Study On Simplified Chemical Kinetic Model Of Gasoline Fuel Substitute Suitable For Lean Combustion

The continuous increase in car ownership has brought severe energy and environmental problems,and strict emission regulations place higher requirements on internal combustion engine technology(high efficiency and low emissions).The combustion mode represented by lean combustion has gradually become the main research direction for efficient and clean combustion in gasoline engines.Lean combustion of a gasoline engine can effectively improve the combustion efficiency of the engine and reduce HC and CO emissions.However,problems such as difficult fire core formation and slow flame propagation limit the development of lean combustion gasoline engines.In this study,cyclohexane was added to gasoline fuel substitutes as a representative of naphthenic hydrocarbons to meet the needs of physical and chemical properties of fuels under lean conditions and improve the gap in laminar flame velocity prediction in multi-component gasoline fuel substitutes in previous studies.The ignition and combustion characteristics of the chemical kinetic mechanism of gasoline fuel substitutes are studied by using the calculation method of simulation.The mechanism can be coupled to three-dimensional computational fluid dynamics for in-cylinder numerical simulation to explore the problems of in-cylinder pressure,heat release rate,and emission.Firstly,based on the physicochemical properties of fuel,cyclohexane was taken as the representative of cycloalkanes in gasoline components.Aiming at the ignition delay time,the cyclohexane mechanism was simplified by using the direct relationship graph error propagating,rate of production analysis,and temperature sensitivity analysis,and a cyclohexane mechanism containing 81 components and 280 elementary reactions is obtained.The experimental and prediction results of various reactors show that the mechanism can predict the ignition delay time,laminar flame speed,and the JSR oxidation species distribution of cyclohexane fuel.The oxidation path analysis shows that cyclohexane was consumed mainly through dehydrogenation reactions,oxygenation reactions,and direct ring opening reactions.At low temperatures,the oxygenation reaction of cyclohexane was dominant,while at high temperatures,the direct ring opening decomposition reaction shows a large reaction rate.On this basis,The toluene reference fuel simplification mechanism,the diisobutylene simplification mechanism of Zheng et al.,and the above cyclohexane simplification mechanism were coupled by the decoupling method.A simplified model of the chemical kinetics of a five-component gasoline fuel substitute was constructed,which was composed of n-heptane,isooctane,toluene,diisobutene,and cyclohexane,representing paraffins,iso-paraffins,aromatics,olefins,and naphthenes in actual gasoline,respectively,containing 115 components and 414 primitive reactions.The comparison between experiments and model calculations shows that the current gasoline fuel substitute mechanism can predict the ignition delay time,laminar flame speed,and the distribution of important intermediate species of pure components and their multicomponent mixtures under lean and stoichiometric conditions.Compared with real gasoline,the prediction of ignition delay time,laminar flame speed,and homogeneous charge compression ignition cylinder pressure curve of three-component,four-component,and five-component gasoline fuel substitutes is discussed.The chemical kinetic model of five component gasoline fuel substitutes constructed in this study has good prediction performance.Based on the chemical kinetic mechanism of gasoline fuel substitutes,the effects of cyclohexane on the ignition,flame propagation,and homogeneous charge compression ignition in the cylinder combustion characteristics of alternative fuels were analyzed.As the proportion of cyclohexane in the alternative fuel increases,the ignition delay time of the fuel gradually decreases at low and high temperatures.In the medium temperature reaction stage,the addition of cyclohexane inhibits the negative temperature coefficient behavior of the alternative fuel,which becomes more and more obvious with the increase in pressure.The laminar flame speeds of alternative fuels with different cyclohexane ratios were similar,and the small difference was attributed to the corresponding decrease in the proportion of n-heptane with higher flame speed with the addition of cyclohexane.At relatively high engine speeds,the five-component alternative fuel containing cyclohexane needs lower intake air temperature to phase combustion at top dead center and is closer to the in-cylinder ignition of real gasoline.Due to the addition of cyclohexane,the concentration of 1,3-butadiene and benzene in five-component fuel at TDC was higher than that of four-component fuel.