43 424 0。0849 10。6 204 。1 0。31 13。6 171 35 1。66

44 416 0。0781 7。7 169 。2 0。31 13。3 186 31 1。71

Figu re 4。  Soot consum ption rat e vs temperatur  e。

As a first step for th e ana lysis an d un derstan ding of th e expe rimenta l results, it is reasona ble to assum e that th e followi ng genera l soot oxidat ion reactions prevail un der real-world conditions:

C + R1O2 f 2(R1 - 0。5)CO2 + 2(1 - R1)CO (1)

C + R2 NO2 f R2 NO + (2 - R2)CO + (R2 - 1)CO2

(2)

where R1 an d R2 ar e indexes of th e completeness of th e reactions,20 chara cterizing th e selectivity towar d CO or CO2 production。

The direct reaction with O2 is ment ioned  here only for reasons of completenessbe cau se it is un likely to be significant at temperatur es below 450 °C, which is th e case in all of our measur ement s。 Therefore, an y soot consum ption will occur from reaction with NO2。 By measur ement of th e exhau st flow rat e m˘ g, th e CO an d NO production in th e filter, it is possible to calculat e R2 of reaction (2) as well as th e app ar ent rat e of soot consum ption by  NO2  as below。

From th e stoichiometr y of reaction  (2):

∆(NO)/∆(CO)

R2 ) 21 + ∆(NO)/∆(CO) (3)

The value of R2 for all measur ement point s is prese nt ed in Table 4。 When th e low-temperatur e conditions  (<280

°C) associat ed with negligible  reactivity ar e excluded, th e index R2 genera lly ran ges betwee n 1。65 an d 1。75。 From th e stoichiometr y of reaction (2), th e rat io of CO: CO2 in th e products will ran ge from 1:1。85 to 1:3。 No systemat ic depe ndence on temperatur e or oth er condi- tions could be foun d from our  data 。

From th e ma ss balance of reaction (2), th e app ar ent rat e of soot consum ption by NO2 in kilogram s per second will be

As expected, th ere is an increasing tr end of th e reaction rat e with temperatur e。 The reaction rat es ar e, in genera l, in th e sam e order of ma gnitu de with th e engine-out soot emission rat es。 The significant scatt er- ing of th e data aroun d th e tr endline is expected if one tak es into account th e lar ge difference of operat ing conditions in each mode, as shown in Table 4。 In oth er words, th e app ar ent reaction rat e is, th erefore, not a single function of temperatur e only but  is  likely  to depe nd on th e NO2 concentrat ion, flow rat e, an d filter soot loading。

It could be possible to obta in a complex app ar ent-rat e exp ressi on that could fit th e expe rimenta lly obse rved reaction rat es as a function of all of th e above-mentioned param eters。 Such an app roach might be mean ingful for th e description of th e reaction process in th e specific engine-filter configurat ion but could be of litt le help towar d un derstan ding th e reaction phenomena an d applying th e  res ults in a  different configurat ion。