Jianguo Yang

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 (NO_x) and soot emissions are both reduced by means of EGR stratification compared to uniform EGR, the trade-off between NO_x 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.

In-cylinder exhaust gas recirculation (EGR) stratification, generally achieved by supplying EGR asymmetrically into intake ports on a four-valve diesel engine, is sensitive to trapped exhaust gas in the intake manifold and intake ports that is caused by the continuous supply of EGR during the valve-close periods of the intake valves. The subject of this study is to evaluate the distribution of trapped exhaust gas in the diesel intake system using commercial Star-CD software (version 4.22.018). Numeric simulations of the intake flow of fresh air and recycled exhaust in the diesel intake system were initialized following previous experiments that were conducted on a reformed six-cylinder diesel engine by supplying CO₂ instead of EGR to the tangential intake port alone to establish CO₂ stratification in the first cylinder. The distributions of the intake CO₂ in the intake manifold and intake ports under the conditions of 1330 r/min and 50% load with different mass flow rates of CO₂ are discussed. This indicates that CO₂ supplied to one intake port alone would escape to another intake port, which not only weakens the CO₂ stratification by diminishing the mass fraction disparity of the CO₂ between the two intake ports of cylinder 1, but also influences the total mass of CO₂ in the cylinder. There is 4% CO₂ by mass fraction in the intake port without CO₂ supply under the condition that the CO₂ mass flow rate is 5 kg/h during the intake process, and 10% CO₂ under the condition of 50 kg/h.