Polycyclic Aromatic Hydrocarbon Generation Mechanism Construction And Multi-dimensional Numerical Simulation Research During Low Temperature Combustion Process Of Diesel Engines

The increase of the environmental concern and the more stringent regulationsabout pollutants emissions from internal combustion engines make necessary the searchof alternatives for current automotive engine combustion processes. One of thetechnologies which is receiving attention is the homogeneous charge compressionignition (HCCI). However, with the development of the studies on HCCI and deeperunderstanding of HCCI combustion, researchers realize that diesel HCCI combustion, ingeneralized concept, does not mean absolute uniform mixture combustion in cylinder,but including a series of new combustion mode, such as stratified combustion,low-temperature combustion and so on, which exist due to stratified concentration incylinder gas mixture. Because this kind of combustion mode is different from thetraditional diesel and gasoline engine combustion, therefore, particulate matter(PM)generation, evolution and physical and chemical properties is inevitable different fromtraditional diesel and gasoline engine. Soot is the main component of PM in dieselengine combustion, which often accounts for50%-80%of the PM. It is determined asinhalable particles and cause serious harm to human health. Especially PAHs which areconsidered as soot precursors are considered highly toxic for human beings and severalPAHs species are carcinogenic, mutagenic, or teratogenic. So, more and more attentionshave been paid on their formation and growth. PAHs formation mechanism and effectsof combustion boundary conditions on the low temperature combustion and PAHsemissions was investigated using fully coupled multi-dimensional CFD and reducedchemical kinetics model.Based on the diesel surrogate fuel (n-heptane and toluene), a new kineticmechanism of diesel surrogate fuel included polycyclic aromatic hydrocarbon (PAHs)formation and growth of up to five aromatic rings was presented. The new mechanismincludes153species and697reactions. Comparisons with various experimental dataavailable in the literature including shock tube, n-heptane premixed flames andhomogeneous charge compression ignition(HCCI) engine show good performance forHCCI combustion prediction of this new mechanism. The presented mechanism can beused as the basis for further reductions and applied the combustion and emissionsimulation by coupling chemical reaction kinetics model with computational fluiddynamics model in internal combustion engines. A reduced mechanis,which could couple with a multidimensional computationalfluid dynamics code,was developed for the chemical kinetics of diesel surrogate fueloxidation in modeling polycyclic aromatic hydrocarbon formation under HCCI dieselengine condition.The complete kinetic mechanism,which comprised697reactions and153species,was reduced to a minor mechanism that included only141reactions and75species by using the sensitivity analysis and reaction path analysis. Validation of thepresent mechanism was also performed with experiments from shock tube available inthe literature and modeling results of detail mechanism in shock tube and HCCI engine,good agreement was obtained. The results showed that this reduced mechanism gavereliable performance for HCCI combustion predictions.Finally, three-dimensional CFD model coupled with reduced chemical dynamicsmodel was established to study combustion and emission process of diesel engine underthe low temperature combustion condition. The results indicate: When intaketemperature was reduced, ignition delay time was longer, at the same time, with intakedensity increasing, it makes airinput and air-fuel ratio increase, the generation of PAHscomponent is relatively low. With the increasing of intake pressure, the generation timeof PAHs components was advanced and the content of PAHs components was reducedin turn. Overall, the emissions of PAHs and soot are at a lower level under the lowtemperature combustion. And the low temperature combustion method can reduce theproduction of PAHs and soot effectively.For pilot injection, smaller oil amount of pilotinjection should be choosed in fixed pre-main injection interval angle conditions,thiscan reduce Nox emission but can not cause increased soot emission. For post injection,larger oil amount of post injection should be choosed in fixed main-post injectioninterval angle conditions,this can reduce soot emission but can not cause increased NOxemission.