This CTS Knowledge Base article describes the fundamentals of diesel particulate filter (DPF) operation. The article provides a summary of the filtration processes occurring in the DPF, various types of DPFs in common use, and differences in filter operating and regeneration strategies.
Diesel Particulate Matter Filtration
Diesel particulate filters operate by trapping soot particles from the engine exhaust, preventing them from reaching the environment. Unlike a catalytic converter which is designed to reduce gas-phase emissions flowing through the catalyst, the particulate filter is designed to trap and retain the solid particles until the particles can be oxidized or burned in the DPF itself, through a process called regeneration.
The most common diesel particulate filters in widespread use are cellular ceramic honeycomb filters with channels that are plugged at alternating ends, as shown in Figure 1. The ends of the filter, plugged in a checkerboard pattern, force the soot-containing exhaust to flow through the porous filter walls. While the exhaust gas can flow through the walls, the soot particles are trapped within the filter pores and in a layer on top of the channel walls. The honeycomb design provides a large filtration area while minimizing pressure losses, and has become the standard, so-called wall-flow filter for most diesel exhaust filtration applications. Ceramic materials are widely used for particulate filters, given their good thermal durability, with the most common ceramic materials being: cordierite, silicon carbide, and aluminum titanate.
The details of the filtration process are illustrated in Figure 1(b), which shows the soot particles trapped along the inlet channel, which is open at the front end but plugged at the back end. DPFs contain several hundred channels or cells per square inch (cpsi), with the most common being 200 cpsi. Since half of the channels are plugged at the front of the DPF and the other half are plugged at the back of the filter, only half of the filter channels accumulate soot or ash. That is, only the channels open on the inlet side are exposed to the “dirty” exhaust flow, while the channels open to the outlet side remain clean. Given the small pore size and design of the honeycomb filters, DPFs can achieve a particle trapping efficiency of 99% or greater . Due to the high trapping efficiency and DPF cell design, no visible soot or ash should pass through the filter walls. Black streaks or visible soot in the outlet channels are a sure sign of filter failure.
Soot particles are captured and retained in the DPF through a combination of depth filtration inside the filter pores and surface filtration along the channel walls. The inset in Figure 1(c) shows these two processes, where a small fraction of the soot initially accumulates in the filter pores (1) and then subsequently builds a layer along the channel walls (2). As the soot load in the filter increases, so too does the filter’s trapping efficiency, as the accumulated soot provides an additional layer to trap incoming particles. The specific soot filtration mechanisms, whether in the pores on the surface of the walls, plays an important role in determining the overall increase in exhaust back pressure (or pressure drop across the filter), shown in Figure 2.
The porosity of most commercial DPFs ranges from around 40% to 60%. The walls of these filters contain a complex network of pores in the range of 10 – 30 micrometers (microns) in diameter . In a new or clean DPF, the surface of the filter is exposed to the exhaust flow and soot rapidly accumulates in the surface pores. Although only a small fraction of the total soot accumulates in the filter micro-pores, it contributes to a steep rise in filter pressure drop shown in Figure 2. Subsequent soot accumulation in the DPF forms a layer (cake layer) along the walls of the channel, and results in a slower and more gradual rise in filter pressure drop . Depending on the soot loading level and filter type, the pore accumulation can account for 50% of the filter pressure drop, or more, in some cases. The non-linear response of the DPF to material accumulation complicates the determination of filter soot or ash loading levels based on pressure drop alone
In order to reduce filter pressure drop due to soot accumulation, the filter is regenerated through a processes the burns off (oxidizes) the soot. There are two broad categories of regeneration processes, although most commercial applications use some combination of the two. This is particularly true with vehicles or equipment experiencing extended periods of low exhaust temperature operation, such as long periods of idle or low speed/load operating cycles.
Active Regeneration requires the addition of heat to the exhaust to increase the temperature of the soot to the point at which it will oxidize in the presence of excess oxygen in the exhaust. The combustion of soot in oxygen typically requires temperatures above 550 °C (1,000 °F). Since these high temperatures generally fo not occur during normal engine operation, a number of strategies are used to actively increase the exhaust temperature . Active regeneration systems may include the use of a diesel burner to directly heat the exhaust entering the DPF or the use of a diesel oxidation catalyst (DOC) to oxidize diesel fuel over the catalyst as a means for increasing the DPF temperature. Use of the DOC also requires excess diesel fuel in the exhaust, which may be accomplished through a fuel injector (hydrocarbon doser) mounted in the exhaust upstream of the DOC, or through late in-cylinder post injection strategies. Other forms of active regeneration include the use of electrical heating elements, microwaves, or plasma burners.
The use of a DOC in combination with some form of exhaust fuel dosing is the most common active regeneration strategy currently used for on- and off-highway applications. The duration of an active regeneration event generally ranges from 20 to 30 minutes on average, under normal operating conditions. In some cases, such as severe DPF soot plugging, a parked regeneration may be required, which can last up to several hours to slowly burn off the soot under more controlled conditions. Regardless of the specific strategy, active regenerations always require additional energy input (additional fuel) to heat the exhaust and the DPF to the required temperature.
Passive Regeneration, as the name implies, does not require additional energy to carry out the regeneration process. Instead, this strategy relies on the oxidation of soot in the presence of NO2, which can occur at much lower temperatures in the range of 250 °C to 400 °C (480 °F to 750 °F). A catalyst is used to convert NO present in the exhaust to NO2. These catalysts require the use of precious metals to facilitate the reaction, platinum (Pt), in particular, which adds additional cost to the system. In some cases the catalyst coating is applied directly to the DPF, as with a catalyzed DPF (C-DPF), or an upstream oxidation catalyst (DOC) may also be used . Many commercial systems utilize a combination of a DOC and C-DPF. Use of the catalysts allows NO2 to be produced and soot to be oxidized at temperatures which occur during normal engine or vehicle operation.
In an ideal case, if engine operation results in a certain amount of time spent within this passive regeneration “temperature window” then active regeneration may not be needed. In reality however, low temperature operation may occur for extended periods of time, such as long periods of idle or low load operation, particularly in cold climates, and some active regeneration may still be needed. In the absence of active regeneration, periods of low temperature operation may be supplemented by periods of high temperature operation (such as extended highway driving) to induce passive regeneration.
In order to reduce fuel consumption, passive regeneration is preferred, although most commercial systems still use active regeneration to varying degrees, depending on the drive cycle and operating conditions. Regardless of the regeneration method, the oxidation of soot (whether active or passive) results in incombustible material, or ash, which can not be burned, and remains in the DPF. Understanding the key differences between ash and soot, as well as their impacts on DPF performance is important when selecting the most appropriate cleaning method for the filter.
Why is this important for DPF ash cleaning?
Understanding the design and operation of the DPF to collect and trap particles, whether in the pores or on the surface, has a large impact on how easily the particles can later be removed. Soot is fundamentally different from ash in that the soot can be oxidized and removed through regeneration, while the ash is incombustible and remains in the DPF until the DPF is serviced for ash cleaning.
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