Wall-Flow Monoliths

W. Addy Majewski

This is a preview of the paper, limited to some initial content. Full access requires DieselNet subscription.
Please log in to view the complete version of this paper.

Abstract: Wall-flow monoliths are by far the most common type of diesel filter. They are derived from flow-through catalyst supports where channel ends are alternatively plugged to force the gas flow through porous walls acting as a filter. Wall flow monoliths are made of ceramic materials such as cordierite and silicon carbide. A number of mechanical and thermal properties have been defined to characterize and compare different monoliths. Filters of different sizes have been developed and are available as standard products for both for light- and heavy-duty diesel engines.


Ceramic wall-flow monoliths, which are derived from the flow-through cellular supports used for catalytic converters, became the most common type of diesel filter substrate. They are distinguished, among other diesel filter designs, by high surface area per unit volume and by high filtration efficiencies. Monolithic diesel filters consist of many small parallel channels, typically of square cross-section, running axially through the part. Diesel filter monoliths are obtained from the flow-through monoliths by plugging channels as shown in Figure 1. Adjacent channels are alternatively plugged at each end in order to force the diesel aerosol through the porous substrate walls which act as a mechanical filter. To reflect this flow pattern, the substrates are referred to as wall-flow monoliths.

[flow pattern image]
Figure 1. Gas flow in a wall-flow filter

(Courtesy of Corning Inc.)

Wall-flow monoliths are most commonly available in cylindrical shapes, as shown in Figure 2, although oval or square cross-section parts are also possible.

Figure 2. Cylindrical wall-flow filters

Left: silicon carbide (segmented); Center & right: cordierite (monoliths)

(Courtesy of NGK)

Wall-flow filter walls have a distribution of fine pores that have to be carefully controlled in the manufacturing process. Total material porosity is typically between 45 and 50% or higher, while the mean pore size (MPS) usually ranges from 10 to 20 µm. Filtration on monolith wall-flow filters occurs through a combination of cake and depth filtration. Depth filtration is the dominant mechanism on a clean filter as the particulates are deposited in the pore network inside the wall material. As the soot load increases, a particulate layer develops along the wall surface in inlet channels and cake filtration becomes the prevailing mechanism. Typically, monolith filters have filtration efficiencies between about 70 and 95% of total particulate matter (TPM) by mass. Higher efficiencies are observed for solid PM fractions—elemental carbon and metal ash—and for solid particle numbers (PN).

Wall-flow monoliths are typically extrusions made from porous ceramic materials. Materials most commonly used in commercial filters include cordierite and silicon carbide (SiC). Cordierite is a synthetic ceramic developed for flow-through catalyst substrates and subsequently adapted for the filter application. Cordierite filters have been used mostly in heavy-duty engine applications. Silicon carbide has been used for a long time in a number of industries such as semiconductors, abrasives, or high temperature/molten metal contact materials. In the mid-2000s, SiC was introduced as a filter material for diesel passenger cars and remains common in light-duty applications. Another commercial filter material, aluminum titanate, is also used primarily for light-duty diesel vehicles.

Table 1
Commercial wall-flow monolith materials
Material Formula Monolith Suppliers
Cordierite 2MgO-2Al2O3-5SiO2 Corning, NGK, Denso, Hitachi Metals
Silicon carbide SiC NGK, Ibiden, Saint-Gobain, LiqTech
Aluminum titanate Al2TiO5 Corning