Diesel engines produce a variety of particles during 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 (black carbon) particles. These particles include tiny nanoparticles—smaller than a thousandth of a millimeter (one micron). 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 (DPF), soot emissions may be decreased to 0.001 g / km or less.
The quality of the fuel also influences the formation of these particles. For example, a high sulfur content diesel produces more particles. Lower sulfur fuel produces fewer particles, and allows use of particulate filters. The injection pressure of diesel also influences the formation of fine particles.
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.
While no jurisdiction has explicitly made filters mandatory, the increasingly stringent emissions regulations that engine manufactures 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 V regulations PSA Peugeot Citroën became the first company to make filters standard on passenger cars.
As of December 2008 the California Air Resources Board (CARB) established the 2008 California Statewide Truck and Bus Rule which—with variance according to vehicle type, size and usage—require that on-road diesel heavy trucks and buses in California be retrofitted, repowered, or replaced to reduce particulate matter (PM) 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:
2001 - Hong Kong retrofit program.
2002 - In Japan the Prefecture of Tokyo passed a law banning trucks without filters from entering the city limits.
2003 - Mexico City started a program to retrofit trucks.
2004 - New York City retrofit program (non-road).
2008 - Milan Ecopass area traffic charge – a hefty entrance tax on all diesel vehicles except those with a particulate filter, either stock or retrofit.
2008 - London Low Emission Zone charges vehicles that do not meet emission standards, encouraging retrofit filters.
Inadequately maintained particulate filters on vehicles with diesel engines are prone to soot buildup, which can cause engine problems due to high back pressure.