Diesel Particulate Filters

You might have read in the news lately that ETS has tested DPF technology on two of its low floor diesel buses. But what exactly is a DPF and what can it do? What can it not do? Find out the facts about the DPF here.

What is a DPF?

Diesel Particulate Filter DPF stands for "Diesel Particulate Filter". It is a device that can be installed into the exhaust system of a diesel engine to "filter out" a portion of the particles of combustion known as "soot" or "particulate matter". These particles are mostly too small to be seen with the naked eye, although on older diesel engines the largest of them are visible as black smoke. The particles are harmful to human health and have been linked to lung ailments, cancer and heart disease.

The DPF is also known by the names "particulate trap" and "trap oxidizer". Probably the most popular DPF is one developed by the Catalytic Systems Division of Johnson Matthey known as the CRT or Continuously Regenerating Trap. This device has the ability to 'clean' itself of the collected particulate automatically.

How does it work?

Most DPF devices employ a round ceramic filter mounted in a metal canister to trap the diesel particulate. To clean or regenerate the filter, the collected particles must be "burned off" or oxidized. In some cases, this involves using electricity and compressed air, and may be accomplished while the vehicle is in motion or at a maintenance facility.

The popular CRT device relies on the principle that particulate matter can be oxidized or "burned off" at lower temperatures in the presence of nitrogen dioxide than in oxygen. It uses two chambers. The first contains a substrate coated with the precious metal platinum. The platinum acts as a catalyst to convert nitrogen oxide in the exhaust stream to nitrogen dioxide by oxidation. The second chamber contains the ceramic filter. When the engine reaches operating temperature and the nitrogen dioxide from the first chamber passes through the ceramic filter, the collected particulate is "burned off". This "burn off" reduces the particle material to gases like carbon dioxide and carbon monoxide and various types of inorganic dusts ("ash") such as iron, zinc, copper and calcium oxides.

The CRT requires very little maintenance. Cleaning of the chamber housing the filter element to remove ash deposits about once a year is the only regular maintenance requirement.

Most particulate traps have a requirement for diesel fuel with reduced sulphur content. The CRT requires ultra low sulphur diesel (ULSD), containing less than 50 ppm sulphur.

DPF First Chamber Reaction DPF Second Chamber Reaction

What does the DPF do for emissions and our health?

The DPF traps diesel soot particles down to about 2.5 microns in diameter. This is the range of particles that most tests measure. The DPF reduces particulate emissions in this size range on the order of about 60-70% over a non-DPF-equipped engine. This reduction in particles has obvious public health benefits, since particulate is one of the two most hazardous emissions released by diesel engines.

The DPF has also been shown to concurrently reduce specific hydrocarbon emissions such as non-methanous hydrocarbons (NMHC) and carbon monoxide. The reduction of these emissions also has health and environmental benefits.

What can the DPF not do, and does it have negative repercussions?

While the DPF does make a meaningful improvement to the emissions profile of the diesel engine, it is important to recognize that it is NOT a 'magic pill' that can turn the diesel into a zero emission engine. Even when equipped with a DPF, diesel engines still produce significant pollution. Because of the cruder nature of diesel fuel, research in the area of exhaust aftertreatment technologies likely has the potential to make a bigger difference in 'cleaning up' gasoline and CNG engines (whose emissions have a lower toxicity in the first place) than diesel engines.

The particles not trapped by the DPF (released into the air) are mostly ultrafine nanoparticles. These are precisely the particles that present the greatest threat to human health. The mucous membranes of the lungs are better at dealing with larger particles than ultrafine ones. Ultrafine nanoparticles pass easily through the lung's mucous membranes, carrying toxins and carcinogens directly into the bloodstream and circulating them throughout the body.

The DPF does not have a significant effect on nitrogen oxide emissions. NOx is the second of the two most harmful constituents of diesel exhaust. NOx has a toxic effect on red blood cells, and it combines with moisture in the lungs to form nitric acid. Exposure to nitrogen oxides can cause or exacerbate respiratory problems. NOx adsorber catalysts to reduce nitrogen oxide emissions are still in the development stage.

Tests show benzene emissions from diesel engines equipped with DPF devices are variable. Benzene is a strong carcinogen. In addition, mutagens (which cause genetic mutations and birth defects) are still detectable in the exhaust of DPF equipped vehicles, although there appears to be some reduction.

The use of DPF devices actually increases carbon dioxide (CO2) emissions. CO2 is the primary gas associated with global warming, and its reduction is targeted by the Kyoto Protocol. The additional CO2 is created both through the operation of the DPF itself, as well as through the production of ultra low sulphur diesel fuel (ULSD). In addition, using ULSD results in slightly poorer fuel efficiency (despite its higher cost), which contributes to higher fuel consumption and an increase in CO2 emissions.

DPF does not reduce the noise associated with the operation of diesel engines in large vehicles such as buses and trucks. The noise associated with diesel engines, particularly in high traffic areas as well as for maintenance personnel, has been identified as a health concern.

The precious metals (like platinum, rhodium, palladium) used in exhaust aftertreatment devices slowly erode over time and are released into the atmosphere. These precious metals have been discovered imbedded in the ice in Greenland. Platinum, the precious metal used in the CRT and many automobile catalytic converters, shows a 40-fold increase since about 1976. It has been demonstrated that these metals have entered the food chain in many parts of the world, making this a global problem.

There is no doubt that Diesel Particulate Filters do reduce some of the harmful effects of diesel emissions, and this is beneficial. But the DPF does not make the diesel bus a substitute for cleaner, quieter and proven electric technologies like trolleybuses and electric light rail. Wherever possible, zero emission technologies using wind, solar or water power, should be actively promoted and implemented to preserve our environment, protect our health, and ensure our quality of life.


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