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Experimental Investigation on Different After-treatment Technologies for the Control of Pollutant Emissions from Automotive Diesel Engines

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High Speed Direct Injection (HSDI) Diesel engines are nowadays spreading in the global market of passenger cars, thanks to undoubted advantages such as higher efficiency (lower fuel consumption and CO2 emissions), drivability, durability and reliability. Increasing concerns regarding the two main pollutants from Diesel engines, NOx and PM, started a progressive process of tightening of emission limits; Tier 2 (USA) and Euro5b/6 (Europe) emissions regulations will definitively bring down NOx and PM, with also the introduction of new limitations such as, for instance, on PN emissions. Efficient and reliable after-treatment technologies for NOx and PM control are going to be necessary. PN emissions from a modern diesel engine were characterized by means of a Scanning Mobility Particle Sizer (SMPS); in addition, the performance of an Advanced Diesel Oxidation Catalyst (A-DOC), with low temperature NOx trapping capability, and a Diesel Particulate Filter (DPF) were assessed. PN emissions were evaluated under normal engine operating mode, as well as under DPF regeneration mode; unimodal distribution were found at both engine outlet and DOC outlet, without any effects of DOC on the emitted particles; conversely DPF reduction efficiency up to 2 orders of magnitude were observed, thus confirming the potential of DPF to be effective also on these small particles. PN emissions were found to increase during DPF regeneration, but it was shown that both DOC and DPF were effective in reducing PN under similar operating conditions. PN emissions were also evaluated during engine warm-up. A new experimental methodology, able to provide fundamental information about the soot loading process inside the DPF, was adopted to load small lab-size SiC DPF samples; advanced electron microscopy provided highly detailed images of soot deposited onto channel walls. While morphology of deposited soot and its compressibility characteristics were found in agreement with previous studies, it was shown that soot did not penetrate into channel walls, differently from what is reported in literature; conversely, a soot cake layer was taking place immediately. Finally, A-DOC technology was tested over NEDC. Reduction of NOx was found to be significantly high, especially during the urban driving; spontaneous release of stored NOx (without periodic rich regeneration such as for LNTs) was observed at high exhaust temperatures typical of extra-urban driving, highlighting the potential for a combined application with a downstream SCR system. Further tests on a modified A-DOC system without PGM loading revealed that the A-DOC might be the result of a combination of different after-treatment technologies such as LNTs and LNCs with also HC trapping capability

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