The reason why heavy duty vehicles emit pollution is because the fuel that typically goes into their engines is inherently dirty. "Clean Fuels" refers to the replacement of regular diesel fuel with something that is cleaner, whether it is a modified form of the same fuel or an outright alternative to diesel. Rather than focusing on the post-combustion phase of the diesel engine, the use of clean fuels targets what goes into the engine to begin with.

Ultra Low Sulfur Diesel (ULSD)

Ultra low sulfur diesel (ULSD) fuel is a specially refined petroleum diesel fuel that has dramatically lower sulfur content than standard on-highway diesel. The sulfur content in ULSD is 30 parts per million (ppm) or less. Standard highway diesel typically has a sulfur content of 500 ppm. Anything less than 500 ppm is considered "low sulfur" diesel. Other than the sulfur content, ULSD meets the same specifications as regular on highway diesel.

Why use ULSD fuel?

Ultra-low sulfur fuel substantially reduces harmful emissions that harm public health. Especially important is the fact that newer particulate control devices for diesel engines require low-sulfur fuel.

When will ULSD fuel be required?

By September 2006, under federal law, all on-road diesel fuel will be limited to a sulfur content of no more than 15 ppm.

Alternative Fuels

Alternative fuels refer to those fuels that are not solely derived from oil. For heavy duty vehicles, there are three commonly accepted alternatives to petroleum-based diesel.


Biodiesel is liquid fuel that can be used as a petrodiesel substitute or additive. Pure biodiesel is different from standard diesel in that it is not made with a petroleum product. Biodiesel is typically made from soybeans, rapseed or sunflower. Biodiesel is made by chemically reacting alcohol with vegetable oils, fats or greases. Biodiesel is often blended with standard diesel to create a biodiesel blend.

Biodiesel can be operated in any diesel engine with no or little modification to the engine or fuel system. Biodiesel in its pure form is called "neat biodiesel". Neat biodiesel is a 100% domestic fuel. Most biodiesel used today is a blend of biodiesel with petrodiesel. The most common is B20 which is 20% biodiesel and 80% petrodiesel.

Environmental Benefits:

Biodiesel reduces nearly all forms of air pollution compared to petrodiesel. Neat biodiesl (B100) is nontoxic and biodegradable. B100 meets low-sulfur, low-aromatic clean diesel standards (see section on ultra-low sulfur diesel).

Pollution Reductions:

Reductions w/ B100
Reductions w/ B20
Carbon Monoxide (CO)
Particulate Matter
Oxides of Nitrogen
+ 5%
+ 1.2%

Other Benefits:

- Safety: Biodiesel has a higher flashpoint and lower votality and does not ignite as easily as petrodiesel.
- Performance: Horespower, torque and fuel economy are similar to those of petrodiesel. Maintenance for requirements for B20 vehicles and petrodiesel vehicles are the same.
- Lubricity: In contrast to ULSD fuel, Biodiesel is a natural lubricant and in fact is sometimes blended with ULSD precisely to add lubricity.
- Oil displacement: Biodiesel fuel helps reduce oil consumption. B20 displaces oil by at least 20%, while B100 offers full displacement. By reducing oil consumption, biodiesel helps improve energy security for the country, since the majority of the oil it uses is imported.

Natural Gas

Natural gas is an odorless, colorless, tasteless gaseous fuel derived primarily from methane. Natural gas generally comes from gas wells, but also can be captured from gases emitted by landfills. Vehicles can use the fuel in two forms: either as compressed natural gas (CNG) or liquified natural gas (LNG). Most people are familiar with natural gas for its use in heating homes. In contrast to biodiesel, natural gas can only be used in vehicles that have been specifically designed to use the fuel.


Like natural gas, propane is a natural colorless, nontoxic gas that can be used as a fuel. Also like natural gas, vehicles must be specifically designed to run on propane. It is a byproduct of natural gas processing and petroleum refining. For fueling and storage purposes, propane is generally liquified (called LPG or liquified petroleum gas). Propane is better known for its use in heating and cooling homes and in powering barbecue grills.