Mechanism Study Of Ultra-low Soot Formation Under Diesel Engine Low Temperature Combustion

The impact of engine-out emissions on human health and atmosphere pollution hadbeen became prominent as the quantity of internal combustion engine increasing inword wide range. As more strict regulations on soot emission with increasingemphasis on the emitted soot particle size have been imposed, diesel engine lowtemperature combustion (LTC) technology had been proven to achieve ultra-low NOxand soot emissions with high thermal efficiency simultaneously, by synergy control ofmixing and chemical parameters. The very low soot emissions call for furtherunderstanding on soot formation mechanism so that to promote the capability ofprediction in CFD code.Based on diesel engine test operation, a new multi-step soot model MSP-CH wasdeveloped in terms of gas-phase chemistry kinetics and particles dynamics, applyingfor soot prediction of diesel engine LTC. Acetylene was taken as the main species ofPAH formation and soot surface growth. So a kinetic model of acetylene wasintegrated into the soot model for acetylene history prediction in complicatedcombustion environment. The modified HACA soot surface growth mechanism wasaccepted as core theory of soot formation. And the effect of UHC on chemicalactivation of soot surface is considered. By correlation study of soot formed andcombustion factor, the parameter of the fraction of active sites αCHwas improved withlocal flame temperature and mixture homogeneity in diesel stratified combustion. Thenew soot model results agreed well with experimental findings in a wide range ofdiesel engine stratification combustion.It was found that soot surface growth plays dominated role on soot formation. Therate of soot surface growth mainly depends on two factors: one is the fraction ofactive sites αCH, another is specific soot surface growth rate RCH. In diesel engine LTCconditions, when EGR ratio increases, although soot precursor formation is increased,the reduction of soot formation is caused by lowered rate of soot surface growth dueto lower combustion temperature. Mixture inhomogeneity promotes the αCHand RCH,leading higher soot surface growth rate and soot formation. For the case of mixture near homogeneous, αCHis mainly decided by combustiontemperature. As mixture inhomogeneity increases, αCHis influenced by both ofmixture inhomogeneity and combustion temperature factors.The effect of thermodynamic parameters and mixture concentrations on the sootformation characteristics was studied under diesel engine “high density-lowtemperature combustion” mode. High charge density was favor to forminghomogeneous mixtures, reducing the degree of soot surface reactivity andconcentration of acetylene. It caused decreased soot surface growth rate. Meanwhile,the higher amount of intake oxygen by increasing charge density accelerated the rateof soot oxidation, which helped reducing engine-out soot emissions.As the intake oxygen decreases, the maximum formation of acetylene was reduced,but the the net formation of acetylene keeps higher during post combustion process.RCHshows decreased determined by the concentration of acetylene and UHC radical,together with combustion temperature. Meanwhile the αCHwas added by rich mixtureof less oxygen case. It was resulted the rate of soot surface growth determined by bothof RCHand αCHwas reduced and then increased as the concentration of intake oxygendecreasing. The degree of “CO cold storage” was enhanced for lower oxygen case,which related with weakened soot re-oxidation. So the emissions of CO and soot weredeteriorated.