A great many different types of aftertreatment systems have been used in connection with internal combustion engines for many years. In many instances, it is desirable to trap particulates in exhaust from internal combustion engines, and exhaust particulate filters or “traps” are widely used for this purpose. While many exhaust particulate filters are quite effective at trapping soot, eventually the quantity of trapped soot reaches a point at which continued operation of the engine becomes problematic or at least less efficient, or risks damaging the filter. “Regeneration” is a term generally used to describe the process of cleansing an exhaust particulate filter of trapped soot. A typical approach involves raising the temperature within the filter to a point sufficient to combust the trapped soot and turn it into less undesirable or more readily treated emissions.
A number of different regeneration techniques are well known and widely used. Among these are the use of catalysts resident in an exhaust particulate filter, borne in the engine fuel or supplied by other means to assist in combusting soot at relatively lower temperatures than what might otherwise be required. Other regeneration techniques rely upon injection of fuel into the exhaust gases, which is subsequently combusted upstream of, or upon entering the exhaust particulate filter, to increase the temperature therein. Still other techniques involve the use of electrically powered heaters and the like, or employ unconventional engine timing and/or fueling techniques. Regeneration technologies relying upon the use of catalysts tend to be quite expensive, whereas techniques employing electric heaters or specialized engine operation tend to siphon off energy from the engine, and fuel injection strategies directly consume fuel.
Regardless of the particular regeneration strategy used, it is desirable to avoid operating an engine with an inordinately packed filter, on the one hand, yet desirable to avoid overuse of energy and reactant-consuming strategies on the other. For these reasons, engineers are continually seeking techniques to more accurately detect an actual amount of trapped soot so that underuse and overuse of regeneration can be avoided.