A Numerical Investigation On Ignition Limit And Knock Of Low-pressure Jet Dual-fuel Marine Engine

With the increasingly strict emission regulations for marine diesel engines and the shortage of petroleum resources,LNG engines are gradually becoming the first choice for future ship power,which have good economy and low emission.Among them,the low-pressure injection micro-ignition marine engine has more outstanding performance in reducing NOx,PM,CO₂ emission.However,at medium or low loads,the combustion stability of the engine may be deteriorated or even misfire.At high load,the combustion heat release rate is too fast,which may lead to knock.Based on the above background,this paper firstly conducted a zero-dimensional theoretical study on the combustion characteristic of diesel/natural gas mixture under engine conditions.Then,the three-dimensional model of the engine is established,and the effects of different factors on the ignition limit and knock are studied.First,based on the condition of engine TDC,a numerical study has been conducted to examine the effects of temperature,pressure,equivalence ratio and blend ratio on IDs of methane/n-heptane mixture using the closed homogenous reactor of CHEMKIN.Results show that the initial temperature and equivalence ratio have great influence on the ID in all research conditions.While the effects of pressure and blend ratio depend on the special condition.It is interesting that the ignition delay map can be divided into four different zones,which can derive from the coupling effect of methane concentration and equivalence ratio.The rate of production and consumption analysis shows that initial temperature and equivalence ratio have great influences not only on the reaction rates but also on the reaction temperature region.Secondly,the effects of the equivalence ratio,fuel injection amount,and fuel injection timing on the engine ignition limit and knock are studied.Results show that:（1）There is a misfire phenomenon under the equivalent ratio of 0.3,and the pressure oscillation can be observed at the monitoring point under the equivalent ratio of 0.5.The main cause of the misfire is that the in-cylinder mixture is too lean,and the methane combustion reaction is slow,so that the temperature is too low to consume CH₂O normally.What’s more,the sensitivity of the reactions related to the CH₂O is large.Thus,there is a misfire phenomenon when these reactions could not proceed normally.The main reason for the pressure oscillation is that under the rich gas condition,the gas flow and flame develop rapidly at the exit of the pre-combustion chamber,which takes away more heat and intermediate components generated by the n-heptane reaction.Because of the rich gas,the reaction rate of combustion is faster,which causes obvious pressure fluctuations in a narrow space near the wall surface.（2）Under lean gas conditions,with the decrease of the pilot fuel quantity,the ignition is more sensitive to the quantity of pilot fuel;When the fuel injection timing retarded to near TDC,the combustion deteriorates.（3）Under the condition of rich mixture gas,the pressure oscillation increases with the delay of injection timing.In addition,the increase in the quantity of pilot fuel provides more ignition energy,which increases the pressure oscillation in the cylinder.In summary,this work studies the ignition characteristics of dual fuel and the ignition limit and knock of dual-fuel low-pressure injection micro-ignition marine engine,which provides a theoretical basis for optimizing the ignition stability and suppressing knock of dual-fuel engines.