Study On Pre-chamber Jet Combustion Characteristics And Its Impact On Engine Lean Burn Performance

The severe challenges brought by energy and environmental crises are constantly affecting the survival and development of mankind.Since 2015,the transportation industry has become the world’s largest emitter of carbon dioxide,with light-duty passenger cars powered by engines playing a key role.In the short term,the engine will continue to be an important power source for vehicles.Lean combustion technology can optimize engine performance,economy,and emission characteristics from multiple dimensions,showing great potential in the engine research process.Due to limitations in ignition energy and combustion stability,the lean combustion limit of conventional spark ignition engines is relatively low,restricting further improvement in engine thermal efficiency.Pre-chamber jet ignition technology combines the advantages of high-energy ignition,dual-fuel combustion,and stratified charge,significantly expanding the lean combustion performance of engines and potentially breaking through the current bottleneck of engine thermal efficiency,achieving efficient and clean combustion.However,existing pre-chamber studies mostly focus on the influence of single structural parameters on performance,the weight of various structures on engine performance remains unknown,and some research conclusions show obvious conflicts.Moreover,the comprehensive study of the flame development process caused by pre-chamber jet injection into the main chamber during ignition is still lacking.Furthermore,the improvement in lean combustion performance of the current pre-chamber jet ignition system is still limited,requiring further multidimensional research to expand its performance.Therefore,this study independently developed a multi-angle visualized constant volume combustion chamber test platform,simulation platform,and engine test platform,with pre-chamber jet combustion characteristics as the main research object,aiming to expand the lean combustion performance of pre-chamber jet ignition systems.For this purpose,systematic studies were conducted on structural parameters,boundary condition parameters,jet collision wall structure parameters,and pre-chamber fuel injection strategies of pre-chamber jet ignition systems.The main research contents and conclusions of this paper are as follows:（1）To explore the influence of pre-chamber structural parameters on jet flame development and lean combustion performance,this study summarized the current geometric structures of pre-chambers in the database,analyzed the common points among various structures and the reasons for these common points,and extracted key structural parameter data.Based on this,various pre-chamber structures containing most existing pre-chamber structural features were redesigned using the Taguchi experimental method.Then,experimental studies on pre-chamber structural parameters were conducted on a self-designed and constructed multi-angle visualized constant volume combustion system test platform.Through signal-to-noise ratio analysis,the influence of the values of various structural parameters on the combustion performance of pre-chamber jet ignition systems was explored,and sensitivity analysis of structural parameters was conducted using variance analysis.The results showed that a larger total cross-sectional area of the orifices is beneficial for expanding the lean combustion limit of pre-chamber jet ignition systems,while a smaller total cross-sectional area of the orifices is conducive to improving flame propagation speed.The 6-orifice pre-chamber structure can achieve large-scale ignition of pre-chamber jet while the single pre-chamber orifice jet intensity remains at a high level.A 60°-120°transition cone angle in the pre-chamber is beneficial for increasing the jet velocity,and the optimal throat diameter of the pre-chamber is approximately 7mm.The most critical parameter in the pre-chamber structure is the total cross-sectional area of the orifices,and its influence weight on both combustion performance and lean combustion limit performance exceeds 80%.By summarizing the pre-chamber jet combustion characteristics under different structural conditions,four pre-chamber jet ignition modes were summarized:jet flame ignition,jet delay ignition,jet direct ignition,and jet wake ignition.Compared with spark ignition,the peak heat release rates of these four ignition modes increased by 15.78%,123.51%,10.89%,and 136.96%,respectively.Among the four ignition modes achieved by structural parameter variations,only pre-chamber jet flame ignition has excellent lean combustion limit performance,with the ability to ignite heavy lean mixtures up toλ=2.77 under test conditions.（2）Boundary condition parameters are key factors affecting the lean combustion performance of pre-chamber jet ignition systems.Based on a visualized constant volume combustion system,this study investigated the influence of boundary condition parameters on the lean combustion performance of pre-chamber jet ignition systems.The study found that under initial test conditions,as the equivalence ratio of the pre-chamber gradually increased(Φ_pre=0.8-2.5),the combustion speed of the main chamber mixture first slowly increased,reaching a maximum value atΦ_pre=1.8,and then gradually decreased.Overall,the lean combustion limit increases with the increase of the pre-chamber equivalence ratio,which is attributed to the significantly increased concentration of the pre-chamber cold jet,reducing the difficulty of jet ignition.There is a special case where whenΦ_pre=1.5,the jet speed is high,but the increase in cold jet concentration is insufficient,resulting in a sharp decrease in lean combustion limit performance.Within the test range,higher initial temperatures and lower initial pressures are conducive to increasing flame propagation speed,while lower initial temperatures and higher initial pressures significantly increase the lean combustion limit of pre-chamber jet ignition systems.The improvement in lean combustion limit performance is attributed to the increase in initial density of the mixture,which increases the flow resistance of the pre-chamber jet,increases the reaction time and collision frequency of the jet with the mixture in the main chamber,reduces the difficulty of jet ignition,and significantly improves the lean combustion limit performance.By adjusting boundary condition parameters,multiple pre-chamber jet ignition modes were switched under a single pre-chamber structure condition.Among them,dual-jet ignition can achieve both high flame propagation speed and high lean combustion limit.（3）For the pre-chamber jet inevitably colliding with the wall in the narrow space of the combustion chamber and the abnormal combustion phenomenon of secondary jet caused by the collision of the pre-chamber jet discovered during the study of boundary condition parameters,this study actively designed various jet collision wall structures,and combined experimental research and simulation calculations to explore the influence of wall structure parameters on the combustion performance of pre-chamber jet ignition systems.The study found that the collision of the pre-chamber jet with the wall has an accelerating effect on the combustion process.This performance improvement gradually increases and then deteriorates rapidly with the shortening of the distance between the wall and the orifice.When the wall is too close to the orifice,the high-speed jet of the pre-chamber with almost no velocity attenuation makes it difficult to form a flame nucleus at the collision point,leading to a significant increase in ignition delay and a decrease in lean combustion performance.When the pre-chamber jet impacts concave wall structures,it has lower ignition delay,higher flame propagation speed,and higher lean combustion limit.This is attributed to the concave wall structure increasing the flow resistance at the collision point,resulting in higher kinetic energy loss,weaker jet wall attachment flow,stronger pressure trapping effect,and higher stagnation temperature.In addition,when the collision point of the pre-chamber jet is a concave structure,it will generate a larger range of strong vortex flow in the main combustion chamber,further accelerating combustion.Conversely,when the pre-chamber jet impacts convex wall structures,the ignition delay is prolonged,the lean combustion limit is reduced,the flame propagation speed is reduced,and the ignition process transitions to delayed ignition attached to the wall,increasing the probability of abnormal combustion of the secondary jet.（4）Based on the combustion visualization research results of the above structural parameters,boundary condition parameters,and wall structure parameters,this study proposed pre-chamber structural design methods,lean combustion performance optimization strategies based on adjusting pre-chamber fuel injection strategies and engine compression ratios,and lean combustion performance optimization strategies based on adjusting pre-chamber orifice orientations and using adiabatic piston technology.It was found that using high fuel injection pressure,a small amount of pre-chamber mixture enrichment,and injecting pre-chamber mixture stratified fuel at a timing located in the middle of the compression stroke can balance lean combustion stability and fuel economy.The position of the pre-chamber orifice affects the stability of heavy lean combustion of the engine significantly.The pre-chamber orifice should be oriented towards the top of the engine piston rather than the cylinder and cylinder head walls.Under high compression ratio conditions,the pre-chamber heat release process exhibits a bimodal heat release:the first peak of the heat release curve is generated by the pre-chamber jet ignition,and the second peak is generated by the spontaneous combustion of the mixture at the end of the combustion chamber.Adiabatic piston technology can slightly improve the stability of heavy lean combustion of pre-chamber jet ignition systems under compression ratios of 13.6-15.4.When the compression ratio is further increased to 16.4,the stability of lean combustion decreases sharply.By synergistically optimizing the above research results,the lean combustion limit of the original spark ignition gasoline engine was successfully expanded fromλ=1.5 toλ=3.1;the indicated thermal efficiency was increased from 41.34%（λ=1.0,compression ratio=13.6）to over 51%（λ=2.236,compression ratio=16.4）.