| As natural gas has the advantages of low price,abundant reserves,good explosion resistance,less particulate matter and carbon dioxide emissions from combustion,it’s the best choice to replace gasoline and diesel when applied in engines.Under lean conditions,natural gas combustion has less heat loss,higher thermal efficiency and less nitrogen oxide emissions.However,the propagation speed of natural gas flame is slow,and it is relatively difficult to ignite,especially in lean conditions.Turbulent jet ignition(TJI)is an effective combustion enhancement technology,which can not only provide higher ignition energy,but also increase combustion pressure and combustion rate,shorten combustion duration.It is an effective way to realize stable ignition of natural gas under lean conditions.Therefore,this paper uses methane,the main component of natural gas as fuel,to investigate the ignition and combustion characteristics of turbulent jet,and to find ways to improve the performance of the prechamber and optimize the combustion characteristics of TJI.First of all,turbulent jet ignition experiments with different apertures and internal structures of prechambers are carried out based on the constant volume combustion bomb test platform.The experiment results show that the jet flame velocity and the combustion pressure increase with the decrease of the prechamber’s apertures,but the too-small aperture will lead to a change in the ignition mechanism,which is not conducive to the combustion of natural gas under lean conditions.The fueled prechamber can expand the lean flammability limit as well as increase the flame propagation velocity and combustion pressure under lean conditions,while using hydrogen as auxiliary fuel in the prechamber can double the jet flame velocity several times,thereby greatly expand the lean flammability limit and further enhance combustion.Proper multi-stage acceleration structure can significantly improve the performance of the prechamber,produce stronger turbulence,enhance the dynamic performance of the flame and improve the heat release rate.Then,a multi-stage acceleration prechamber with the best performance was used in the spark-ignition single-cylinder engine to investigate the effects of TJI on the power,emission performance and combustion characteristics of the engine.The use of TJI can effectively extend the lean flammability limit of methane in the engine,shorten the combustion delay period and combustion duration,and increase the heat release rate.The excess air coefficient of 1.5 is the best lean burn condition of methane TJI,with which the fuel consumption is the lowest and the NOx emission is almost zero.Because the fuel consumption of TJI is high under the condition of equivalent combustion at high load and lean combustion at low load,the engine fuel consumption is greatly reduced by increasing the intake pressure at high load and reducing the throttle valve at low load to maintain the combustion condition near the excess air coefficient of 1.5.Finally,hydrogen is used as the prechamber’s fuel and lean methane / air mixture as the fuel in the main combustor of the above-mentioned single-cylinder engine to investigate the combustion characteristics of hydrogen-assisted jet ignition.Compared with the combustion characteristics of methane TJI engine,hydrogen-assisted jet ignition can further expand the lean flammability limit,shorten the ignition delay and combustion duration,increase the heat release rate,and improve the performance of TJI.In this paper,a series of experiments on the jet ignition combustion of methane using prechambers were carried out on the constant volume combustion bomb and single cylinder engine,which has a certain guiding significance for the application of the TJI system in the natural gas engines... |
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