Abstract
External exhaust gas recirculation (EGR) stratification in diesel engines contributes to reduction of toxic emissions. Weak EGR stratification lies in that strong turbulence and mixing between EGR and intake air by current introduction strategies of EGR. For understanding of ideal EGR stratification combustion, EGR was assigned radically at −30 °CA after top dead center (ATDC) to organize strong EGR stratification using computational fluid dynamics (CFD). The effects of assigned EGR stratification on diesel performance and emissions are discussed in this paper. Although nitric oxides (NOx) and soot emissions are both reduced by means of EGR stratification compared to uniform EGR, the trade-off between NOx and soot still exists under the condition of arranged EGR stratification with different fuel injection strategies. A deterioration of soot emissions was observed when the interval between main and post fuel injection increased, while NO emissions increased first then reduced. The case with a 4 °CA interval between main and post fuel injection is suitable for acceptable NO and soot emissions. Starting the main fuel injection too early and too late is not acceptable, which results in high NO emissions and high soot emissions respectively. The start of the main fuel injection −10 °CA ATDC is suitable.
Highlights
IntroductionIn-cylinder temperature and fuel/oxygen ratio (strongly dependent on fuel character [1]), fuel injection, and air entrainment [2,3] during the total injection-mixing-burning sequence in a diesel engine, are important in determining nitric oxides (NOx) and soot emission levels
In-cylinder temperature and fuel/oxygen ratio, fuel injection, and air entrainment [2,3] during the total injection-mixing-burning sequence in a diesel engine, are important in determining nitric oxides (NOx) and soot emission levels
For internal exhaust gas recirculation (EGR), exhaust gas retention controlled by a large negative-valve overlap (NVO) can result in significant stratification in the temperature and EGR distributions for a wide operation range of Homogeneous charge compression ignition (HCCI) combustion
Summary
In-cylinder temperature and fuel/oxygen ratio (strongly dependent on fuel character [1]), fuel injection, and air entrainment [2,3] during the total injection-mixing-burning sequence in a diesel engine, are important in determining NOx and soot emission levels. Local high EGR in the cylinder improves EGR effectiveness and avoids reduction of combustion efficiency and fuel consumption due to low oxygen [26,27]. For internal EGR, exhaust gas retention controlled by a large negative-valve overlap (NVO) can result in significant stratification in the temperature and EGR distributions for a wide operation range of HCCI combustion. It causes high in-cylinder temperature and less fresh charge due to hot EGR [28]. The effects of the start of main fuel injection and interval between main and post fuel injection on EGR stratification combustion were discussed in this work
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