| Due to the severe energy environment situation and stringent ship emission regulations,the technology of natural gas engine has been payed more attention.Diesel/natural gas dual fuel engine is widely used in marine engine market because of its high energy density and low emission.In addition,given the emission problems of conventional diesel combustion,low temperature combustion technology has been widely concerned in order to reduce emissions of diesel/natural gas dual fuel engines while ensuring high thermal efficiency.Low temperature combustion mode of dual fuel engine can reduce NOx emission simultaneously and guarantee high thermal efficiency.The RCCI combustion mode can achieve good control of combustion phase and heat release rate,which is conducive to the expansion of the engine load.Therefore,the RCCI combustion mode has the potential to meet more stringent emission regulations in the future.However,the problems of RCCI combustion mode,such as working roughly under high load and high HC and CO emission at low load have not been solved effectively,there are still few detailed studies on the effects of diesel fuel injection parameters on the stratification and combustion of the in-cylinder mixture,the understanding of the ignition and combustion mechanism is still not clear.Therefore,it is of great significance to optimize the combustion process and emissions of dual fuel engine in RCCI mode to construct a more accurate diesel/natural gas reaction model,establish the corresponding combustion chemical kinetic mechanism and comprehensively study the influence of injection parameters with RCCI mode.Firstly,according to the physical and chemical properties of diesel and natural gas,n-dodecane,methylcyclohexane and toluene were selected as the reference fuel of diesel and methane as the reference fuel of natural gas.According to the working condition range data of the studied dual fuel engine,the working condition range of simplified mechanism was determined.The detailed mechanism of each component was simplified at the selected operating point by using the DRG,DRGEP and FSSA method as the combination of mechanism simplification methods,then,based on the experimental data of ignition delay,laminar flame speed and important intermediate concentration,the simplified mechanism of each component was verified.By combining the reduced mechanism of grouping components,a multi-component diesel/natural gas dual fuel combustion mechanism with 150 components and 776 reactions was constructed.Secondly,taking the cross reactions between the diesel components as the research object,selecting the simplified mechanism model,the main components involved in the cross reactions were obtained through the reaction path analysis and the corresponding cross reactions were determined.According to the thermodynamics theory,the cross reaction coefficients in the mechanism model were supplemented,and 71 cross reactions were generated.Thus,a simplified dual fuel mechanism with cross reactions was constructed,which contains 150 components and 847 elementary reactions.The influence of cross reaction on ignition process of diesel components was studied under different temperatures,pressures and equivalence ratios.It was found that the influence of cross reaction on ignition was more obvious at low temperature,and such influence would be weakened with the increase of initial temperature.In addition,the influence of cross reaction on ignition process is not sensitive to the change of initial pressure.With the increase of equivalence ratio,especially at low temperature,the influence of cross reaction will also be enhanced.In order to further study the mechanism of the effect of the cross reaction on the ignition process,the differences of the main evolution paths of diesel with or without cross reactions were determined by calculating the reaction fluxes of diesel components.Based on the sensitivity analysis of the key intermediate products,the key elementary reactions of the cross reactions on combustion were determined,the variations of the main evolution pathways of diesel components with or without cross reactions were also analyzed.The results show that when the reactivity of diesel is lower at low temperature,the cross reactions can promote the formation of diesel intermediates and the reaction rate,especially for n-dodecane.The cross reactions can greatly increase the ratio of the important secondary products,accelerate the formation rate of small active molecular in the later stage of combustion process,and thus reduce the ignition delay;however,at higher temperature,the reaction rate of elementary elements is greatly increased,so the cross reactions do not significantly improve the reaction speed of intermediate products,the influence of cross reactions on ignition is very limited.Finally,the geometric model of 6K micro ignition diesel/natural gas dual fuel engine was established by using 3D modeling software,and the numerical simulation of the dual-fuel engine under 25%and 75%load conditions was carried out by using CFD software.The results show that the simplified mechanism model with cross reactions can more accurately describe the combustion process of the dual fuel engine.Combined with the analysis of the main pollutant generation path of diesel and natural gas components at different temperatures,the effects of different diesel injection parameters on the combustion and emission characteristics of micro ignition diesel/natural gas dual fuel engine were studied.The first injection timing was from 60°CA BTDC to 20°CA BTDC,second injection timing was from 15°CA BTDC to 5°CA ATDC,and ratio of first injection diesel was changed from20%to 80%.The results show that,under low load,the advance of first injection timing promoted the diffusion of OH ions generated by low-temperature reaction of preinjection diesel,and CH4 was fully burned.The temperature field in the cylinder was relatively uniform,which inhibited the generation of NO.With the increase of the ratio of pre-injection,the increase of the ratio of diesel premixed combustion promoted the low temperature oxidation path of the fuel,and the phase of the combustion starting point was advanced,but the combustion center was premature and too much negative work was generated.Under high load,due to short combustion duration and high combustion temperature,the reaction path at high temperature was strengthened,and the combustion process difference caused by different first injection timing is low.The high ratio of first injection will lead to the delay of the combustion,and the lack of ignition energy of the second injection diesel will also lead to the increase of CO.When the ratio of first injection is too small,the concentration of high reactive active components in the cylinder is low,and the effect of diesel first injection on the main combustion period is reduced,resulting in the increase of unburned CH4.Under different loads,with the advance of the second injection timing,the combustion duration was greatly advanced,and the peak cylinder pressure also increases.When the timing of the second injection lags to or after the top dead center,although the NO emission will be reduced,the lower combustion temperature also limits the fuel's high-temperature reaction path,the unburned CH4 and CO emissions will be increased due to the serious post-combustion. |
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