Assignment

The particle filter should capture the harmful particles in the exhaust gas from the Diesel engine and burn them of in certain regeneration intervals allowing only harmless components to be released into the open air.Through the EU-decisions in 2005, the discussions arising about fine dust emissions in the cities, have become particularly heated.

Function

Due to the fact that the actually harmful portion of the Diesel exhaust gas is not the soot, but the accompanying hydrocarbons, the respective filter is not called a soot-filter, but a particle filter. Now one can take measures to avoid all sorts of internal engine soot build-up, or filter this out at the end of combustion. Such equipment has been around for a long time, e.g., in city busses. However, these had to be previously, in a daily- or weekly rhythm, freed from the accumulated solids. It's difficult to carry out a controlled burning out of the filters before they become clogged. At least what we have here, is lightly compacted carbon dust, which in other areas, leads to explosions. Because a regeneration cycle lasts several minutes, e.g., an excessive concentration of oxygen in the exhaust gas must be avoided, once the afterburning has started. At temperatures of over 1300°C, even ceramic material cannot survive without being damaged.

Apart from the particles, also the nitrogen-oxides are EU-certified. When designing an engine, the manufacturer must decide between either a hot and economic combustion with a high nitrogen content or a less hot combustion with a higher particle emission. Although through a special regulation of the cooled exhaust gas recirculation a lower particle- and NOX-emission is possible, there are however, disadvantages as far as the consumption is concerned. Basically, e.g., an SCR-Cat-System reduces nitrogen oxides, and the particle filter reduces particles. In this case, one makes a distinction between open- and closed systems. Open systems are found exclusively in after-market equipment. These are laid out as metal catalysers and have duct openings with an air-duct leading to the soot deposits. It is particularly important that they cannot be clogged.

Closed filters are different. Here is either a sensor in front of a particle filter, which measures against external pressure, or a second thereafter. They inform the control device if the filter threatens to become clogged, or through the temperature sensor if it overheats (on the top of the catalyser, see above figure). The electronic Diesel control then initiates a combustion in the filter through an additional injection (Toyota). In addition, the proximity to the engine and the oxidation catalyser helps, these are often found in the same housing (see picture on the left). Without this type of facility, the low exhaust gas temperature of the Diesel engine (often less than 200°C) is mostly not sufficient.

To raise the temperature further by 100°C, apart from the oxidation catalyser, an 'Eolys' (picture above) called additive is used. It lowers the temperature necessary for the burning off of the particles. The disadvantage of this system is, after travelling a certain distance (e.g., 80,000 kms), a servicing with regeneration/replacement of the filter and a small refilling of the small tank is necessary.

The problem of finding the one or two of each oxygen atoms during the injection, arises in the short period when the C- and H-atoms in the fuel are available. The shorter the period, the lower the air excess and the worse the mixing, the larger the development of particles and particularly the adhering hydrocarbons. The situation can be alleviated e.g., by the multiple-injection of modern common-rail- or pump-nozzle-systems, because of the dependance on the cam, in the latter only up to 50° after TDC is possible.

Dangerous particles can be reduced/avoided by...
- drivers who seldom drive flat out, thus causing the engine to operate with a high air excess,
- manufacturers who do not exploit the potential of the engine and regulate it down when there is still a relatively high air excess,
- specific swirling of the in-streaming air through special forming/guiding of the intake paths (swirling-ducts),
- even higher injection pressures,
- spray jets in the hole-type nozzles with the smallest possible diameter,
- filtering and afterburning.

According to the Euro 4 ordinance, the latter appears to be necessary for a great deal of mid-range motor cars. Lighter vehicles will probably only be forced by Euro 5 into taking these measures. At least one thing is certain, a filter system with afterburning gives the manufacturers a certain amount of guarantee that they will fulfil the next, and the following exhaust gas regulations as far as particles are concerned, (not the NOx regulations!). It looks very much like - also because of the high exhaust pollution in the cities - a very generalised introduction.

If one looks at the latest developments in the Diesel engine, the most effective ethods seem to be the high pressure and/or the afterburning. Thereby, as always, the basic policy is: rather to avoid the development in the combustion, than to find ways to treat it afterwards. Thus, there are already individual systems with pressures of way over 2000 bar. Whether less PAH (Polycyclic Aromatic Hydrocarbons) actually develop, or are only so strongly isolated, that they fall below the detection limit, is debatable. It is also uncertain, whether these are less dangerous for people or not. It seems to be a continuous race between medical science, ingenious measuring methods and the industry which is bound by mandatory regulations.

Basically there are, apart from the integrated systems with a link-up to the engine electronics, also systems which are installed afterwards. At the moment (2005) the integrated systems achieve a limit-value of 5-, the latter only manage 8,5 milligrams/km.

Important

The regeneration can also be accompanied by the switching on of the glowing equipment. In this case, the installation of glow-plugs with a ceramic heating element is a good bet. For a complete check of the equipment, there is also a broad-band Lambda sensor installed (see picture on the top, far right hand side).