Diesel Filter Systems

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Abstract: Diesel filter systems are designed by combining filter materials with regeneration methods. The biggest challenge in the system design is to ensure adequate regeneration and durability. Based on the principle of regeneration, filter systems are classified into passive and active. Considering low exhaust temperatures in many diesel applications, both light- and heavy-duty, filter systems for new engines and vehicles typically utilize active strategies to support regeneration. Passive filter systems have been used for selected retrofit applications.

Classification of Systems

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. The alternative non-regenerating approach involves the use of disposable or manually cleanable/washable filters—a solution that has been accepted only in certain very specialized niche market applications.

The majority of regenerating diesel filter systems, Figure 1, utilize thermal regeneration, during which the particulates are oxidized to produce gaseous products. However, the temperature of diesel exhaust gas is typically 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 types of mechanisms (as discussed under Diesel 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, and/or (c) using an NO₂ generating catalyst upstream of the filter.

Increasingly, the term passive regeneration is used to refer to soot oxidation by NO₂. Systems using fuel borne catalysts (FBC) are rarely used in new applications and the effectiveness of catalyzed soot oxidation via oxygen is debatable. Thus, the vast majority of new passive systems rely on an oxidation catalyst that promotes the production of NO₂. However, for the purposes of this discussion, passive systems will also include those using a FBC as well as systems with catalysts that promote oxidation via oxygen—the key distinguishing feature being sufficiently high rates of soot oxidation under exhaust temperature conditions typically encountered in normal operation, regardless of the oxidation mechanism.

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 a heat-up 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 also be utilized to trigger regeneration, rather than heated exhaust gas.

In most cases, active systems raise the exhaust temperature high enough to promote the uncatalyzed oxidation of soot by oxygen (i.e., > 550°C). In most diesel engine applications, such high temperatures can only be achieved via active management of the exhaust gas temperature. Thus the term active regeneration is commonly used to refer to filter regeneration via this mechanism. However, it is important to note that exhaust gas thermal conditions can also be actively managed to promote soot oxidation via NO₂. While strictly speaking these are active systems, the exhaust temperature requirements are lower and can often be achieved via engine management measures without having to resort to significant amounts of direct exhaust gas heating.

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 diesel vehicle 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.

Sometimes active systems that rely on active exhaust gas management measures to promote soot oxidation via NO₂ are referred to as active-passive or passive-active systems. While this may be consistent with the usage of the term passive regeneration via soot oxidation via NO₂, it is not consistent with the more general usage of the terms adopted here.

Acknowledgements

We extend our appreciation to Rahul Mital of General Motors who provided the SEM images that are used in Figure 9.