Diesel

Diesel is a mixture of hydrocarbons distilled from petroleum and is used as fuel in a particular type of engine invented by German engineer Rudolf Diesel. When compressed to a very high pressure, diesel automatically ignites and burns, releasing energy to power the vehicle. This is in contrast to a gasoline engine, which requires a spark plug to ignite the fuel. The diesel engine is more efficient than the normal gasoline engine, and when combined with the high energy content of diesel fuel, this means that a diesel vehicle can achieve up to 40% better mileage per gallon in comparison to a gasoline car. This increased fuel economy also results in reduced CO₂ emissions. However, a diesel engine does produce more toxic emissions, in particular, oxides of nitrogen (NOx) and particulate matter (PM10).

Diesel has a Cetane Number (CN) rather than the Research Octane Number (RON) or Pump Octane Number (PON) that gasoline has. In the US, diesel at the pump can be found in two CN ranges: 40-46 for regular diesel, and 45-50 for premium. Diesel engines tend to run best with a CN between 45 to 50; however, there is no performance or emission advantage when the CN is raised past 50.

Diesel is high in sulfur; this acts as a lubricant for the engine, but is harmful to the environment and can prevent filters from controlling diesel particulate emissions. There are number of different types of diesel sold at the pumps; these include low sulfur diesel and ultra low sulfur diesel.

Low Sulfur Diesel

Low sulfur diesel (LSD) contains up to 500 ppm sulfur, which leads to significantly higher exhaust emissions than those seen with Ultra Low Sulfur Diesel (ULSD).

Ultra Low Sulfur Diesel (ULSD)

ULSD is now the normal diesel fuel sold in the US, and contains less than 15 parts per million of sulfur. ULSD gives good rate of miles per gallon, low CO₂ emissions, but fairly high emissions of particulate matter and oxides of nitrogen.

Ultra-low sulfur diesel was proposed by EPA (Environmental Protection Agency) as a new standard for the sulfur content in on-road diesel fuel sold in the United States since October 15, 2006, except for California and rural Alaska. California required it since September 1, 2006, and rural Alaska will transition all diesel to ULSD in 2010.