during the fuel combustion process throughout the
soot particles, results in regeneration temperatures
as low as 350°C. 132'14°J'~'161 Although stochastic
(erratic) regenerations may be observed even down to
200°C under favorable engine and filter operating
conditions (Fig. 21)) 43 Minor secondary effects that
may arise by the use of catalytic fuel additives include
incomplete filter cleaning and filter backpressure
increase, due to the retaining of fuel additive ash
after regeneration.
Erratic regeneration behavior observed at low
temperatures ]4°'143 has its roots, in part, in the
presence of the volatile hydrocarbon. The volatile
fraction originates from unburned fuel and lubri-
cating oil and is found adsorbed/condensed on-the
soot particles at temperatures below 190°C (this
part is completely gasified and desorbed at higher
temperatures). 132't61 In this case, the regeneration
initiates in specific channels of the monolith, where
the local soot loading and temperatures are favorable.
The associated heat release, apart from the soot,
gasifies and ignites the volatile hydrocarbons, thus
enhancing the propagation of the reaction. These,
unpredictable, low-temperature regenerations may
also be 'ignited' by hot particles originating from
engine deposits formed by the additive.143
Several mechanisms have been suggested to explain
the catalytic activity of metal compounds on carbon/
soot oxidation/combustion. 162:63 Due to the strong
oxygen concentration dependence of the action of
transition metal oxides, a redox mechanism 164-16s is
usually invoked to explain their catalytic activity: the
additive stores and exchanges oxygen atoms with the
surrounding carbonaceous matrix and gas. However,
additional mechanisms involving catalyzed thermal
decomposition of water vapor and soot oxidation by
hydroxyl radicals, 164 or electron exchange among
additive and carbon atoms resulting in a weakening of
the carbon bonds in the boundaries of the carbon
matrix facilitating this reaction with oxygen, ~62 have
also been proposed.
The commonly used transition metal additives for
promoting filter regeneration, form more than one
type of oxide, corresponding to the possible valence
states they can assume. We can, therefore, distinguish
between the metal being in 'higher' or 'lower'
oxidation state. The approach presented here refers
to regeneration catalyzed by transition metal fuel
additives, existing as well dispersed oxides with
varying valence states inside the porous soot deposit
in the filter. Soot oxidation by the catalyst oxides,
triggers the ignition of the remaining soot. The fuel
additive participates in the combustion process,
leaves the combustion chamber and accumulates in
the filter together with the emitted soot. Typical
filtration efficiencies for additives are usually over
95%. 139'143'161 We can assume that during this process
each metal additive molecule is bonded with a number
of soot constituents, such as carbon and hydrocarbon
molecules. By reaching the filter the metal additive is