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)

1