| Under the current energy background of the "dual carbon" policy,increasingly stringent emission regulations have gradually tightened the emission requirements of engines under transient conditions,especially in the future,the emission limits of particle quantity and quality in the National Seventh Emission Regulations will become more and more important.At the same time,the particulate emissions of automotive diesel engines represented by compression ignition engines are the main source of particulate emissions in the transportation industry,so the optimal control of compression ignition engine performance under transient conditions and particulate emissions is very important.In this study,a 2.8L vehicle compression-ignition supercharged diesel engine was used as the research and modification object,and the control system,the self-built intake port and in-cylinder dual-injection system,the constant volume bomb spray test platform and simulation software to carry out research on the combination of thermodynamic engine bench test and simulation simulation,to study the variation law of the performance,combustion,heat transfer and emission parameters of the compression ignition engine under typical transient conditions,from the characterization parameters Starting from the macroscopic and microscopic perspectives of the mechanism,it reveals the reasons for the deterioration of the transient performance of the compression ignition engine and particulate emission,and according to the causes of the deterioration of the particulates,a layer-by-layer optimization is formulated from the three aspects of engine injection parameters,fuel physicochemical properties and combustion mode.The main research contents and conclusions are as follows:1.In order to clarify the parameter differences between steady-state and transient loading conditions and the particulate emission characteristics under different transient loading conditions,this paper firstly studies the effects of different loading conditions(loading time,initial speed,initial load and EGR state).The influence of particulate emission characteristics was explored,and the differences in performance,combustion,heat transfer and emission parameters between steady-state and transient operating conditions were explored,and the causes of particulate emission deterioration were systematically summarized from a macro perspective.The study found that in the transient loading condition,the air supply delay caused by the turbo lag of the supercharger first made the oil supply and air supply responsiveness mismatched.Compared with the steady state condition of the same load,the intake air volume was reduced,so that the combustion parameters in the cylinder were reduced.The pressure and heat release rate decrease,the average combustion temperature in the cylinder decreases,the combustion rate slows down and the combustion phase lag causes the combustion performance to deteriorate,which further leads to the increase of the temperature of the inner cylinder wall and the cylinder head,the increase of the heat transfer coefficient,and the increase of the proportion of heat transfer.,the combustion thermal efficiency decreases,and the heat transfer loss increases.Finally,due to the difference between the performance,combustion and heat transfer parameters of the transient operating conditions and the steady state,the boundary conditions of the in-cylinder environment change,resulting in a sharp deterioration of particulate emissions under transient operating conditions.At the same time,for different loading conditions,when the loading rate is larger,the initial rotation speed and initial load are smaller,and when a large EGR rate is introduced,the particle deterioration is more serious,the number concentration of small-sized nucleated particles and the mass concentration of larger-sized aggregated particles occur Significant deterioration.2.From the microscopic point of view of chemical reaction mechanism,the effects of differences in environmental conditions(temperature,pressure and equivalence ratio)between steady-state and transient conditions on the formation of initial flame and the generation of aromatic hydrocarbon precursors and soot precursors PAHs were studied.,and further explore the influence mechanism of the most sensitive environmental oxygen concentration on PAHs,and summarize the range of equivalence ratio to control the deterioration of particles and meet the requirements.The results show that the oxygen concentration is the most sensitive and the most important influence on the aromatic hydrocarbon precursors and PAHs,while the difference of temperature and pressure is the secondary factor affecting the formation of soot particles.The relative yield of the bulk increases,resulting in an increase in PAHs,which in turn leads to a sharp deterioration in the number of particles.3.Based on the above conclusions,the fuel injection parameters were firstly optimized,and the effects of main injection timing,fuel injection pressure,and post-injection parameters on particulate emissions during transient loading were studied by single-factor experiments.combustion and PAHs emissions.Then,the orthogonal experiment was designed by Taguchi method,and the optimal control combination with specific particle quantity and mass concentration as target parameters was given with the help of signal-to-noise ratio and variance analysis,and the weight ratio of each control parameter to particle emission was clarified.The results show that under transient loading conditions,the PAHs and particulate emissions can be reduced by advancing the main injection timing based on the original MAP,increasing the rail pressure,and introducing post-injection with appropriate proportions and intervals.At the same time,the number and mass concentration of particles and the optimal combination of injection parameters for different modes of particles are different.Generally speaking,there is a trade-off trade-off relationship between the influence of control parameters on nucleated and aggregated particles,and the influence of injection parameters The larger weights are fuel injection timing,that is,the influence of fuel injection timing and main post-injection interval.4.In order to further improve the degree of fuel mixing and reduce the local over-rich area in the cylinder,five kinds of petrochemical diesel fuels with different ignitability and volatility were selected,and the effects of fuel physicochemical properties on spray characteristics and particulate emission characteristics were experimentally studied.The study found that increasing the rail pressure and reducing the environmental back pressure can increase the spray penetration distance,spray area and spray cone angle;although the highly volatile fuel reduces the spray penetration distance,the increase of the spray cone angle increases the total spray area,which is beneficial to Improve oil and gas mixing.For fuels with different ignitability,it is advisable to use moderate cetane number for small loads,fuel with lower cetane number for medium and large loads to reduce the number of particulates,and fuel with higher cetane number to improve particle quality.For fuels with different volatile and aromatic contents,low-load fuels with moderate volatile and aromatic contents should be used,and the influence of aromatics under medium-to-large loads increases,so the fuel with the smallest aromatics content should be used.Under transient loading conditions,the use of fuel with higher cetane number,better volatility and lower aromatic content can appropriately reduce particulate emissions.5.The above research found that by optimizing the fuel injection and fuel properties,the degree of improvement of particulates is only within the same order of magnitude,meanwhile,new combustion modes represented by low-temperature combustion have been shown to have more potential in terms of energy saving and soot emissions,but particulate emissions cannot be directly inferred from soot emissions measurements alone.This section combines injection parameters and fuel properties,and based on the concept of improving fuel premixing,three dualfuel active stratified combustion modes with different premixing degrees are proposed for full load conditions.The transient operating condition test was carried out to explore the particle emission characteristics of three transient operating points of low,medium and high load under different combustion modes,and to determine the correlation between the soot precursor PAHs and particles of different particle sizes.The results show that the fully premixed,highly premixed and partially premixed dual-fuel combustion modes all significantly improve particulate emissions compared to the original engine.During transient loading in the small load range,a highly premixed dualfuel combustion mode should be used in medium and small loads(10%-60%),and a partially premixed dual-fuel combustion mode should be used in larger loads(60%-90%).At the same time,in the soot precursor PAHs,A1 mainly affects the formation of nuclear particles,while A4 mainly affects the aggregation of particles.In summary,this study has revealed the transient combustion and particulate deterioration mechanism of compression ignition engines through thermodynamic engine tests,optical spray characteristics tests,chemical kinetics simulations,etc.,and proved that the critical boundary conditions affect transient conditions.The chemical reaction mechanism of the generation of particles under variable conditions has clarified the optimization range and criteria of sensitive distortion parameters and combustion boundary conditions,and proposed a technical approach to suppress the combustion deterioration and particulate deterioration of the compression ignition engine under transient conditions,and at the same time solve the problem The reaction activity control compression ignition combustion mode lays the foundation for the key engineering problem of stable operation under transient conditions. |
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