Effect of exhaust gas recirculation (EGR) on diesel engine carbonaceous PM emissions.

Abstract

As one of the most effective ways of reducing nitrogen oxides (NO_x) emission, exhaust gas recirculation (EGR) has been widely used in diesel engines. However, EGR generally shows adversely effective in particulate matter (PM) emissions. The chemical composition of the PM with the application of EGR is not well identified because few previous publications focus on this topic, especially for high EGR rate cases. In this paper, emission characteristics of organic carbon (OC, OC₁-OC₄), elemental carbon (EC, EC₁-EC₂), and particulate semi-volatile organic compounds (SVOCs) including 18 n-alkanes and 20 polycyclic aromatic hydrocarbons (PAHs) for a common-rail diesel engine at mild and high EGR rate conditions (up to maximum achievable level while maintaining stable combustion) were analyzed at four steady-state conditions comprehensively. It can be clearly observed that EGR rate instead of load and speed significantly affects the EC emission under the experimental conditions. EC emission increase with increasing EGR rate, which is divided to two sections, i.e., slight increase from 0 to 30% (mild EGR rate) and sharp increase from 30 to 45% (high EGR rate). TC is dominated by OC₁, OC₂, and EC₁ at low EGR rate, and the fraction of EC₁ evidently increases with increasing EGR rate. It is observed that TC is heavily dominated by EC₂ at highest EGR rate ranges, which corresponds to the lower heat release rates (lower HRR_max at higher EGR rate) and lower air-fuel ratio at these conditions. All the target PAHs increase with increasing EGR rate at the four operation modes. The adverse effect of EGR on PAH emission is less significant than EC emission. Moreover, the effect of EGR rate on the PAH ring distribution is not significant. Both of total ∑C₁₆-C₂₅ and ∑C₂₆-C₃₃ emission rates evidently increase at high EGR rate condition in comparison with those at baseline and mild EGR condition cases, which indicates that both fuel-derived and oil-derived n-alkanes exhibit higher emission rate at high EGR conditions compared with those at baseline and mild EGR condition. The application of EGR helped with other controlling strategies (e.g., fuel injection, after-treatment device) is suggested to suppress the carbonaceous PM formation for the modern common-rail diesel engine.