Effect of after-treatment systems on particulate matter emissions in diesel engine exhaust

Diesel engine exhausts from a common rail 3.0 L F1C diesel engine were analyzed at two different load conditionsof the WLTC testing cycle downstream of both the diesel particulate filter (DPF) and selective catalyticreactor (SCR) to verify their effect on the characteristics of carbon particulate matter. An array of chemical,physical and spectroscopic techniques (gas chromatography coupled with mass spectrometry (GC-MS), mobilityanalyzer, UV-Visible absorption and fluorescence spectroscopy) was applied for characterizing polycyclic aromatichydrocarbons (PAH), heavy aromatic compounds and soot, constituting the particulate matter (PM)sampled from the exhaust.The engine was operated in half load (HL) (188 Nm, representing the more common condition for engine inurban traffic) and full load (FL) (452 Nm, representing the best performance of the engine operation) conditions,at the same engine speed (2000 rpm). Soot formation was enhanced in HL condition, with respect to FL, but, justbecause of the much lower soot amount, the after-treatment systems in this last condition resulted to be lessefficient in the soot abatement. Indeed, the abatement through DPF was about 40% lower in the FL conditionwith respect to HL condition, and any significant further concentration decrease was found after SCR, in bothconditions. By contrast, PAH concentration after DPF abatement was found to be higher in the HL with respect toFL condition. A further PAH concentration decrease of about 30% was found after the SCR in the HL conditionwhereas in FL the reduction was only about 5-6%. Also the heavy aromatic compounds having molecular weightabove the GC-MS detection limit (300 u), were mitigated by SCR. Therefore, SCR did not cause a further sootreduction, whereas it was effective in largely reducing PAH and heavy aromatics emissions, especially in thelower temperature condition featuring the half-load condition, when combustion efficiency is worse. Moreover,SCR system reduced the emission of small particles probably due to an enhanced agglomeration of particles, withbeneficial effect on the harmfulness to human health.