Under the conditions met in diesel exhaust systems regarding exhaust flow rate and oxygen content, the required reaction rates for complete filter regeneration 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. 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. Figure 20 presents
simplified mechanisms of soot and additive deposition
and combustion in diesel particulate filters, catalyzed
or assisted by the use of additives mixed in the fuel or
in the exhaust stream