디젤기관 금속폼 필터의 입자상물질 저감특성에 관한 연구

Abstract

The DOC(Diesel Oxidation Catalyst) and DPF(Diesel Particulate Filter) have been widely used to reduce the emissions from vehicles which are powered by diesel engine. As for a filter type of the DPFs, wall flow type ceramic honeycomb filters have been commonly used and developed steadily so far. However, there are still some needs to improve the current DPFs characteristics such as high cost for mass production and durability in terms of performance. When compared to the current DPFs which have above weak-points, metal foam is rising as the one of promising alternative materials because of its cost effectiveness, good thermal conductivity and high mechanical reliability. The purpose of this research is to analyze the P.M. reduction characteristics of some variant rolled-metal foam filters, which consist of a respectively different pore size and rolling thickness. A number of metal-foamed filters consisting of a different pore size(450㎛, 580㎛ or 800㎛) and rolling thickness (20mm or 30mm) were tested with 1.9 L diesel engine on an engine bench so that P.M. reduction characteristics could be evaluated in combination with each different metal-foamed filter as a partial DPF(PDPF). In addition, the metal-foamed PDPF consisting of different pore layers, which have been applying in used vehicle, was tested on road and on the chassis dynamometer using 2.6 liter diesel vehicle to investigate the field applicability. In engine test, the various prototype of PDPFs were inspected and examined in the area of PM reduction rate, HC / NO2 conversion rate, pressure drop and the effect of catalyst loading. Through the test, Pt-catalyzed filter showed more than 80% of P.M. reduction rate and a filter with 800㎛(pore size) did demonstrate relatively low reduction rate by the effect of bypassed exhaust gas through big size pore. In the HC reduction rate test, Pt-catalyzed filter showed 80% reduction efficiency at 80km/h and 120km/h speed conditions where exhaust gas temperature could be over light-off temperature. On the other hand, the catalyzed filter didn't show higher reduction rate at 120km/h in which the exhaust gas temperature is higher than at 80km/h speed condition due to high space velocity which represents the relation between volumetric flow of exhaust gas and filter volume In diesel particulate filter. At 40km/h condition, NO2 rate in NOx was lower than the value of engine tailpipe without filter due to the effects of high light-off temperature of NOx and low exhaust gas temperature at 40km/h. The back pressure of bare filter showed higher than Pt-catalyzed filter under the influence of increased filtering efficiency and non-oxidation of soot by catalyst. Nano-particles from engine were also measured by using SMPS (Scanning Mobility Particle Size) to prepare for the upcoming future regulation and the experimental result showed that Pt-catalyzed filter had low reduction rate of particle number compared to bare filter. Because depth filtering efficiency was worsen on the influence of bare filter's surface changing to be smooth by catalyst loading and increasing of through-out flow rate of exhaust gas. In the field application test under suburban and urban driving conditions, the investigations was performed through on-road testing of the metal-foamed PDPF consisting of a various foam porosity in order to validate the potential of enhanced passive regeneration activity. The on-road test demonstrated that the pressure drop was pretty slow as a result of superior passive regeneration at field operating condition in which the accumulated daily profile of exhaust gas temperature above 290℃ was more 10%. The P.M. reduction rate was achieved to 77% at CVS-75 mode after 80,000km running and the mass emission of P.M. met the requirement (0.013g/km) for Euro Ⅳ. On the other hand, PDPF is known to exhibit “blow-off”behavior (negative efficiency) at a certain soot loading and flow conditions as soot has been accumulated in the PDPF filter. Therefore, in this study, blow-off rate was inspected through measuring smoke value at both upstream and downstream of PDPF on the test rig consisting of artificial carbon loader and air blower. As the result, lower smoke level was measured in the downstream of metal-foamed PDPF compared to codierite PDPF consisting of cross open cell after rapid increase of air flow rate because of uniform soot distribution inside PDPF that may be caused by the piled structure of foam sheets and the such graded foam structure which allocates higher porosity layers near the filter entrance, while allocating lower porosity layers near filter exit. In order to inspect and analyze SLI(Soot Load Index) and verify the regeneration performance, the additional field running test was conducted over four hours in artificial patterns composed of low and high speed conditions respectively. And the result showed that the increasing rate of SLI was reduced to 30% compared to the cordierite DPF in low speed patterns. As same as the result of cordierite DPF, the SLI in high speed pattern was lower than in low speed condition.