electromagnetic valve and extends the application scope on high speed engine;2)Based on the buffer mechanism of the variable orifice,some valve cushions are proposed so as to control seating speed of valve and further decrease its impact;3)For purpose of reducing the system energy consumption,some key parameters of FEVVA system are optimized using the best optimization methods;4)In order to ensure the accuracy and robustness of the valve movement,variable valve timing and lift control strategy are proposed after the effects of back pressure of valve,system leakage and instability of hydraulic oil on valve movement are researched and controllability of variable valve timing and continuous variable valve lift are discussed.The thesis research results present that:1)Experiments show that duplex compression braking strategy is better than the single tank compression brake strategy in terms of energy recovery efficiency.Studies based on GT-POWER show that the maximum pressure of low pressure tank decreases with the increase of speed in 100 cycles.Besides,the pressure of high pressure tank at the end of the cycle decreases with the increase of speed in 100 cycles.2)After key components of FEVVA system has been designed,the numerical model is established based on MATLAB/Simulink platform and its results show that: a)the value of VED can be controlled effectively by high and low pressure rotary valve opening time;the phase difference angle of rotary valve,which has no influence on characteristics of opening section of lift and the MVL value,can change VED so as to delay the valve closing time;b)the MVL can be adjusted by changing supply pressure without variation of valve closing time and opening duration;c)the leakage of flow control rotary valve break the state when valve lift keep maximum and lead to a second valve open.As temperature of oil increase,valve opening and closing time are in advance,the MVL increases and the speed of valve seat has a tendency to increase.Intake process has little effect on valve movement while exhaust process has a great influence on it.Back pressure of exhaust valve lead to its opening and closing time lag and reduces the MVL;d)Not only does engine speed affect the characteristic of valve openning section and closing section,but also it changes valve closing time.With the increase of engine speed,MVL has a tendency to decrease,and the valve closing time delays.Besides,the value of VED increases,but sectional area of smaller valve lift emerged.3)Three different types of buffer instruments are presented to control valve seating velocity.A dynamics evaluation mechanism of FEVVA system is established.Then the results show that the diameter of the hydraulic cylinder has a greater impact on the valve seating velocity;the spring stiffness has little effect on the valve seating velocity.With the valve spring preload increasing,the hydraulic piston opening resistance increases and thus acceleration reduces during the valve opening.Consequently,the valve speed is reduced.As a whole,the engine speed is not sensitive to valve seating velocity.4)Energy Consumption Index(ECI)to evaluate system energy consumption is proposed.The results show that the ECI of case B is smaller than case A;the ECI of case C is the least by comparison to the other two cases.Optimization parameters including pretension force of spring,spring stiffness,radius of the flow control rotary valve shaft,diameter of hydraulic cylinder,initial volume and initial pressure in accumulator have been determined using disturbance method of single factor which evaluates effect of system parameters on ECI.The optimization results show that power for valve reduces by 26.5% at speed of 2000r/min.5)The experimental results of FEVVA system show that it can realize flexible adjustment of the variable valve timing,VED and variable valve lift.For 60 cycles,the cycle differences of the valve opening time maintain within ±5 ° CA,and the cycle differences of the value of VED keep within ±5 ° CA.Besides,the cycle differences of the MVL maintain within ±0.3 mm.What is more,valve seating velocity keeps within allowed range and meets the requirement of the system.From above research results,FEVVA system can adjust valve movement flexibly.Not only does it provide substantial technical support for the gas exchange process of air hybrid,but also it lays the foundation for practical application and industrialization of FEVVA system.In addition,the research results reveal and analyse the effect of system parameters and the disturbance factors on performance of the FEVVA system.Based on these research results,general rules for electro-hydraulic system are concluded to provide theoretical supports for the design and development of fully flexible electro-hydraulic variable valve actuation system of the same type. |
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