Diesel particulate filter

A diesel particulate filter, sometimes called a DPF, is device designed to remove diesel particulate matter or soot from the exhaust gas of a diesel engine, most of which are rated at 85% efficiency, but often attaining efficiencies of over 90%. A diesel-powered vehicle with a filter installed will emit no visible smoke from its exhaust pipe.

In addition to collecting the particulate, a method must be designed to get rid of it. Some filters are single use (disposable), while others are designed to burn off the accumulated particulate, either through the use of a catalyst (passive), or through an active technology, such as a fuel burner which heats the filter to soot combustion temperatures, or through engine modifications (the engine is set to run a certain specific way when the filter load reaches a pre-determined level, either to heat the exhaust gases, or to produce high amounts of NO2, which will oxidize the particulates at relatively low temperatures). This procedure is known as "filter regeneration." Fuel sulfur interferes with many "regeneration" strategies, so almost all jurisdictions that are interested in the reduction of particulate emissions, are also passing regulations governing fuel sulfur levels.


Variants of DPFs

Unlike a catalytic converter which is a flow-through device, a DPF cleans exhaust gas by forcing the gas to flow through the filter. 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 (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, and is more of an issue with passive systems than with active systems, unless there is a system break down.

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 (1700°C) 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.



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 a current can be passed through the monolith to heat the core for regeneration purposes. Metal fiber cores tend to be more expensive than cordierite or silcon carbide cores, and generally not interchangeable with them.



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 of a store.



Partial filters

There are a variety of devices that produce over 50% particulate matter filtration, but less than 85%. Partial filters come in a variety of materials. The only commonality between them is that they produce more back pressure than a catalytic converter, and less than a diesel particulate filter. Partial filter technology is popular for retrofit.



Filter usage

A properly designed filter will have little effect on fuel usage, however improper installation can be catastrophic, which is why automobile and truck engine manufacturers have avoided the use of filter technology until now. It was first offered as standard by the French manufacturer PSA Peugeot Citroën in early 2000, and has been a huge success. Slow adoption by the German car industry sparked local protests in March 2005.



Maintenance

Filters require more maintenance than catalytic converters. Engine oil ash builds up on the surface of the inlet face of the filter, and will eventually clog the pores. This increases the pressure drop over the filter, which when it reachs 100 inches of water or higher is capable of causing engine damage. Regular filter maintenance is a necessity.



Regeneration

Regeneration is the process of removing the accumulated soot from the filter. This is done either passively (by adding a catalyst to the filter) or actively. On-board active filter management can use a variety of strategies:

* 1) Engine management to increase exhaust temperature
* 2) A fuel burner to increase the exhaust temperature
* 3) A catalytic oxidizer to increase the exhaust temperature
* 4) Resistive heating coils to increase the exhaust temperature
* 5) Microwave energy to increase the exhaust temperature

All on-board active systems use extra fuel, whether through burning to heat the DPF, or providing extra power to the DPF's electrical system. Typically a computer monitors one or more sensors that measure back pressure and/or temperature, and based on pre-programmed set points the computer makes decisions on when to activate the regeneration cycle. The additional fuel can be supplied by a metering pump. Running the cycle too often while keeping the back pressure in the exhaust system low will use extra fuel. The reverse runs risk of engine damage and/or uncontrolled regeneration and possible DPF failure. Quality regeneration software is a necessity for longevity of the active DPF system.

Diesel particulate matter combusts at when temperaures above 600 degrees celsius are attained. The start of combustion causes a further increase in temperature. In some cases the combustion of the particulate matter can raise temperatures above the structural integrity threshold of the filter material, which can cause catastophic failure of the substrate. Various strategies have been developed to limit this possibility. Note that unlike a spark-ignited engine, which typically has less than 0.5% oxygen in the exhaust gas stream before the emission control device(s), many diesel engines run above 15% oxygen pre-filter. While the amount of available oxygen makes fast regeneration of a filter possible, it also contributes to runaway regeneration problems.

Some applications use off-board regeneration. Off-board regeneration requires operator intervention (i.e. the machine is either plugged into a wall/floor mounted regeneration station, or the filter is removed from the machine and placed in the regeneration station). Off-board regeneration is not useable for on-road vehicles, except in situations where the vehicles are parked in a central depot when not in use. Off-board regenration is mainly used in industrial and mining applications. Coal mines (with the attendant explosion risk from coal damp) use off-board regeneration if non-disposable filters are installed, with the regeneration stations sited in an area where non-permissible machinery is allowed. Many forklift makes also use off-board regeneration - typically mining machinery and other machinery that spend their operational lives in one location, which makes having a stationary regeneration station practical. In situations where the filter is physically removed from the machine for regeneration there is also the advantage of being able to inspect the filter core on a daily basis (DPF cores for non-road applications are typically sized to be useable for one shift - so regeneration is a daily occurrence.



Diesel Oxidation Catalyst (DOC)

DOCs is the other diesel retrofit system. They are devices that use a chemical process to break down pollutants in the exhaust stream into less harmful components.