Design And Application Study On A Cam Driven Hydraulic Variable Valve Mechanism For Gasoline Engine

Increasingly stringent fuel consumption and emission regulations have brought unprecedented challenges to vehicle gasoline engine,and their energy conservation and emission reduction are imperative.The Miller/Atkinson cycle has the characteristic of expansion ratio?ER?greater than compression ratio?CR?.It can not only reduce the pump loss of part load and increase the thermal efficiency,but also reduce the effective CR and in-cylinder temperature at the valve closing time and thus reduce the detonation tendency and NOx emission which is fit for the high geometric CR,high turbocharged,downsized GDI engine.Therefore,it is necessary to study intensively.The Miller/Atkinson cycle is mainly realized by the increasingly flexible variable valve actuation?VVA?which includes the type of mechanical,electro-hydraulic,electromagnetic and pneumatic.Among so many VVA mechanisms,the research on mechanical and electro-hydraulic type is the most extensive.The mechanical type has high reliability and control precision,but the adjustment degree of freedom is low.The electro-hydraulic type can realize the free adjustment of single parameter and have better engineering application value,but it is not efficient and accurate,the seating velocity is hard to control and the solenoid valve response frequency restricts the maximum engine speed and so on,which limits its further development.In view of the above problems,a cam driven hydraulic variable valve mechanism and one-way throttled soft seating mechanism had been designed.The cam was instead of solenoid valve to increase permissible speed,the valve lift?VL?and closing time?CT?can be continuously adjusted by changing the setting pressure?SP?of direct relief valve or the throttle valve opening?TVO?,the one-way throttled soft seating mechanism is in charge of reducing the valve seating velocity?VSV?.The key configuration parameters of the system had been confirmed using one-dimension SimHydraulic simulation platform of the mechanism.This paper puts forward the evaluation index of pressure fluctuation and valve movement cyclic variation which are closely related to mechanism reliability and combustion variation.The valve movement characteristics,seatting velocity,system hydraulic fluctuation and cyclic variation characteristics of valve movement are studied experimentally and the reduction measure of valve movement cyclic variation is given.The energy saving potential of early intake valve closing?EIVC?load control method?LCM?of gasoline engine was explored by comparing with original LCM,and the influence law of different intake valve control strategies on in-cylinder airflow was studied by AVL-Fire.The one-dimension simulation model of the designed mechanism was established by using the SimHydraulic simulation module of Simulink,which proved the mechanism feasibility to realize EIVC and the key parameters of the mechanism had been comfirmed.The study results showed that the properly increased plunger diameter could decline the highest working pressure of the system and then the hydraulic fluctuation.The smaller diameter of connecting pipe caused the greater pressure loss between the tappet and valve cavity,and the larger diameter reduced the overall system stiffness.Finally,the connecting pipe diameter of 6.0mm was selected,and the tappet and valve piston diameter were 17 and 14.5mm respectively.The diameter changing shape and direction changing shape of the pipeline had some effects on the pressure fluctuation and the smooth transition could appropriately reduce the system pressure fluctuation.Reducing the diameter of the thin-wall orifice could reduce the VSV.When its overflow area was 2.0 mm²,the VSV of 4000r/min was less than 0.5m/s,and the corresponding diameter and thickness were 1.6and 0.6mm respectively.The orifice overflow area of the selected throttle valve was linearly related to its opening,VL and CT were gradually decreased with the increased TVO.The core displacement and outlet flow of the direct-acting relief valve had an oscillation process of about40ms at the beginning of the pressure adjustment and then gradually stabilized.The process would increase the system pressure shock which exceeded the required time for valve movement at high speed conditions?14ms,4000r/min?.EIVC was realized by building reverse towing bench based on direct acting relief valve?scheme A?and throttle valve?scheme B?respectively,and the corresponding valve movement characteristics were studied.Relief valve and throttle valve could realize continuously variable VL and CT by limiting the highest pressure and chaging the reflux quantity of high pressure system,respectively.With other parameters constant,the maximum VL and CT were just related to the SP,showing a trend of gradual raise with the SP increase.Under the same SP,the maximum VL,the corresponding crank angle?CA?of maximum lift and the CT all had a trend of gradually increase,and the difference was obviously reduced when the SP was high.The influence of oil supply pressure on the maximum lift was very small.Increasing oil supply pressure was equivalent to reducing the pretightening force of valve spring,and the valve opening time was advanced.With the same motored speed,oil supply pressure,and oil temperature,the larger TVO corresponded to smaller VL and valve CT.The smaller the throttle orifice diameter of the seating buffer mechanism would cause that the valve CT of high speed was after top death center?TDC?.When the throttle valve was closed,the maximum VL difference of different motored speeds was small,and it increased obviously with the increase of motored speed under the same TVO.The maximum lift and CT of the valve were approximately linear with TVO at the constant motored speed.With the increase of hydraulic oil temperature,the corresponding maximum VL of same TVO was gradually decreased,and the fitting curve was almost a horizontal straight line when throttle valve was closed,the slopes of the other opening had a little difference.The VSV of different buffer modes was compared,and the cyclic variation?COV?of valve movement and system pressure fluctuation were studied.The singlehole buffer and multihole buffer were all belongs to slender hole which coulud not provide effective valve seating buffer for high motored speed,but the multihole buffer was obviously better than the single one.The VSV increased first then decreased with the enlarged TVO under the same motored speed.The valve movement velocity and the peak braking pressure increased with the rise of the motored speed,the VSV of 2400r/min,3600r/min and 4800r/min had little difference for the same TVO.One-way throttled buffer mechanism could make the 0⁴800r/min VSV less than 0.5m/s.The VSV was insensitive to the hydraulic oil temperature and had good consistency.The valve opening buffer had a larger velocity gradient which was resulting in system pressure to produce compression and expansion waves,the two waves caused pressure shocks after several reflection and superposition in the pipeline.The system pressure was a low-frequency base vibration,and the high-frequency resonance was superimposed on it with the increased motored speed.This high frequency vibration was the result of the interaction between fluid pressure fluctuation with wall and pipeline.With the increase of TVO,the valve chamber pressure decreased gradually,and the maximum pressure and maximum VL had the same crank angle.The valve chamber pressure gradually emerged fluctuation when the motored speed was increased,and the higher the speed was,the stronger the pressure fluctuation was.For the constant speed,the corresponding CA of wave peak and trough gradually enlarged with the increase of TVO,and the pressure increase speed was obviously reduced.At high speed,valve movement would appear transient uncontrollable phenomenon,the minimum pressure increased gradually with continuely increased TVO and this phenomenon was improved.Our mechanism had better controllability due to the fact that the oil drain could be realized in the whole valve action process instead of a specific time.With the increase of motored speed,the value of wave peak and trough increased and decreased respectively,the total pressure fluctuation amplitude increased,and the number of wave peak and trough decreased.Reducing the weight of the inlet one-way valve and the stiffness of the return spring could improve the response speed of one-way valve and shorten the oil filling time.Increasing the piston top area,and reducing the total liquid volume of the system,the quality of moving parts,the peak speed value of the cam motion and the throttling of system could increase the natural system frequency which could reduce the system hydraulic fluctuation,thus the allowable speed of the system was improved.The chi-square test results of maximum VL,corresponding CA of maximum lift,opening time,CT and open duration had the same significant level which was a normal distribution,therefore a certain parameter could be selected to evaluate valve action COV.The COV of the system pressure leaded to the cyclic variation of valve action.With the increase of TVO,the COV of maximum lift(COV_ML)increased under the same motored speed.When the TV was closed,COV_ML had small difference and was less than 1%,and it was reduced with the increase of motored speed for the same TV.Increasing the hydraulic oil temperature,COV_ML raised for the same TVO,shortened oil return pipe madeCOV_ML less than 1.5%.The energy-saving potential of EIVC and EIVC+throttle load control modes was compared with the original engine.Secondly,the in-cylinder air flow with different valve control strategies was explored.Compared with the throttle load control mode?LCM?,the LCM of EIVC and EIVC+Throttle could effectively improve engine fuel consumption.The fuel consumption of 15%load?EIVC+Throttle?reduced for 4.1%and 5.9%for the speed of 2400r/min and 3000r/min respectively,and the energy saving potential decreased when the load was continuously increased.The energy saving potential of EIVC was the biggest.At the part load,bigger throttle opening was needed when using bigger throttle valve opening.The LCM of EIVC and EIVC+Throttle could reduce the pump loss of part load,thus reduce the fuel consumption.The fuel into the cylinder from the upper edge was obviously more than the lower edge,and there was an obvious concentration difference in the cylinder around the BDC.Rich region and lean region were on the side of exhaust valve and intake valve respectively,the tumble flow was broken up in the compression stroke and squish flow emergenced in the later,and the global in-cylinder equivalent ratio near the TDC was around 1.The airflow velocity increased with piston downward and there was an obvious velocity gradient in the cylinder at the intake stroke,and the airflow velocity in compression stroke decreased obviously.The average turbulent kinetic energy?TKE?in the cylinder presented double peak structure,its peak values appeared at the early stage of intake stroke and the end of compression stroke,and it increased obviously with the speed increasement,but the first peak was obviously delayed at the moment,and the second peak appeared to be independent of the speed.The average in-cylinder TKE had a slowly decrease tread at the end of intake stroke and at the beginning of compression stroke?90²40^°CA?.The intake valve opening time had no obvious effect on the fuel-gas mixing of port fuel injection gasoline engine,and the airflow velocity and the average in-cylinder TKE could be improved when the valve opening time was delayed.The effect of EIVC on the tumble flow of intake stroke was obvious,the airflow patial scale of intake valve side increased with the advance of valve CT,the velocity of the tumble flow center was reduced and the region with a speed of 0 emergenced.The tumble flow of intake valve side near intake bottome death center?BDC?was largely damaged and nearly disappearred for NO.6 lift curve.The tumble flow of compression stroke gradually disappearred due to the extrusion of the piston upward,the earlier the intake valve closed,the sooner the tumble flow would disappear.As the gradual advance of intake valve closing time,average TKE,tumble flow strength and tumble ratio peak all showed the trend of increasing first and then decreasing.The post-throttle pressure had no obvious effect on the airflow velocity and the flow field line of intake stroke.The peak of average TKE and tumble flow ratio could be increased by reducing the post-throttle pressure,and the corresponding CA of average TKE peak was also gradually increased.Compared with the LCM of throttle and EIVC,EIVC+Throttle could effectively increase the in-cylinder average TKE and tumble ratio peak.