Study On The Influence Of Cylinder Chemical Atmosphere On Combustion Process Of Diesel Piloted Natural Gas Engine

In the face of stricter automobile emission regulations,the application of alternative fuel natural gas in internal combustion engines has attracted much attention.Diesel piloted natural gas engine has the characteristics of high compression ratio,which makes it have the space to further improve the engine brake thermal efficiency.As we all know,there are characteristics of compression ignition,pilot ignition and flame propagation in the combustion process of diesel piloted natural gas engine.The cylinder chemical atmosphere not only directly affects the pilot diesel compression ignition and the natural gas flame propagation process,but also indirectly affects the natural gas combustion process through the pilot diesel combustion.At present,there are few studies on the influence mechanism.In order to better understand the combustion process of diesel piloted natural gas engine,this paper intends to carried out the following four parts of research.Firstly,the influence law of pilot diesel combustion characteristics on natural gas combustion process was carried out.Secondly,the influence of cylinder chemical atmosphere on the pilot diesel combustion process was analyzed.Thirdly,the influence of cylinder chemical atmosphere on natural gas combustion process was studied.Lastly,the influence mechanism of cylinder chemical atmosphere on diesel piloted natural gas engine combustion process was proposed.The research results can provide theoretical guidance for the combustion process organization of diesel piloted natural gas engine and dual fuel engine.In this study,the engine experimental platform and numerical simulation platform were built.At the same time,the representative non combustible components（"monatomic molecule Ar","diatomic molecule N₂"and"triatomic molecule CO₂"）and combustible components（"oxygen"and"hydrogen"）were selected as the research objects for the cylinder chemical atmosphere.The main contents and conclusions are as follows:1)On the research platform of diesel piloted natural gas engine,the influence of pilot diesel combustion characteristics on the later natural gas combustion process was studied under 1335r/min speed and 375N.m condition.The results show that the spatial distribution of pilot diesel in the cylinder at the time of natural gas ignition has an important impact on the natural gas combustion performance.The spatial distribution of pilot diesel determines the ignition area of natural gas.By adjusting the diesel injection time,the spatial distribution of pilot diesel can be controlled in a wide range.So the multi-point ignition of natural gas around the oil beam can be realized,and the local space regional ignition of natural gas in the cylinder can also be realized.2)On the research platform of diesel piloted natural gas engine,the influence of cylinder chemical atmosphere on pilot diesel combustion process was carried out under1335r/min speed and 375N.m condition.The results show that the introduction of non combustible components has induction and inhibition effects on the pilot diesel combustion process,and the influence mechanism of oxygen and hydrogen on the combustion process of pilot diesel is different.They also determine the variation law of pilot space which characterizes the pilot capacity.Ar makes the pilot diesel combustion phase move forward,and it makes the range of pilot space changes toward a narrower trend.However,the introduction of N₂ or CO₂ will slow down the pilot diesel combustion speed,and the change of pilot space is small.With the decrease of oxygen concentration,the combustion phase is delayed and the range of pilot space has a broader trend.With the introduction of hydrogen,the pilot diesel combustion process has little change,the change of pilot space also varies little.3)The direct influence of cylinder chemical atmosphere on the natural gas combustion is represented by the change of the natural gas flame propagation speed.So,the influence of cylinder chemical atmosphere on the natural gas combustion process was carried out under 1450r/min condition on the research platform of single ignition source spark ignition natural gas engine.The results show that the non combustible components also have induction and inhibition effects on the natural gas combustion process,and the influence of combustible components on the natural gas combustion process is different from that of pilot diesel.Ar accelerates the natural gas flame propagation speed.However,N₂ or CO₂ will slow down the natural gas flame propagation speed.With the decrease of oxygen concentration（excess air coefficient shall not be less than 1.0）,the natural gas flame propagation speed increases.Hydrogen can also accelerate the flame propagation speed of natural gas.4)On the research platform of diesel piloted natural gas engine and selecting1335r/min speed and 375N.m condition,the synergistic mechanism effect of chemical atmosphere and pilot diesel combustion on natural gas combustion was revealed.The results show that after introducing different components,the combustion overlap period of pilot diesel and natural gas and the change of combustion amount of natural gas in the overlap period are different,which can characterize the influence of pilot diesel combustion on natural gas combustion.And the natural gas combustion process change is different.When Ar is introduced,the combustion overlap period decreases,and the combustion amount of natural gas in the overlap period decreases.At this time,the start of natural gas combustion is slightly advanced and the combustion speed is accelerated.After introducing CO₂ or N₂,the combustion overlap period increases and the combustion amount of natural gas in the overlap period gradually increases.At this time,the start point of natural gas combustion is delayed and the combustion speed is slowed down.When the oxygen concentration decreases（excess air coefficient decreased from2.2 to 1.0）,the change of combustion overlap period is small and the combustion amount of natural gas in the overlap period increases.At this time,the start of natural gas combustion is delayed,but its combustion speed increases first and then decreases slightly.And it begins to decrease slightly when the excess air coefficient is 1.0.With the introduction of hydrogen,the combustion overlap period is basically unchanged and the pilot diesel has almost no effect on the natural gas combustion process.At this time,the starting of natural gas combustion is also basically unchanged,but the end of natural gas combustion tends to advance.In terms of improving the diesel piloted natural gas engine thermal efficiency,the introduction of Ar can be the first choice.At the low load operating point in this study,the indicated thermal efficiency can be increased from36.05%to 48.07%.