Numerical Simulation Of The Effect Of Turbulent Jet Ignition On Lean Combustion Characteristics In Gasoline Engine

In pursuit of higher thermal efficiency of engines,lean burn technology has become one of the most promising technologies.However,factors such as poor ignition stability and slow flame propagation in lean-burn limit its large-scale use in engines.To solve this problem,turbulent jet ignition(TJI)has become a current research hotspot.The turbulent jet ignition technology utilizes a small-volume pre-chamber to ensure the reliability of in-cylinder ignition,and the high-temperature active products carried by the jet flame accelerate the combustion of the mixture.Due to the complexity of the jet ignition process in the engine and its many influencing factors,the current research on the turbulent jet ignition technology needs to be improved.Based on the self-designed jet ignition device,this paper employs the CFD software to analyze the jet ignition process and its influencing factors in the engine.Firstly,the research on the jet ignition process in the engine is carried out.In this paper,three-dimensional model of jet ignition engine is established based on CONVERGE software,and RNG k-ε turbulence model,KH-RT spray model and GEquation coupled SAGE combustion model are selected.The results show that the model can accurately predict the working process of jet ignition engine;due to the active fuel injection of the pre-chamber,there will be three stages of interaction between the pre-chamber and the cylinder.In the initial stage,the mixture in the cylinder enters the pre-chamber due to the entrainment phenomenon caused by the high-speed oil beam.The pressure change between the pre-chamber and the cylinder leads to the generation of flow in the second and third stages;the mixture in the pre-chamber can be divided into three areas at the ignition timing: local rich mixture,homogeneous mixture and incylinder flow to the pre-chamber mixture.Among them,the generation of the local over-rich area is caused by the flow in the pre-chamber;the jet ignition process can be divided into three phases based on the temperature at the nozzle and the change of the fuel quality: the cold jet、the jet flame ignition、the in-cylinder mixture is fully burned.A reverse jet will be formed during the third phase.The jet ignites the in-cylinder mixture from the jet head;the division of the jet ignition process in the experimental study can be realized based on the in-cylinder heat release rate derivative.The jet ignition process defined by this method has good agreement with the simulation analysis.Then,the effects of injection timing,injection quantity and ignition timing on the jet ignition in the engine are studied in the fuel-injected pre-chamber device.The results show that the advance of fuel injection in the pre-chamber will cause the mixture to diffuse into the cylinder,while the delayed fuel injection timing will lead to the delay of fuel evaporation in the pre-chamber,both of which will worsen the pre-chamber combustion.-180 °CA is the best choice,when the jet is formed the fastest and the jet ignition effect is optimal;the pre-chamber mixture burns rapidly at the stoichiometric ratio and can quickly form a jet.However,when the pre-chamber mixture is slightly richer than the stoichiometric ratio,the high-temperature active OH radical are most generated,so the effect of igniting the in-cylinder lean mixture is ideal.When the prechamber mixture is too rich,the incompletely combusted mixture will enter the cylinder with the jet;the change of the ignition timing will affect the mixture density in the prechamber and the initial pressure and temperature,which will further affect the jet formation and the jet ignition effect.Delaying the ignition timing can accelerate the formation of the jet and increase the jet ignition energy to a certain extent;the NOx in the jet ignition engine is mainly generated in the pre-chamber under the lean combustion condition,and the Soot mostly comes from the cylinder.Changes in fuel injection quantity and ignition timing will affect the combustion temperature in the prechamber and thus change the amount of NOx generated.Finally,the effects of scavenging concentration and scavenging timing in the scavenging pre-chamber device on the jet ignition in the engine are analyzed and explored.The results show that the three-dimensional numerical model can accurately capture the jet ignition process of the scavenging pre-chamber,and the jet ignition of the scavenged and fuel-injected pre-chamber is basically the same;The scavenging prechamber can sweep the exhaust gas in the pre-chamber under high EGR rate conditions to ensure ignition reliability.The increase of the scavenging concentration increases the equivalence ratio of the mixture in the pre-chamber,accelerates the combustion rate,accelerates the jet formation,and increases the number of high-temperature active products in the jet.However,the improvement of the scavenging concentration on the combustion rate in the pre-chamber is limited,which is determined by the distribution of the mixture in the pre-chamber.Under the condition of high EGR rate,the delay of the scavenging timing reduces the carbon dioxide in the pre-chamber,which is beneficial to the spark ignition.At the same time,the equivalence ratio of the mixture in the central area of the pre-chamber will also decrease,resulting in a decrease in the flame propagation speed in the pre-chamber;delaying the scavenging timing increases the quality of fuel and oxygen in the pre-chamber,increases the high-temperature active radicals in the jet,and enhances the ability of the jet to ignite the mixture in the cylinder.