A diesel particulate filter (DPF) is a device designed to remove diesel particulate matter or soot from the exhaust gas of a diesel engine. With the introduction of new legislations and environmental standards in Australia, diesel particle filters are commonplace on newer vehicles.
Wall-flow diesel particulate filters usually remove 85% or more of the soot, and under certain conditions can attain soot removal efficiencies of close to 100%.
Unlike a catalytic converter, which is a flow-through device, a DPF retains bigger exhaust gas particles by forcing the gas to flow through the filter.
Some filters are single-use, intended for disposal and replacement once full of accumulated ash.
Others are designed to burn off the accumulated particulate either passively through the use of a catalyst or by active means such as a fuel burner which heats the filter to soot combustion temperatures; programmed to run when the filter is full in a manner that elevates exhaust temperature or produces high amounts of NOx to oxidize the accumulated ash, or through other methods. This is known as "filter regeneration". Cleaning is also required as part of periodic maintenance, and it must be done carefully to avoid damaging the filter.
Types of Diesel Particulate Filters
There are a variety of diesel particulate filter technologies on the market. Each is designed around similar requirements:
- Fine filtration
- Minimum pressure drop
- Low cost
- Mass production suitability
- Product durability
Cordierite wall flow filters
The most common filter is made of cordierite,a ceramic material that is also used as catalytic converter supports (cores).
Cordierite filters provide excellent filtration efficiency, are (relatively) inexpensive, and have thermal properties that make packaging them for installation in the vehicle simple.
The major drawback is that cordierite has a relatively low melting point (about 1200 °C) and cordierite substrates have been known to melt down during filter regeneration. This is mostly an issue if the filter has become loaded more heavily than usual.
Cordierite filter cores look like catalytic converter cores that have had alternate channels plugged - the plugs force the exhaust gas flow through the wall and the particulate collects on the inlet face.
Silicon carbide wall flow filters
The second most popular filter material is silicon carbide (SiC). It has a higher melting point (2700 °C) than cordierite, however it is not as stable thermally, making packaging an issue.
Small SiC cores are made of single pieces, while larger cores are made in segments, which are separated by a special cement so that heat expansion of the core will be taken up by the cement, and not the package.
SiC cores are usually more expensive than cordierite cores, however they are manufactured in similar sizes, and one can often be used to replace the other.
Silicon carbide filter cores also look like catalytic converter cores that have had alternate channels plugged - again the plugs force the exhaust gas flow through the wall and the particulate collects on the inlet face.
Ceramic Fiber Filters
Fibrous ceramic filters are made from several different types of ceramic fibers that are mixed together to form a porous material. This material can be formed into almost any shape and can be customized to suit various applications. The porosity can be controlled in order to produce high flow, lower efficiency or high efficiency lower volume filtration.
Fibrous filters have an advantage over wall flow design of producing lower back pressure.
Ceramic wall-flow filters remove carbon particulates almost completely, including fine particulates less than 100 nanometers (nm) diameter with an efficiency of >95% in mass and >99% in number of particles over a wide range of engine operating conditions. Since the continuous flow of soot into the filter would eventually block it, it is necessary to 'regenerate' the filtration properties of the filter by burning-off the collected particulate on a regular basis.