Research On The Influence Of Fuel Injection Strategy On Diesel Engine Emission Performance Under Transient Operating Conditions

Diesel engines are widely used in the field of heavy machinery.Increasingly stringent emission regulations place higher requirements on diesel engine emissions,and diesel engine emissions increase sharply under transient operating conditions compared to steady-state operating conditions.The current fuel injection strategy is the focus of research on reducing diesel engine emissions.Based on a light-duty diesel engine,this paper tests the diesel engine with constant-rotation and increased-torque transient conditions as a typical operating condition,and studies the changes in macroscopic parameters of automotive diesel engines.Combined with CONVERGE software for simulation research,the distribution of the diesel engine cylinder field and the deterioration of transient conditions are studied and the degradation mechanism is revealed.The boundary conditions of the simulation model under transient conditions are the throttle valve with the shortest response time.The opening signal is extracted as a reference.According to the deterioration mechanism,the influence of fuel injection strategy on the combustion characteristics and emission laws of automotive diesel engines is studied by adjusting fuel injection parameters.The specific conclusions are as follows:（1）The generation boundaries of NO_x and Soot are temperature T>2200K,equivalent ratioφ<1 and T=1400K～2500K,φ>2,respectively.When the fuel injection timing is advanced,the _maximum temperature T_max in the cylinder increases,the turbulent kinetic energy decreases first and then increases,the temperature range of T_max=1400K～2500K in the cylinder decreases,and the high temperature range of T_max>2200K increases.In the cylinder field at 5°CA/ATDC,the area of T>2200K expands sharply,and the expansion trend of T=1400K～2500K is not obvious;the O₂concentration decreases with the advance of the fuel injection timing;the velocity field in the cylinder increases first Then it decreases;the equivalent ratio in the cylinder first expands and then decreases sharply in theφ>2 area;it is smaller in theφ<1 area.NO_x emissions have gradually increased,and particulate emissions have increased first and then decreased.Properly advance the fuel injection timing,you can get a compromised emission performance.（2）Increasing fuel injection pressure NO_x emissions increase as fuel injection pressure increases,and particulate matter emissions decrease as fuel injection pressure increases.The range of T_max=1400K～2500K decreases.In the in-cylinder field at 5°CA/ATDC,the area of T>2200K expands slightly,and the area of T=1400K～2500K expands more obviously.The O₂concentration increases with the advance of the injection timing.Slightly reduced;the equivalent ratio in the cylinder is reduced in the area ofφ>2,and the area ofφ<1 is enlarged.The area of T=1400K～2500K expands,the area ofφ>2 decreases,and the Soot emission decreases.The oil-gas mixing effect plays a major role in restraining the production of Soot.The use of as large a fuel injection pressure as possible within the NO_x emission limit and technically permitted can effectively reduce the emission of particulate matter.（3）Increasing the interval between pre-injection and main injection will not affect NO_xemissions.In the case of medium and small pre-main spray intervals,increasing the pre-main spray interval will reduce particulate emissions.At-5°CA/ATDC crankshaft angle,the range of T>2200K decreases.The oxygen concentration in the cylinder first decreases and then increases;the gas flow rate in the cylinder decreases;the equivalent ratioφ<1 in the cylinder increases.In the cylinder field under the crankshaft angle of 5°CA/ATDC,the temperature range of T>2200K and T=1400K～2500K in the cylinder first increases and then decreases;the velocity field first increases and then decreases;φ<1 The area decreases first and then increases with the increase of the pre-main spray interval;the area ofφ>2 decreases first and then increases,but does not change much from 20°CA to 25°CA.At this time,the generation of Soot is mainly affected by temperature boundary conditions.influences.In order to simultaneously control the emission of particulate matter,the pre-main spray interval should be controlled within 15°CA to 20°CA.（4）Increasing the pre-injection rate increases NO_x emissions and reduces particulate emissions.The 1400K<T<2500 and T>2200K areas in the cylinder both show an increasing trend;the average turbulent kinetic energy of the pre-injection section in the cylinder increases significantly.In the cylinder field under 5°CA/ATDC,T>2200K and 1400K<T The<2500K area shows an expanding trend and the equivalent ratio field shows a decreasing trend in theφ<1 area andφ>2 area.A compromise choice of 10%-15%pre-injection rate can get a better NO_x and particulate matter emission effect.（5）After adding the post-injection,more oxygen will diffuse towards the injection area within the crankshaft angle of the main-post-injection interval,and the entrainment effect will greatly improve the oil-gas mixing effect at the end of the injection period.As the interval between main injection and post injection increases,and the trends of NO_x and particulate emissions first decrease and then increase.Control the main after-spray interval to about 15°CA to obtain better NO_x and particulate matter emission effects.With the increase of the post-injection rate,the average turbulent kinetic energy in the cylinder shows a decreasing trend in the main injection section and an increasing trend in the post-injection section;the velocity field in the cylinder field under 25°CA/ATDC increases,and the temperature distribution T>2200K area shrinks.Both NO_x and particulate matter are reduced,and the reduction of NO_x is relatively small and nearly unchanged.Using a larger post-injection rate can effectively reduce particulate matter emissions and NO_x emissions.