Simulation Research On Lean-Burn Characteristics Of DI CNG Engine

In order to deal with the energy and environmental crisis,and to satisfy increasingly stringent emissions regulations,developing energy-saving and environmentally friendly alternative fuel has become an urgent demand.Using natural gas as fuel can not only ease the energy crisis but also reduce vehicle emissions effectively.The direct injection（DI）technology can effectively improve the volumetric efficiency of the CNG engine.And engine power will be further improved when the compression ratio is increased appropriately.It is a good way to reduce the fuel consumption and emissions by the lean-burn technology.So,by simulation method,this paper researches the lean combustion characteristics of the DI CNG engine with high compression ratio.（1）According to the structure parameters of the original CNG engine,one-dimensional and three-dimensional simulation models are built by AVL Boost and Fire simulation platform.Then these models are verified by the experimental data.On the basis of these models,the paper finishes the DI CNG engine model by increasing the compression ratio to 12:1 and installing an injector on the cylinder head.Calculation condition: n=1600r/min,MAP=169k Pa,WOT,φ_a=1.5.（2）The results of single injection show that: In the intake stroke,the earlier the CNG injected,the more homogeneous the mixture will at in 20° CA BTDC.When the injection time delayed,the uniformity of mixture is deteriorated.While in the compression stroke,under the influence of high pressure direct injection of natural gas,mixture can also be homogeneous when inject earlier.And the average velocity rate is higher than injecting in the intake stroke.But the mixture is not as homogeneous as injecting in the intake stroke.Volumetric efficiency of injecting in the compression stroke is biggest and is higher than injecting in the intake stroke of 13.2%.the volumetric efficiency of port injection is slightly higher than the volumetric efficiency of the intake stroke of the early injection,but it is lower than the early injection in the intake stroke.As for a single injection,different injection time have different ignition advance angle.When θ_ig is the best ignition advance angle,the pressure peak and temperature of injecting in the compression stroke is higher than these when injecting in the intake stroke.No matter inject in the intake stroke or in the compression stroke,there are both best injection time.When θ_ig is the best ignition advance angle,the indicated thermal efficiency of the two injection time has little difference.But the IMEP of injection in the compression stroke is 8.11% higher than injection in the intake stroke.（3）The results of two-stage CNG injection show that: In order to achieve the lean combustion,after the first injection,the mixture in cylinder should be homogeneous in the vicinity of the ignition timing.When the second injection is completed,there should be a good stratification in cylinder.And the mixture should be thick near the spark plug,while the mixture concentration should be decreased gradually away from the spark plug area.When the first time and injection ratio are fixed,there is an optimal time of second injection to make the mixture form a good stratification.As for the two-stage CNG injection,different second injection time have different ignition advance angle.When θ_ig is the best ignition advance angle,there is an optimal time of second injection to make the indicated thermal efficiency and IMEP maximum and ISFC minimum.When the injection ratio is 4: 1,the best second time of injection is the 45 ° CA BTDC.When the two times of injection are fixed,here is an optimal time of second injection to make the mixture form a good stratification.And different injection ratio has different ignition advance angle.When θ_ig is the best ignition advance angle,with the increase of injection ratio,the indicated thermal efficiency increases and then reduce.When the ratio is 5:1,the indicated thermal efficiency is highest.Different injection ratio has different injection time.And with the increases of the injection ratio,the optimal time of second injection is advanced.With the optimal time of second injection,when the ratio increases,the indicated thermal efficiency increases,but not by much.（4）The paper analyzes the different influence on lean combustion characteristics of the signal injection,two-stage injection and port injection.The results show that: when θ_ig is optimum ignition advance angle,the indicated thermal efficiency of two-stage injection is highest of all,while port injection is slightly higher than single injection.but because of the influence of volumetric efficiency,IMEP of two-stage injection is 10.81% higher than port injection.During combustion,flame propagation of two-stage injection is fastest of all.While in the early and end of combustion,flame propagation of port injection is faster than single injection.Therefore,the average temperature in cylinder of two-stage injection is the fastest-rising of all and is the first one to reach the maximum.And two-stage injection’s peak temperature is highest and port injection’s is lowest.In the end of combustion,the temperature of two-stage injection and port injection dropfaster,and single injection’s temperature drops slowly.As the two-stage injection’s temperature rises rapidly,the NO mass fraction also grows fast.When the average temperature in cylinder reaches a maximum value,NO also reaches the maximum mass fraction and remains the same.At last,the NO mass fraction of two-stage injection is highest,followed by single injection,port injection is minimized.