Numerical Investigation Of Flow Characteristics Around Intake Valve In A Supercharged Diesel Engine

The intake process of diesel engines strongly depends on the geometry of intake ports,valve rods and complex operation conditions,which result in backflow and flow separation at the valve gap.Afterwards,it may worsen the charge performance and combustion efficiency.Therefore,there is an urgent need to understand the flow characteristics near the intake valves.In this work,large eddy simulations(LES)of a four-stroke single-cylinder supercharged diesel engine were performed.The main conclusions are:Firstly,the flow around the intake valve was investigated.The results show that:(1)backflow is the main reason for the non-uniform distribution of the velocity around the intake valve curtain under high valve lifts.Specifically,under a low valve lift,the velocity distributes around the intake valve curtain uniformly.As the lift increases,the backflow was gradually enhanced,resulting in non-uniform distribution of velocity around the valve curtain.For different geometries of intake ports,backflow was only observed in the valve seat at the outlet of the tangential port.In contrast,at the outlet of the helical port backflow could occur at the valve stem,valve seat,and valve sealing.(2)Backflow at the intake valve curtain has a minor effect on the performance of the intake flow.During the engine operation,the maximum mass flow rate ratio of backflow through the tangential port and helical port to the whole flow entering into the cylinder are 0.614% and 0.718%,respectively.(3)The charge coefficient increases gradually with the increase of the boost ratio,but the effect of the boost ratio on the swirl ratio is negligible.At medium engine speed(2000 rpm),both the backflow velocity and the backflow area decrease with the increase of the boost ratio.In addition,at a high engine speed(4000 rpm),with the increase of the boost ratio,the magnitude and the distribution of the mean velocity around the intake valve curtain keep nearly unchanged during the intake process.Secondly,the characteristics of the intake jet and its influence on the in-cylinder flow were investigated.The results show that:(1)the increase of boost ratio can change the direction of intake jet,but the evolution of the direction is not change.During the intake stroke,the jet angle gradually shifts from the bottom of the cylinder head to the center of the cylinder,and finally swerves to the valve gap centerline.(2)Supercharging has a significant influence on the velocity in the intake jet centerline during the initial and middle stage of the intake stroke,but no obvious effects are observed during the later intake stroke.At the initial stage,the flow velocity in the intake jet centerline decreases rapidly for both the tangential port and the helical port.However,at the middle stage of the intake stroke,the flow velocity in the intake jet centerline through the tangential port still decreases rapidly,and the penetration depth of the jet decreases correspondingly.In contrast,the velocity decreases slowly through the helical port,and the penetration depth of the jet increases.(3)A strong correlation between the intake jet and the flow structure during the initial and middle stage of the intake stroke is found.The influence of the intake jet on in-cylinder flow decreases during the later stage of the intake stroke,due to the decrease of the intake jet velocity.In addition,the breakup of the intake jet and the cyclic variation of the vortex center is one of reasons for the increase of the turbulent kinetic energy during the intake stroke.