Construction Of Low-temperature Oxidation Mechanism Of N-heptane And Simulation Of Combustion Process In Cylinder

High-strength diesel engines are widely used in engineering vehicles and special vehicles with high power density requirements.Under high-strength conditions,in order to ensure the strength of the body,diesel engines often use a smaller compression ratio,resulting in a lower temperature in the cylinder at the end of the compression stroke.It brings challenges to the ignition,cold start and low temperature stability of diesel engines.In-depth exploration of the low-temperature combustion process of diesel has a theoretical significance to guide the development of the cold engine working strategy of high-strength diesel engines.Diesel contains a variety of components and often uses marked fuels to study its combustion mechanism.N-heptane has a cetane number similar to diesel,and is often used to predict the ignition process of diesel.In this paper,based on the gas kinetic analysis software CHEMKIN-Pro,the time history of the main elements in the low-temperature oxidation process of n-heptane,the conversion of hot and cool flames,and the ignition delay are studied.A simplified mechanism of 69 speciess and 102 reactions was constructed,and based on this,the zero-dimensional single-zone diesel engine combustion process was studied.A comparative study of the 12 widely recognized oxidation mechanisms of n-heptane was carried out.Based on literature data,the time history and ignition delay period of elemental concentrations predicted by these mechanisms at different temperatures,pressures and equivalent ratios were analyzed.The results show that the detailed reaction mechanism of the Seiser from Lawrence Livermore National Laboratory(LLNT)has the most accurate prediction accuracy under low temperature conditions,and the simplified reaction mechanism of the Patel from university of Wisconsin-Madison performs well in high-temperature environments.The prediction accuracy of the remaining models was insufficient.Based on the simulation analysis of the low-temperature oxidation process of n-heptane,the skeleton mechanism of the core element reaction is determined.Through the yield analysis,sensitivity analysis and path analysis,the key elementary reactions that have a greater impact on the conversion of hot and cold flames are added as the main body of the mechanism,and based on the sensitivity analysis,a high-temperature oxidation reaction with a large influence on the negative temperature coefficient region and the hot flame was added as a supplement,and a "BJT simplified mechanism" containing a 102-step reaction was obtained.The mechanism in the initial temperature range of 600-900 K is extremely consistent with the prediction result of the LLNL detailed mechanism of the lag period.At 900-1000 K,the deviation of the lag period is slightly larger and the maximum error is not more than 11%.Based on the IC reactor model,coupled with the simplified mechanism of BJT,the low temperature cylinder combustion process of diesel was simulated and analyzed.The simulation results show that the increase of the intake air temperature inhibits the specific product-related reactions in the low-temperature oxidation reaction,and at the same time promotes the hightemperature oxidation reaction rate,the phenomenon of premature ignition and increased heat release occurs.The adjustment of the equivalence ratio directly affects the fuel and oxygen content,and has a great influence on the combustion characteristics of the fuel.The excessively rich or lean mixture gas concentration will cause the ignition moment to deviate from the top dead center and affect the normal combustion and working efficiency.However,enhancing the intake pressure in a small range hardly changes the heat release,and has little effect on improving combustion characteristics.