The thr ee operat ing point s betwee n 2800 an d 4400 s ar e chara cterized by low flow rat e, temperatur es be- twee n 340 an d 380 °C, an d quite high NOx emissions。 Combined with th e favora ble temperatur e for oxidat ion cata lyst activity, th e NO2 concentrat ion at th e filter inlet reaches values close to 400 pp m。 The backpr ess ur e shows a very sma ll decreasing tr end (bar ely visible becau se of th e gra ph scaling)。 Howeve r, th e soot ma ss assess ment indicat es a net soot consum ption of about
Figu re 2。 Tes t protocol: typical recordings dur ing a series of steady-stat e point s operat ion。
2 g。 The soot consum ption is also visible by compar ing th e measur ed backpr ess ur e at th e reference point at 2800 an d 4400 s。
From 4700 to 8200 s, th e engine is operat ed at conditions of high temperatur e (>400 °C)。 The flow rat e is moderat e-to-high, an d th e NOx emissions ar e betwee n 700 an d 900 pp m。 The NO2 concentrat ion at th e filter inlet is always aroun d 200 pp m。 Looking at th e back- press ur e tra ces, espe cially at th e reference point as well as th e es timat ed soot ma ss evolut ion, it is clear that
th ese operat ing point s ar e chara cterized by net soot accumu lat ion, even though th e temperatur e is high。
Fina lly, betwee n 8500 an d 9600 s th e two operat ing point s ar e chara cterized by low flow rat e, high NOx an d NO2 emissions (250-360 ppm), an d temperatur es on th e order of 370-390 °C。 The backpr ess ur e indicat ions an d th e soot assess ment show clear ly a significant net soot consum ption。
Summar izing th e findings of this expe riment, th e NO2 reactivity with soot an d th e net res ult on filter soot
Figu re 3。 Ope rat ing point s on th e engine speed -load ma p。
loading depe nd on a num ber of param eters in a non- tr ivial way。 The bes t regenerat ion beha vior see ms to
occur at low-flow-rat e conditions, high NO2 concentra - tion in th e filter, an d temperatur es betwee n 350 an d 400 °C。 A thorough un derstan ding of th e phenomenon would be possible by describi ng th e relat ed phenomena in a mechan istic math emat ical model, validat ed by expe rimenta l data 。 This is actua lly th e scope of th e work describe d onwar d。
To generat e a sufficie nt ly lar ge expe rimenta l data - base, 44 operat ing point s ha ve bee n tes ted covering a wide ran ge of combinat ions of different conditions regar ding temperatur e, flow rat e, NO2 concentra - tion, an d soot loading。 Figur e 3 shows th e operat ing point s on th e speed -load diagram of th e engine。 More details about th e conditions prevailing at each operat ing point ar e give n in Table 4。 The num bering of th e operat ing conditions does not refer to th e order in which th ey ha ve bee n conducted。 The soot loading ment ioned in th is ta ble was assessed at th e begi nn ing of each tes t point by th e inverse soot ma ss calculat ion as discussed previously。 It was avoided to work at temperatur es higher than 450 °C to minimize th e contr ibut ion of th erma l soot oxidat ion with O2。 Table 4 also includes th e NO an d CO production in th e filter, as well as th e index of th e NO2-soot reaction selectivity R2, defined below。
Tab le 4。 Tes t Mat ri x
filter inlet temp (°C)
exhau st flow rat e (kg/s)
NOx/soot rat io
NOx filter inlet (g/h)
NO2/NOx
rat io
es timat ed soot loading (g)
∆(NO) in filter (pp m)
∆(CO) in filter (pp m)
index R2
(eq 3)
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