Experimental Study On Lean Combustion Characteristics Of Natural Gas Based On Optical Engine

With the increase of the number of automobiles,environmental problems become more and more serious.Meanwhile,the consumption of a large number of fossil fuel and the uneven distribution of oil resources lead to an increasingly serious energy crisis,so it is urgent to develop clean and large reserves of alternative fuels.As the most potential alternative fuel for automobile,natural gas has the advantages of abundant reserves,clean combustion and high heat value.The combination of lean combustion and high compression ratio can greatly reduce the emission of NOx and improve the thermal efficiency of the engine.However,the cycle fluctuation caused by the difficulty of ignition and slow burn rate of natural gas under lean combustion condition is large,and the high exhaust temperature also restricts the development of natural gas engine.Based on a single cylinder spark ignition engine,the effects of hydrogen injection,high energy ignition and large spark plug gap on lean burn characteristics of methane were studied.During the test,the pressure in the cylinder and the optical flame picture are measured synchronously to analyze the flame propagation process and engine performance.In addition,an empirical formula is proposed to evaluate the initial combustion process based on the initial flame image to quantify the burned mass fractionThe experimental results show that:(1)Under lean burn condition,the indicated mean effective pressure(IMEP)of methane will decrease,and the cyclic variation will increase.With the increase of combustion duration,direct injection of hydrogen can significantly improve the lean burn stability of methane.Optical pictures show that the formation of flame kernel and the initial flame propagation are significantly accelerated.Further analysis shows that hydrogen can significantly improve the early combustion process and reduce the modification Cycle change of the process.(2)Under the same excess air coefficient(λ),the ignition energy increases,the cycle variation decreases,the IMEP increases,and the combustion phase is optimized;the reduction of combustion duration mainly comes from the reduction of CA10-CA50 stage,but for CA50-CA90 duration,the effect of high energy ignition is not obvious;the flame picture shows that the increase of ignition energy can promote the formation of the initial flame kernel and accelerate the early flame propagation process,and reduce the cycle change of the process.(3)Under high ignition energy condition,with the increase of spark plug gap,the heat release rate increases,the combustion duration shortens,correspondingly,IMEP increases,and the cyclic variation decreases;under ultra-lean condition(λ > 1.5),with the combination of ultra-high ignition energy(200 m J)and large spark plug gap,the lean burn limit of methane can be successfully extended to 1.55;the flame images show that at the same ignition energy,the large spark plug gap can form a large flame kernel,which can directly promote the flame propagation process;the quantitative evaluation of the early combustion process shows that the synergistic effect of ultra-high ignition energy and large spark plug gap can make the engine obtain a faster and more stable initial flame propagation under the condition of lean combustion,so as to improve the combustion stability and thermal efficiency of the engine.In conclusion,the effects of direct-injected hydrogen and high ignition energy on the ignition process and flame propagation of natural gas under lean burn conditions are systematically studied.It is found that direct injection of hydrogen and high-energy ignition can significantly improve the lean burn stability and thermal efficiency of methane,and ultra-high ignition energy combined with large spark plug gap can significantly expand the lean burn limit of methane.The results of this study can be considered as an effective way to reduce cycle variation and improve thermal efficiency of natural gas engine.