The combination of a diesel particulate matter filter and ultra low sulfur diesel fuel is often referred to as Clean Diesel Technology. However, purchasing new buses equipped with oxidation catalysts is a cost-effective bus replacement strategy. The U.S. Environmental Protection Agency often has funding available to assist with diesel retrofits and other clean diesel technology. The EPA also has a listing of verified retrofit products that are available and a listing of retrofit, fuel, and engine manufacturers.

    Diesel Oxidation Catalysts
    Diesel oxidation catalysts are devices that use a chemical process to break down pollutants in the exhaust stream into less harmful components. Diesel oxidation catalysts can reduce emissions of particulate matter (PM) by 20 percent and hydrocarbons (HC) by 50 percent and carbon monoxide (CO) by approximately 40 percent. Oxidation catalysts cost about $1,000 to $2,000, can be installed on any new or used bus, and run on regular diesel fuel. Although installation time can vary, field experience suggests it takes about 1 to 3 hours to install an oxidation catalyst.

    Diesel Particulate Matter Filters
    Diesel particulate matter filters are ceramic devices that collects the particulate matter in the exhaust stream The high temperature of the exhaust heats the ceramic structure and allows the particles inside to break down (or oxidize) into less harmful components. They can be installed on new and used buses, but must be used in conjunction with ultra-low sulfur diesel (ULSD) - fuel with a sulfur content of less than 15 parts per million. The combination of PM filters and ULSD can reduce emissions of PM, HC, and CO by 60 to 90 percent.

PM filters come in a kit that includes mounting brackets and an electronic monitoring device. The cost of the kit can range from $5,000 to $10,000. Some buses may need the more expensive filters to compensate for lower exhaust temperatures. PM filters work best on engines built after 1995. As a result, knowing the age and type of each engine in the fleet as well as the drive cycles of the buses is an important part of any retrofit project. These factors determine whether or not buses are candidates for filters. Although installation time can vary, field experience suggests it takes about 6 to 8 hours to install the filter.

Ultra-Low Sulfur Diesel (ULSD)
ULSD will be available nationwide in June 2006, but currently is available in certain parts of the country. The primary purpose of ULSD is to enable or improve the performance of aftertreatment technologies such as a PM filter. The quantity of emissions reductions from the use of ULSD alone will vary depending on the application, level of sulfur reduction, and other fuel characteristics of the replacement fuel (e.g., cetane number, aromatics, PNA). Some case studies suggest that the use of ULSD alone can reduce emissions of PM between 5 and 9 percent. While ULSD-only emission reductions for PM are relatively modest on a per-vehicle basis compared to aftertreatment retrofit, the emission reductions can be significant if an entire fleet is fueled with ULSD. The price differential between ULSD and regular diesel fuel varies by location but ranges between 8 and 25 cents per gallon. In 2006, when ULSD is available nationwide, the cost differential will be much less.

Compressed Natural Gas
Another replacement option is to purchase new buses that run on compressed natural gas (CNG) and have been fitted with an oxidation catalyst to reduce pollution. CNG is a mixture of hydrocarbons, mainly methane, and is produced either from gas wells or in conjunction with crude oil production. Vehicles powered by natural gas perform just like vehicles powered by diesel fuel. The majority of natural gas consumed in the United States comes from sources within North America. Increased use of clean, domestically produced fuels helps reduce our reliance on oil from overseas, which is good for our nations energy security. Natural gas buses can reduce emissions of particulate matter (PM) by about 70 to 90 percent if they meet Clean Fueled Fleet (http://www.epa.gov/otaq/cff.htm) requirements or have catalysts. CNG engines that do not have catalysts, may have higher formaldehyde, hydrocarbon, and ultra-fine PM emissions than Clean Diesel engines meeting EPA's 2007 emission standard for PM. The cost of natural gas varies, but generally is comparable to the cost of regular diesel fuel. However, a new compressed natural gas school bus costs about $30,000 more than a new diesel school bus. Additionally, CNG buses require special refueling facilities as well as a special maintenance facilities, both of which are very expensive.

Funding for Clean Diesel Technology

Please check the Environmental Protection Agency's Air and Radiation web site for information about funding that may be available for diesel retrofits, biodiesel programs, etc..

This information comes from the Environmental Protection Agency's Clean School Bus USA Program.

Links on This Clean School Bus Site