Study On The Flame Propagation Characteristics And NO Generation Pattern Of Direct Injection CNG Engine

As the automobile enterprise in our country develops rapidly, our country importsmore petroleum from foreign countries and low-carbon becomes the direction of theautomobile enterprise. Natural Gas has many advantages, such as lower emissions,better economical efficiency, better anti-detonating quality, better security and so on.So it is the most popular automobile alternate fuel. As a result, Compressed NaturalGas (CNG) engine develops quickly. When CNG engine chooses the port-injectionmethod, the volumetric efficiency will decline and the dynamic will also drop.However direct-inject CNG engine can improve the dynamic and it has some otheradvantages such as more accurate injection control. As a result, direct-inject CNGengine has a bright future. When the lean-burn technology is used in DI-CNG engine,the emissions will be much lower, but there are some issues. Such as how to controlthe stability of the combustion process and how to reduce NOxfurther. As a result, inthis paper the flame propagation mechanism and NOxgeneration pattern during thecombustion process of the DI-CNG engine were researched.The principal factors that influence the flame propagation velocity are equivalenceratio, temperature, pressure and turbulence fluctuation. The combustion system modelwas set up on the basis of the earlier experimental research of single cylinder DI-CNGoptic engine in our research group. This model was set up by3d simulation softwareAVL-FIRE. And the compression and power stroke are simulated. The2D results suchas combustion heat release rate, cylinder mean pressure and turbulence kinetic energyrepresent the macroscopic part. And3D physical fields represent the microscopic part.The influence of the equivalence ratio, ignition mode, injection time, swirl andcombustion chamber shape on the flame propagation characteristics and NOgeneration pattern was analyzed according to the macroscopic and microscopiccalculation results. After that, the flame propagation mechanism and NO generationconditions were summarized. And these will be the theoretical foundation for combustion process optimization and NOx emission reduction.The research results are shown below.Firstly, when comes to CNG engine which has a bigger cylinder diameter, twoignition plugs are better than one ignition plug. When two ignition plugs are used,sparking at different time is better than sparking at the same time. And the flamepropagation speed is related to the promoting effects of the pressure field of flame2.When the ignition time is earlier, the flame propagation speed will be faster, but NOquantity will improve.Secondly, when lean-burn technology is used in DI-CNG engine, if the injectiontime is much earlier, the gas and air in the cylinder are like the cloud, and they don’tmix up with each other. This phenomenon is not benefit for the flame formation and itspropagation. If the injection time is delayed properly, the mixture in the cylinder isrelatively homogeneous. If the injection time is much delayed, the mixture in thecylinder is gradient distribution, the lower the thicker. This phenomenon will improvethe flame propagation speed and heat release rate and NO quantity will be controlled.Thirdly, the combustion chamber shape and the swirl mainly influence theturbulence fluctuation intensity. There is the best swirl when the average equivalentration and combustion chamber shape are certain. Compared to canister combustionchamber, basin combustion chamber’s heat release is earlier, and NO quantity is lower,but the peak of heat release is slightly lower.Lastly, combustion temperature in the cylinder and the mixture’s distributiondetermine the NO generation rate. And NO generation rate is sensitive to oxygenconcentration. NO generates quickly by the temperature range of2550K2750K andthe equivalent ratio range of0.81.0. And the more the temperature and the equivalentratio deviate from the best range, the less NO will generate. When the equivalent rationis greater than1.4or less than0.7, or the temperature is lower than1900K, NO hardlygenerates. NO generation quantity depends on NO generation rate, the duration of NOgeneration reaction and the size of the reaction field.