| Stratified EGR technology can flexibly control the spatial distribution of oxygen concentration in the cylinder.By reasonably controlling the oxygen concentration and distribution form in the cylinder,NOx emission can be greatly reduced without causing the increase of particulate emissions.Meanwhile,a large number of studies have shown that oxygenated fuel can significantly reduce the carbon emission of diesel engines.Therefore,different molecular structure of oxygenated fuel and the distribution of oxygen concentration will be combined together in this study,to explore the effect of oxygenated fuel(fuel-oxygen)and the distribution of oxygen concentration(ambient-oxygen)composite mechanism on combustion and emissions for compression ignition engine.By this way,the effect of stratified activation on transient combustion and particles emission could be revealed.Then,it is also possible to improve the transient combustion and to put forward the way to realize High-Efficiency Clean Combustion(HECC).In this paper,the four-valve high-pressure common rail diesel engine is taken as the original engine.Through the transformation of the air intake system,the stratified EGR control system is developed.Stratification Activation Controlled Compression Ignition and emission characteristics measurement platform is established,so as to realize the active control and regulation of the spatial and temporal distribution of element-oxygen and active components.The original generation field of in-cylinder emissions and the method of target exhaust gas introduction are obtained by CFD simulation analysis.On this basis,the effects of different oxygen concentration distributions on combustion and emission of diesel engines were investigated.The results show that,compared with the homogeneous exhaust gas introduction method,the distribution of oxygen concentration in the cylinder formed by the exhaust gas introduced by the spiral tube has a certain impact on the combustion in the cylinder,which will reduce the maximum combustion temperature in the cylinder.Moreover,under a certain range of oxygen concentration in the cylinder,the thermal efficiency can be guaranteed,NOx and particle emissions can be reduced at the same time.In this paper,the effects of oxygenated fuel and oxygen concentration stratification on combustion and emission were studied.It is found that oxygen concentration stratification can inhibit the problem of higher combustion temperature in cylinder caused by n-butanol/diesel mixtured fuel,and thus control the increase of NOx emission.The oxygen concentration stratification can inhibit the increase of the concentration of nuclear particles and total particles caused by n-butanol oxygenated fuel and further reduce the number of accumulated particles.Finally,under the synergistic action of the oxygenated fuel and oxygen concentration stratification,the total number of particles and the number of accumulated large particles can be reduced simultaneously.Combined with the two respective contribution to the pollution control level,the two kinds of effect factors influence degree under different load presents certain difference.With the increase of load,the influence of oxygenated fuel on NOx and soot emission is relatively large,while the effect of oxygen concentration stratification is weakened,and the trend is more obvious with the increase of the proportion of n-butanol blending.Oxygenated fuel form of oxygen bond has some influence on combustion and emissions,by comparing the oxygen levels is the same with different oxygen bond forms of B15 and D05 two oxygenated mixed fuel can be found that the oxygen concentration stratification is not sensitive to oxygenated fuel molecule structure,compared with B15 fuel,the effect of D05 fuel on carbon smoke emission reduction is more apparent.Under the combined impact of oxygenated fuel and oxygen concentration stratification,compared with diesel fuel and B15 fuel,burning D05 fuel under heavy load can reduce the total particle mass and the number of particles at the same time,and will not cause the increase of NOx emission,and its effect on the improvement of accumulated particle emission is more than 80%.The influence of oxygenated fuel and in-cylinder oxygen concentration stratification on combustion and emission under transient conditions was studied experimentally,and the adaptability of oxygenated fuel and oxygen concentration stratification to transient combustion process was confirmed.The results showed that in different loading rate of transient process,the oxygen concentration stratification improves the degree of premixed combustion,improves the transient combustion process,can alleviate the transient conditions carbon smoke emissions increased dramatically,in the process of optimization of transient combustion has certain advantages.Oxygenated fuel can effectively improve the problem of sharp increase of smoke degree caused by oil and gas mismatch under transient conditions,so that the peak value of accumulated particles in the late loading stage of transient process is significantly reduced,and the effect becomes more obvious with the increase of loading rate.The molecular structure of oxygenated fuel has a certain influence on smoke emission in the transient process,but has little influence on NOx.On the one hand,oxygen concentration stratification can further reduce the emission of accumulated particles in the transient process,and the effect becomes more obvious with the extension of loading time.On the other hand,it also inhibited the increase of nuclear particles after adding oxygenated fuel.Therefore,under the coordinated control of oxygenated fuel and oxygen concentration stratification,transient combustion can be optimized to reduce smoke and particulate matter emissions under transient conditions. |
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