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Diesel particulate filter (DPF) systems are designed by combining different filter materials with selected regeneration methods. A classification of diesel filter systems based on the principle of regeneration is shown in Figure 1. A diesel filter system must provide reliable means of regeneration, preferably a fully automatic regeneration occurring without an intervention (or even knowledge) of the vehicle operator. An alternative would be to use disposable filters—a solution that has been accepted only in certain very specialized niche market applications.
Figure 1. Classification of Filter Systems by Regeneration Method
The majority of regenerating (as opposed to disposable, Figure 1) diesel filter systems utilize thermal regeneration, during which the particulates are oxidized to produce gaseous products. The temperature of the diesel exhaust gas is, however, too low to sustain auto-regeneration of the filter. That problem may be solved by either (1) decreasing the required soot oxidation temperature to a level which is reached during regular engine operation or (2) increasing the temperature in the filter to the point where the trapped soot starts oxidizing. The first approach is used in passive filter systems, the second in active filter systems.
Passive Systems. In passive systems the soot oxidation temperature is lowered to a level allowing for auto-regeneration during regular vehicle operation—a task commonly achieved by introducing an oxidation catalyst to the system. The catalyst can promote oxidation of carbon through two mechanisms (as discussed in the paper on filter regeneration):
Through different placement of the catalyst and different system configuration one can utilize either one of these mechanisms or their combination. Three major approaches have been used: (a) adding a catalyst precursor to the fuel as an additive, (b) placing the catalyst directly on the filter media surface, or (c) using an NO2 generating catalyst upstream of the filter.
Active Systems. The second approach is to actively trigger regeneration by raising the temperature of soot trapped in the filter through the use of an outside energy source. There are two obvious energy sources that are available on-vehicle: diesel fuel and electricity. The energy from fuel combustion can be used to increase exhaust gas temperature by either (1) in-cylinder engine management methods, such as late cycle injection of additional fuel quantities, or (2) injection and combustion of fuel in the exhaust gas. If exhaust gas combustion is used, fuel can be burned in a fuel burner or else oxidized over an oxidation catalyst, in a catalytic combustion process.
Electric heating can be used in a number of configurations, such as—for example—placing an electric heater upstream of the filter substrate, incorporating heaters into the filter media, or using electrically conductive media (such as metal fleece) which can act as both the filter and the heater. A stream of heated air can be also utilized to trigger regeneration, rather than heated exhaust gas.
The third category of systems in Figure 1 utilize the combination of passive and active regeneration, where a catalyst-based filter is also equipped with some kind of an active regeneration system. Some authors refer to this approach as the passive-active system, others call it the quasi-active filter, still others simply consider it a form of the active regeneration system. The use of catalyst allows to perform regeneration at a lower temperature and/or to shorten the regeneration time period, compared to non-catalytic active systems. In either case, the fuel economy penalty associated with active regeneration can be minimized (at an added cost of the catalyst). Regeneration at a lower temperature also results in lowering thermal stress and increasing lifespan of the filter media.
Passive-active combinations, depending on the type and loading of the catalyst, may be able to sustain fully passive operation during periods of increased exhaust temperature. For instance, a catalyzed filter in a passenger car may regenerate passively during fast highway driving, but will depend on active regeneration—which could be triggered by an engine management strategy—during low speed city driving.
We extend our appreciation to Rahul Mital of General Motors who provided the SEM images that are used in Figure 10.