United Nations Environment Programme


Facts on Emissions Control Technologies


In the remaining sections, basic information on emissions control technologies for diesel vehicles is provided. Most of these technologies can be successfully retrofitted into older vehicles.

Diesel oxidation catalysts (DOCs)
DOCs oxidize hydrocarbons (HC), carbon monoxide (CO) and the soluble organic fraction of particulate matter (PM). A DOC can work at sulphur levels higher than 500 ppm, but there is a risk that sulphur contained in the fuel will also oxidize and form sulphate and thus actually increase the total emissions of particulate matter. The lower the fuel sulphur level, the more efficiently the DOC functions. Emissions reductions from DOC use is estimated to be around 20-50% for PM and 60-90% for HCs and CO. Below is an animation on how a DOC works.

Diesel oxidation catalysts

DOCs are the most common diesel emissions control technology available for retrofitting, with over 20 000 installed on buses and long-distance driving trucks in the US and the EU.

DOCs replace the muffler (they are the same size and weight) and take about two hours to install. Installation is straightforward, as no other changes – apart from replacing the muffler – are needed. The cost of retrofitting a DOC varies from $600 up to $2000 for heavy trucks.

Diesel particulate filter (DPFs)
DPFs function by capturing particulate matter in a filter and oxidizing (i.e. burning) the particles. Two types of DPFs exist, and vary by how the particulate matter is oxidized. In a passive filter, a catalyst is used that lowers the temperature needed for the exhaust gases to oxidize the particulate matter. In an active system, particulate matter is oxidized by heat as soon as the filter is full. Heat is supplied from outside sources, such as via an electric heater. Today, a common model is to combine a DOC and a DPF in one package. DPFs (especially passive systems) usually require very low sulphur levels (<15 ppm) in order to prevent contamination by sulphur. However, some newer DPFs with active regenerating systems are being developed that can tolerate sulphur levels up to 500 ppm. A DPF with an oxidizing catalyst (passive system) can reduce HC, CO and PM by 90%.For DPFs, measurement of the exhaust gas temperature is usually required beforehand, in order to see whether an active or passive filter can be adopted. Below is an animation about how a particulate filter works.

Diesel particulate filter

DPFs are more complex systems than DOCs, but can be retrofitted into older vehicles equipped with an electronic injection system (usually meaning trucks manufactured after 1994). In Sweden, over 7000 passive DPFs have been retrofitted since 1996. In the US, DPFs have been retrofitted since the beginning of the 1990s. For example, New York City Transit has retrofitted over 2000 buses with DPFs. Retrofitting requires measurement of exhaust gas temperatures to find the right DPF. Retrofitting a DPF takes 5-6 hours and is thus more expensive. Equipping heavy duty trucks with DPFs is estimated to cost between $3500 and 9,000, depending on the size of the truck, the regeneration system and whether the DPF is installed in a new truck or is retrofitted in an older one. For light and medium duty trucks prices are from $1000 and up. Although installation or retrofitting with a DPF seems expensive, it should be compared with the cost of a new truck. Also, with advances in technology and stricter regulations, prices are expected to drop.

Diesel Particulate Filter. Photo: MECA

In the US, DPFs will be mandatory by 2007 in order to meet new regulations, and the same is likely in the EU with the upcoming Euro V restrictions. Taking a proactive approach to this matter – such as through investment in these technologies and/or vehicles with technologies that exceed current regulations– is strategic, as many cities have introduced restrictions for trucks entering densely populated urban areas.

Flow through filters (FTFs)

FTFs are a relatively new technology for reducing diesel PM emissions. Unlike a DPF – in which only gases can pass through the filter components – the FTF does not physically "trap" and accumulate PM.  Instead, exhaust flows through a medium (such as wire mesh) that has a high density of interrupted flow channels, thus giving rise to turbulent flow conditions. 

The filtration efficiency of an FTF is lower than that of a DPF, but the FTF is much less likely to clog under unfavourable conditions (e.g. high PM emissions, low exhaust temperatures and emergency circumstances). Consequently, the FTF is a candidate for use in applications that are unsuitable for DPFs. The cost of retrofitting an FTF is estimated to be between $3500 and $5000.

NOx and diesel

Because diesel engines have a different air-to-fuel ratio than petrol engines, 3-way catalytic converters do not function in diesel engines. Diesel engines run with an excess of air (lean combustion), which makes NOx reduction more difficult. Instead, newer techniques and catalysts for diesel engines that target NOx have been developed, such as exhaust gas recirculation (EGR), selective catalytic reduction (SCR), emulsified diesel, lean NOx catalysts, and NOx adsorbers.

Exhaust gas recirculation (EGR)
EGR is used primarily to reduce NOx emissions. Because the formation of NOx is highly temperature dependent, EGR works by recirculating 5-10% of the exhaust gases back to the engine air inlet in order to lower the combustion temperature (several hundred degrees), and thus lower these emissions. EGR systems can be either a high or a low-pressure system. Low-pressure systems can be retrofitted since they require no engine modifications. DPFs are an integral part of any low pressure system, ensuring that particulate matter is not being circulated back into the engine. EGR can reduce NOx by about 40 -50%.
EGR has been installed in many new trucks in order to reduce NOx emissions. The extra costs of installation have not been disclosed and the system is well-integrated into the overall engine design, both of which make costs difficult to estimate. The cost of retrofitting a low-pressure EGR into an older truck is estimated to be between $13,000 -$20,000. This has been done in very few cases.

Selective catalytic reduction (SCR)
SCR operates much like a DOC but uses an outside agent to catalyze and reduce the NOx to harmless nitrogen and water. SCRs use ammonia (in the form of urea) that is supplied in small doses. This requires a careful monitoring system, as the amount of urea needs to be finely tuned to the amount of NOx. If this is not properly monitored, excessive ammonia, which is a toxin, will be emitted. Urea is supplied from a tank in the vehicle that needs to be refilled. SCRs reduces emissions of NOx by 75-90%.
Typical cost for an SCR system retrofit is $13,000. If the SCR system is combined with a DPF the price range is between $16,000 and $20,000 per heavy duty vehicle.

Lean NOx catalysts
Lean NOx catalysts function much like a SCR, i.e. using an outside agent to convert NOx to nitrogen and water. Lean NOx catalysts use an extra shot of diesel fuel injected into the catalyst. The advantages are that the diesel is already at hand in the vehicle; however this lowers fuel economy and requires advanced monitoring systems. Lean NOx catalysts reduce NOx by approximately 30%, and up to 50% under the right conditions.

NOx adsorber
A NOx adsorber stores NOx under lean combustion conditions and when storage capacity has been reached, the NOx is catalytically converted to nitrogen and water. NOx adsorbers have proven effective in combination with DPFs. NOx adsorbers easily become poisoned by sulphur and therefore require ultra-low sulphur diesel (<10ppm). NOx adsorbers are NOT retrofittable. NOx adsorbers reduces NOx substantially (by over 90%), but are very sensitive to sulphur poisoning. This technology is still under development.
SCR, lean NOx catalysts and NOx adsorbers have yet to be used in large-scale applications. However, with increasingly strict emissions regulations, their introduction to the commercial market is expected in the next 2-5 years. Also, because the application is new and still under development, the costs are difficult to estimate. However, preliminary estimates put the cost at approximately $6000-$10000 for a truck. These costs are expected to drop quickly as the technologies become mandatory.

More information and price indications for various retrofitting options can be found in the EU Report on supporting technical measures to reduce emissions from existing heavy duty vehicles (available at www.airqualitypolicy.co.uk/content/view/19/49/ ).