Under the conditions met in diesel exhaust systems
regarding exhaust flow rate and oxygen content, the
required reaction rates for complete filter regenera-
tion are attained at temperatures above 550°C.
Exhaust temperatures of that order are observed
only at high load operation of the diesel engine, which
are scarcely attained in the driving cycles of the official
tests (e.g. ECE-EUDC, FTP-75 etc.). Thus, special
regeneration techniques are employed, that fall into
three broad categories:
• thermal regeneration by use of engine measures or
by the supply of external energy;
• catalytic regeneration (catalytically coated filter or
fuel doping);
• aerodynamic cleaning (using compressed air to
remove the soot).
In the first category, a significant fuel consumption
penalty must be foreseen to supply the additional
energy required for regular thermal regeneration
during city driving. Catalytic regeneration, on the
other hand, is based on the use of catalysts to
achieve the onset of regeneration at significantly
lower temperatures. The catalyst may impregnate the
porous ceramic wall or be used as a fuel additive,
which is emitted and accumulated in the filter together
with the particulate. The use of catalysts is critical to
the design of a successful diesel filter system, because
it overcomes both problems mentioned above: namely
that of minimizing backpressure levels and that of
sustaining regeneration at low temperatures. 139-145
The use of some catalytic fuel additives results in
regeneration temperatures as low as 350°C, although
stochastic regenerations may be observed even down
to 200°C for high filter loadings.143 Figure 20 presents