Diesel particulate filter


A diesel particulate filter is a device designed to remove diesel particulate matter or soot from the exhaust gas of a diesel engine.

Mode of action

Wall-flow diesel particulate filters usually remove 85% or more of the soot, and under certain conditions can attain soot removal efficiencies approaching 100%. 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. This is accomplished by engine programming to run in a manner that elevates exhaust temperature, in conjunction with an extra fuel injector in the exhaust stream that injects fuel to react with a catalyst element to burn off accumulated soot in the DPF filter, or through other methods. This is known as. Cleaning is also required as part of periodic maintenance, and it must be done carefully to avoid damaging the filter. Failure of fuel injectors or turbochargers resulting in contamination of the filter with raw diesel or engine oil can also necessitate cleaning. The regeneration process occurs at road speeds higher than can generally be attained on city streets; vehicles driven exclusively at low speeds in urban traffic can require periodic trips at higher speeds to clean out the DPF. If the driver ignores the warning light and waits too long to operate the vehicle above, the DPF may not regenerate properly, and continued operation past that point may spoil the DPF completely so it must be replaced. Some newer diesel engines, namely those installed in combination vehicles, can also perform what is called a Parked Regeneration, where the engine increases RPM to around 1400 while parked, to increase the temperature of the exhaust.
Diesel engines produce a variety of particles during the combustion of the fuel/air mix due to incomplete combustion. The composition of the particles varies widely dependent upon engine type, age, and the emissions specification that the engine was designed to meet. Two-stroke diesel engines produce more particulate per unit of power than do four-stroke diesel engines, as they burn the fuel-air mix less completely.
Diesel particulate matter resulting from the incomplete combustion of diesel fuel produces soot particles. These particles include tiny nanoparticles—smaller than one micrometre. Soot and other particles from diesel engines worsen the particulate matter pollution in the air and are harmful to health.
New particulate filters can capture from 30% to greater than 95% of the harmful soot. With an optimal diesel particulate filter, soot emissions may be decreased to or less.

History

Diesel particulate filtering was first considered in the 1970s due to concerns regarding the impacts of inhaled particulates. Particulate filters have been in use on non-road machines since 1980, and in automobiles since 1985. Historically medium and heavy duty diesel engine emissions were not regulated until 1987 when the first California Heavy Truck rule was introduced capping particulate emissions at 0.60 g/BHP Hour. Since then, progressively tighter standards have been introduced for light- and heavy-duty roadgoing diesel-powered vehicles and for off-road diesel engines. Similar regulations have also been adopted by the European Union and some individual European countries, most Asian countries, and the rest of North and South America.
Whilst few jurisdictions have explicitly made filters mandatory, the increasingly stringent emissions regulations that engine manufacturers must meet mean that eventually all on-road diesel engines will be fitted with them. In the European Union, filters are expected to be necessary to meet the Euro.VI heavy truck engine emissions regulations currently under discussion and planned for the 2012-2013 time frame. In 2000, in anticipation of the future Euro 5 regulations PSA Peugeot Citroën became the first company to make filters standard on passenger cars.
As of December 2008, the California Air Resources Board established the 2008 California Statewide Truck and Bus Rule which—with variance according to vehicle type, size and usage—requires that on-road diesel heavy trucks and buses in California be retrofitted, repowered, or replaced to reduce particulate matter emissions by at least 85%. Retrofitting the engines with CARB-approved diesel particulate filters is one way to fulfill this requirement. In 2009 the American Recovery and Reinvestment Act provided funding to assist owners in offsetting the cost of diesel retrofits for their vehicles. Other jurisdictions have also launched retrofit programs, including:
Inadequately maintained particulate filters on vehicles with diesel engines are prone to soot buildup, which can cause engine problems due to high back pressure.
In 2018, the UK made changes to its MOT test requirements, including tougher scrutiny of diesel cars. One requirement was to have a properly fitted and working DPF. Driving without a DPF could incur a £1000 fine.

Variants of DPFs

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 material before exiting; however, the DPF does not retain small particles. Maintenance-free DPFs oxidise or burn larger particles until they are small enough to pass through the filter, though often particles "clump" together in the DPF reducing the overall particle count as well as overall mass. There are a variety of diesel particulate filter technologies on the market. Each is designed around similar requirements:
  1. Fine filtration
  2. Minimum pressure drop
  3. Low cost
  4. Mass production suitability
  5. Product durability

    Cordierite wall flow filters

The most common filter is made of cordierite. 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 and cordierite substrates have been known to melt during filter regeneration. This is mostly an issue if the filter has become loaded more heavily than usual, and is more of an issue with passive systems than with active systems, unless there is a system breakdown.
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, or SiC. It has a higher melting point 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.
The characteristics of the wall flow diesel particulate filter substrate are:
  • broad band filtration
  • high filtration efficiency
  • high refractory
  • high mechanical properties
  • high boiling point.

    Ceramic fiber filters

Fibrous ceramic filters are made from several different types of ceramic fibers that are mixed together to form a porous medium. This medium 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. Fibrous ceramic filters remove carbon particulates almost completely, including fine particulates less than 100 nanometres diameter with an efficiency of greater than 95% in mass and greater than 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. Soot particulate burn-off forms water and CO2 in small quantities amounting to less than 0.05% of the CO2 emitted by the engine.

Metal fiber flow-through filters

Some cores are made from metal fibers – generally the fibers are "woven" into a monolith. Such cores have the advantage that an electrical current can be passed through the monolith to heat the core for regeneration purposes, allowing the filter to regenerate at low exhaust temperatures and/or low exhaust flow rates. Metal fiber cores tend to be more expensive than cordierite or silicon carbide cores, and are generally not interchangeable with them because of the electrical requirement.

Paper

Disposable paper cores are used in certain specialty applications, without a regeneration strategy. Coal mines are common users – the exhaust gas is usually first passed through a water trap to cool it, and then through the filter. Paper filters are also used when a diesel machine must be used indoors for short periods of time, such as on a forklift being used to install equipment inside a store.