Numerical Study On The Combustion Process Of Gasoline Compression Ignition Engines

Gasoline compression ignition?GCI?has been proved to be able to achieve high thermal efficiency and low emission in a wide range of operating conditions through reasonable manipulation of combustion.Researches on GCI numerical simulation mainly focus on the influence of fuel injection strategy and boundary conditions on combustion modes,pressure rise rates and emissions.Three-dimensional CFD software CONVERGE and chemical kinetics software CHEMKIN are combined in this paper to explore the chemical kinetics of the gasoline compression ignition combustion process including elementary reactions as well as path ways,which provides a crucial method for analyzing chemical dynamic characteristics,and is of great significance for understanding the combustion process of gasoline compression ignition.This paper divided the combustion process of GCI with PRF70 as fuel into ignition preparation stage and ignition stage.The main exothermic reactions and the consuming path of main reactants in the ignition preparation stage were analyzed.Results show that the heat released by different reactions contributes differently to the total heat releaserate.HCO+O₂=CO+HO₂,CH₂O+OH=HCO+H₂Oand CH₂CCH₂OH+O₂=CH₂OH+CO+CH₂O are the three reactions contributing the most heat.Four radicals HO₂/OH/H/O are involved in the H abstraction reactions.It shows that the proportion of hydrogen abstraction reaction by HO₂ with both NC₇H₁₆ and IC₈H₁₈ is always dominating as the combustion proceeds.The proportion of hydrogen abstraction reaction participated by H/O increases with the crank angle increasing while the proportion of OH counterpart decreases.With the change of crankshaft angle,the temperature in the cylinder increases,high-temperature pyrolysis proportion of IC₈H₁₈increases,and the proportion of low-temperature oxidation reactions decreases.As combustion proceeds,ignition stage begins.The results show that the three reactions with the greatest contribution to the total exothermic rate in the ignition stage are HO₂+O=OH+O₂,H+O₂?+M?=HO₂?+M?and O+H₂O=2OH,which all belong to the H₂-O₂ reaction system.OH was selected as the flame maker to study the flame propagation.It is observed that there is a flame propagation mode similar to spark ignition engines during the initial stage of the gasoline compression combustion process.Pre-injection mass ratio of 25%and 50%cases were compared and analyzed.It is found that the higher the pre-injection mass ratio,the smaller the stratification of the mixture concentration in the cylinder,and the more uniform the mixture in the cylinder resulting in more ignition spots occuring.