Research On The Influence Of The Structure Of The Intake Port Of A Diesel Engine On The Flow Field In The Cylinder Near The Wall

For diesel engines,the earlier injection of fuel into the cylinder technology can achieve the purpose of promoting the mixing of fuel and air,improving combustion in the cylinder,and reducing pollutant emissions.However,because the fuel is injected into the low-pressure low-temperature combustion chamber under the earlier injection timing conditions,it becomes a common phenomenon that the fuel hits the combustion chamber wall.As a result,most of the fuel is spread on the side wall of the pit of the combustion chamber,and the area and thickness of the oil film increase.As a result of the wall quenching effect in the cylinder,a non-combustible or incompletely combusted quenching layer with a thickness of about 0.1-0.2 mm will be formed on the cylinder wall surface,increasing the pollutant emissions of the direct injection engine.Therefore,it is necessary to analyze the airway-cylinder airflow movement characteristics and the near-wall surface flow field in the cylinder to grasp the influence of the near-wall surface airflow movement law and the near-wall surface flow field on the combustion and pollutant emissions in the cylinder.Build an airway steady flow test bench for visualizing the near-wall flow field with the wire method / particle method.Visualized the flow field of the near wall surface in the cylinder,using CFD(Computational Fluid Dynamics)to numerically simulate the air flow movement and evolution process of the near wall surface in the cylinder,and verified the test results and simulation results mutually.The results show that under steady flow conditions,the wire movement characteristics of the cylinder wall can intuitively reflect the movement and changes of the air flow in the cylinder near the wall.The movement direction of the thread near the wall of the cylinder is affected by the vortex generated by the spiral air passage,so that the direction of the thread movement is consistent with the direction of the vortex.The flow field near the wall on the tangential airway side is affected by the airflow in the tangential airway.At the same time,under the joint action of the vortex motion,the velocity field on the near wall surface on the tangential airway side is the largest,which makes the wire swing angle maximum.When the valve lift is 8mm and 12 mm,the wire swing angle is 110 °and 90 °,respectively.The steady-state calculation and transient calculation of the in-cylinder flow field evolved from multiple small vortices in different directions into asingle rotating vortex.The transient calculation of the in-cylinder flow field has vortex and tumble flow motions,so the in-cylinder flow field merges into a single vortex motion more quickly.In the study of the influence of the valve structure of different air passages on the flow field in the cylinder and the flow field near the wall surface in the cylinder,different valve structure schemes were designed.Combining particle method near-wall flow field test and CFD simulation calculation.The results show that the particle method visualization of the flow field of the cylinder near the wall and the steady flow experiment can clearly show the airflow movement of the cylinder near the wall of the diesel engine.The intake valve structure has a great influence on the flow field in the cylinder.The decrease in the intake valve cone angle and the increase in the arc on the back of the intake valve increase the in-cylinder flow coefficient;the increase in the intake valve cone angle and the round of the intake valve back Decreasing the arc increases the in-cylinder swirl ratio.The optimization of the intake valve structure can simultaneously increase the in-cylinder flow coefficient and the swirl ratio,the spiral air intake valve cone angle increases,and the tangential air intake valve back arc increases,the optimization is 6.3% and 10.5%,respectively.The optimization of the intake valve structure promotes in-cylinder combustion.Compared with the original engine,the in-cylinder combustion pressure increases by 6.5%,the cumulative heat release increases by 8%;the in-cylinder Soot decreases by 9.1%,and the NOx(Nitrogen oxide)generation increases by 5.7%.In the analysis of the flow field near the wall surface of the diesel engine to the combustion and emissions in the cylinder,the velocity field in the 6 cylinder of the scheme is as follows.The airway increase by 33.8%.It avoids the airflow interference between the tangential airway and the spiral airway between the two airways.For the distribution of the pressure field in the cylinder,the air flow in the tangential air passage X slice,spiral air passage X slice and cylinder Y slice are lower than the original engine pressure,so that the air flow resistance of the air flow into the cylinder is reduced,and the intake of the diesel engine is increased.Soot in the cylinder is mainly distributed in the combustion chamber.However,due to factors such as the air flow in the cylinder and the downward movement of the piston,the Soot in the cylinder moves from the beginning to the uniform distribution in the combustion chamber to thecenter of the cylinder,and finally to the tangential air passage side combustion chamber.NOx will also be affected by the airflow in the cylinder and the downward movement of the piston.As a result,NOx is evenly distributed in the cylinder when the piston descends to the bottom dead center.In the Z-axis slice in the cylinder,the NOx concentration field in the cylinder is distributed at the 4mm slice position with the largest amount.The CO(Carbon monoxide)concentration field is mainly distributed at 8mm and 12 mm slice positions.The CO2(Carbon dioxide)concentration is the largest at the 4mm section.The OH concentration field in the cylinder is mainly distributed at the 4mm slice position.Because OH has strong oxidizing properties,as the combustion in the cylinder progresses,the amount of OH will be quickly consumed,so the OH concentration field will decrease at the 8mm and 12 mm slice positions.