Diesel engines are significant contributors to air pollution. Exhaust from diesel vehicles contains high levels of nitrogen oxides, fine particulate matter and a mixture of other toxic gases. Many of the gases are known or suspected cancer-causing agents. Diesel emissions are hazardous to health, and exposure can result in serious medical conditions.

While new EPA regulations will require diesel engines to use low-sulfur fuel and reduce emissions by 2007, vehicles purchased before this year are not required to meet new emission standards. Diesel engines last a long time, many running for 25-30 years. In order to reduce air pollution from emissions, diesel engines can be retrofitted with after-treatment pollution control devices to achieve sufficient reductions. Many retrofit technologies have shown significant emission reductions, on both stationary and mobile diesel engines. Installing retrofit technologies is beneficial to the environment and human health.

Diesel Oxidation Catalyst (DOC)

In most systems, Diesel Oxidation Catalysts (DOCs) consist of a stainless steel canister that contains a honeycomb structure called a substrate or catalyst support. It contains no moving parts, only an interior surface coated with catalytic metals such as platinum or palladium. The catalyst oxidizes the soluble organic fraction (SOF) of particulate matter, specifically carbon monoxide, gaseous hydrocarbons and the liquid hydrocarbons absorbed on carbon particles. The soluble organic fraction is converted into carbon dioxide and water.

The level of total particulate reduction is influenced by the percentage of SOF in the particulate. A Society of Automotive Engineers Technical Paper reported that oxidation catalysts could reduce the SOF of the particulate by 90 percent under certain operating conditions, and could reduce total particulate emissions by 40 to 50 percent. Destruction of the SOF is important since this portion of the particulate emissions contains numerous chemical pollutants that pose significant health concerns.

Combining an oxidation catalyst with engine management techniques can be used to reduce NOx emissions from diesel engines. This is achieved by adjusting the engine for low NOx emissions which is typically accompanied by increased carbon monoxide (CO), hydrocarbons (HC), and particulate emissions. An oxidation catalyst can be added to offset these increases, thereby lowering the exhaust levels of all the pollutants.

Fact sheet: "Questions and Answers on Using a Diesel Oxidation Catalyst in Heavy-Duty Diesel Trucks and Buses", (June 2003, EPA420-F-03-016)

Diesel Particulate Filter (DPF)

Diesel particulate matter filters are ceramic devices that collect particulate matter in the exhaust stream. The high temperature of the exhaust heats the ceramic structure and allows the particles inside to break down (oxidize) into less harmful components. They can be installed on new and used buses, but they must be used in conjunction with ultra-low sulfur diesel (ULSD) - fuel with a sulfur content of less than 15 parts per million. The sulfur in diesel fuel significantly affects the reliability, durability, and emissions of catalyst-based diesel particulate filters. Sulfur inhibits the movement of catalytic materials on the filter. It also interferes with chemical reactions intended to reduce pollutant emissions and creates particulate matter through catalytic formation.

The combination of particulate matter filters and ULSD can reduce emissions of particulate matter (PM), hydrocarbons (HC) and carbon monoxide (CO) by 60 to 90 percent. There is a virtually complete reduction in odor and SOF. However, some catalysts may increase sulfate emissions. By using the lowest possible level of sulfur in the fuel, the emissions can be reduced dramatically.

Fact sheet: "Questions and Answers on Using a Diesel Particulate Matter Filter in Heavy-Duty Diesel Trucks and Buses", (June 2003, EPA420-F-03-017)

For a list of U.S. EPA approved retrofit technology, please visit: www.epa.gov/otaq/retrofit/retroverifiedlist.htm

For a list of California Air Resources Board's (CARB) approved retrofit technology, please visit: http://www.arb.ca.gov/diesel/verdev/currentlyverifiedtech.htm

Selective Catalytic Reduction

Selective Catalytic Reduction (SCR) is used to control NOx emissions from stationary sources. In addition, SCR can be applied to mobile sources including trucks, marine vessels, and locomotives. When applied to a diesel-powered vehicle, it simultaneously reduces NOx, particulate matter, and hydrocarbon emissions.

Similar to an oxidation catalyst, SCR uses a catalyst to cause chemical reactions. However, SCR requires a reductant in the exhaust stream to convert NOx to nitrogen and oxygen in an oxidizing environment. The reductant is usually ammonia or urea. This device reduces 75 to 90% of NOx emissions, 50 to 90% of hydrocarbon emissions, and 30 to 50% of particulate matter emissions.

For further details and structural information, please visit: http://www.epa.gov/airmarkt/arp/nox/scrfinal.pdf