| Compared with traditional gasoline spark ignition mode,gasoline compression ignition mode can significantly increase its thermal efficiency and reduce NOx and soot emissions simultaneously.It was demonstrated that low octane number fuel was more suitable than regular gasoline for the compression ignition mode.However,the load ranges in the low octane number fuel studies were quite narrow,and it was still unclear about the ignition behavior and combustion kinetics of the low octane number fuel,like naphtha.This paper investigates how to extend the load range of compression ignition with low octane number fuels on a single cylinder engine and the ignition behavior of naphtha and its surrogates on a rapid compression machine through both experiment and numerical simulation.Based on the Multiple Premixed Compression Ignition(MPCI)and Partially Premixed Compression Ignition(PPCI)modes,this study controls the mixture concentration,the degree of premixing,and temperature simultaneously in order to extend the high and low loads.It is found that naphtha MPCI can operate stably with Indicated Mean Effective Pressure(IMEP)in a range of 0.4 to 1.4 MPa by optimizing the injection strategies and using moderate Exhaust Gas Recirculation(EGR).In the low load extension research of dieseline namely gasoline and diesel blended fuel Partially Premixed Compression Ignition(PPCI),it is demonstrated that increasing intake pressure,reducing injection pressure,and adopting low octane number fuels are effective ways to extend the low load limit.G80 blended fuelwhich contains 80 vol% gasoline and 20 vol% diesel is used to compare different compression ignition modes at three dedicated operating points.The criterion to choose the combustion mode is concluded,which is to achieve the proper stratified concentration firstly,then to increase the degree of premixing,and to make the combustion occur in different regions finally.According to this multi-mode combustion strategy,G80 is operated in a wide range with high efficiency and low emissions.Compared with the original diesel results,G80 can significantly reduce NOx,soot emissions,and the fuel consumption,but with a penalty of high CO and HC emissions.To better understand the combustion process in MPCI and PPCI mode,the two modes are compared by a Computational Fluid Dynamics(CFD)code.It is found that MPCI mode is not only temporal multi-stage combustion,fuel injected at different stages also burns at different spatial regions in the cyclinder,which provides low temperature and more premixture with low NOx and soot emissions.The flame propagation occurs in the MPCI mode,but chemical reactions still domain the combustion process.The ignition comes from the auto-ignition at several spots without flam propagation,while the flame front can be found in the two-stage combustion process.According to the composition of naphtha,a three-component surrogate of the methylcyclohexane reference fuel(MRF)includeing n-heptane,iso-octane,and methylcyclohexane is proposed.The ignition delays of naphtha and its surrogates are measured on a rapid compression machine.It is found that primary reference fuel has shorter first-stage ignition delay than naphtha,while MRF matches the total ignition delay of naphtha well.It is also demonstrated that matching research octane number and H/C of the target fuel is more accurate to emulate the ignition delay of naphtha than matching research octane number and motor octane number in determining the ratio of each components in MRF.And the former method also has a smaller uncertainty.A reduced MRF mechanism containing 95 species and 356 reactions is developed according to Directed Relation Graph with error propagation,sensitivity analysis,reaction path analysis,and isomers lumping.The reduced mechanism is validated in several oxidation conditions,such as shock tube,rapid compression machine,plug flow reactor,premixed flame.Overall,the mechanism can well predict the ignition delay,laminar flame speed,and the major species concentration... |
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