The best product to use when an extremely high reduction in particulate matter is required on a long term basis is known as a ‘wall-flow’ filter.
Wall flow filters derive their name from the fact that all the exhaust gas is required to pass through the porous walls of the filter.
As it does so, particles larger than the pore size are left behind, and filtration efficiencies of more than 99% by mass are possible.
Wall flow filters are made from a variety of materials, with the most common being cordierite, silicon carbide and sintered metal.
As the filtration efficiency is so high, any filter will rapidly become blocked unless the accumulated diesel particulate matter is removed by oxidation at regular intervals. This process is known as ‘regeneration’ and is conventionally divided into two categories referred to as ‘passive’ and ‘active’.
Passive regeneration means that the process relies entirely on the heat of the exhaust gas, whilst active regeneration means that additional heat has been provided from another energy source.
These days most wall flow diesel particulate filter systems make use of both types of regeneration, with passive regeneration doing most of the work and active regeneration serving as a back-up.
Original equipment diesel particulate filters are normally closely integrated with the engine management system, however when designing a retrofit system it is often preferable to keep this separate from the engine management or CANBUS systems.
(Photo: Long term filters > 20kW Schematic)
