Jump to main content or area navigation.

Contact Us

Region 1: EPA New England

Retrofits & Cleaner Fuels

A retrofitted engine is cleaner because it either has been fitted with a device designed to reduce pollution and/or it uses a cleaner fuel. There are a variety of ways to retrofit a diesel engine. Retrofit options range in cost as well as effectiveness and some come with special requirements. Through EPA's Voluntary Diesel Retrofit Program, EPA verifies retrofit technology and certifies cleaner fuels to ensure that they produce the emission reductions advertised by their manufacturer.

The following chart summarizes some clean fuel and clean technology options.

Clean Fuel / Clean Technology Options Type of Engine Percent Reduction in Emissions of Particulate Matter Percent Reduction in Emissions of Nitrogen Oxides Approximate Cost of Technology
Ultra-Low Sulfur Diesel (ULSD) Nonroad Diesel Engine About 5 to 9%

Enables the PM filter technology to work

N/A 8 to 20 cents per gallon more than regular diesel now
Particulate Matter Filter New or Used Diesel Engine - 1995 or newer models 60 to 90% N/A

$5,000 to $10,000

Must use ULSD fuel

Oxidation Catalyst New or Used Diesel Engine 20 to 30% N/A $1,000 to $2,000 and can be used with regular diesel
Compressed Natural Gas (with an oxidation catalyst) New CNG Engines 70 to 90% if using catalyst technology to reduce ultra fine PM, formaldehyde, and methane - otherwise, methane and aldehydes will be much higher than diesel engines About 60% reduction but are highly variable (sometimes increases occur)

$30,000 more than a diesel bus (cost of CNG fuel similar to regular diesel fuel)

Very expensive special re-fueling infrastructure and maintenance facilities are required

Biodiesel Fuel

B20: 20% biodiesel, 80% regular diesel

B100: 100% biodiesel

New or Used Diesel Engine B20 - 10%

B100 - 40%

Biodiesel increases emissions of NOx slightly.

B20 blend +2%

B100 fuel +10%

B20 - 5 to 30 cents per gallon more than regular diesel

B100 - 75 cents to $1.50 per gallon more than regular diesel (B100 may not be an option in the winter season)

Emulsified Diesel Fuel New or Used Diesel Engine About 50% About 10% 20 cents per gallon more than regular diesel fuel


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.

Top of Page

Diesel Particulate Matter Filters

Diesel particulate matter filters are ceramic devices that collect 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.

Top of Page

Ultra-Low Sulfur Diesel (ULSD) Fuel

As of October 2006, ULSD is available at the pump in most of New England. 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.

Top of Page

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 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, the cost of a new compressed natural gas vehicle can be much higher than a comparable diesel vehicle. Additionally, CNG buses require special refueling facilities as well as a special maintenance facilities, both of which are expensive.

Top of Page

Biodiesel Fuel

Biodiesel is a domestically produced, renewable fuel that can be manufactured from new and used vegetable oils and animal fats. Biodiesel is safe, biodegradable, and reduces air pollutants such as PM, CO, HC and air toxics. However, emissions of NOx increase with the concentration of biodiesel in the fuel. Some biodiesel produces more NOx than others, and some additives have shown promise in modifying the increases.

Blends of 20% biodiesel with 80% petroleum diesel (B20) can be used in unmodified diesel engines. Biodiesel can be used in its pure form (B100), but may require certain engine modifications to avoid maintenance and performance problems. Pure blends of biodiesel may not be suitable for the winter season. B20 reduces emissions of PM by about 10 percent. However, B20 also increases NOx emissions by approximately 2%. The B20 blend costs about 15 to 30 cents per gallon more than regular diesel fuel. B100 reduces emissions of PM by roughly 40 percent and costs about 75 cents to $1.50 more than regular diesel fuel.

Top of Page

Emulsified Diesel Fuel

Emulsified diesel is a blended mixture of diesel fuel, water, and other additives that reduces emissions of PM as well as NOx. Emulsified diesel can be used in any diesel engine, but the addition of water reduces the energy content of the fuel, so some reduction in power and fuel economy can be expected. Emulsified fuel will stay mixed for a fairly long time. However, if a vehicle sits dormant for months at a time the water can settle out of the fuel and possibly cause problems. Therefore school buses may want to phase out the use of the fuel towards the end of the school year and then reintroduce it in the fall. Case studies suggest that emulsified diesel can reduce emissions of smog-causing NOx by about 10 - 20 percent and PM by about 50 - 60 percent. Emulsified diesel costs roughly 20 cents more per gallon than regular diesel fuel.

Top of Page

Jump to main content.