• Diesel oil is fuel specifically produced to power diesel engines. Most diesel oils are derived from petroleum, but some are produced using gases, plants and other biological material. Somewhat denser than regular gasoline, diesel is categorized according to qualities including heaviness, purity and source. Each type has different properties and performs differently in engines.
  
  

Conventional and Low-Sulfur Diesel

• Diesel fuel, while allowing diesel engines to get greater fuel economy than regular gasoline engines, also results in a higher production of nitrogen oxide and soot, mainly due to its higher sulfur content. In order to meet environmental requirements in many countries, some of this sulfur must be removed before the diesel can be used as a fuel in car engines. Conventional diesel has approximately 500 parts per million of sulfur.
  
  

Ultra-Low-Sulfur Diesel

• Since 2006, the United States has required that all diesel engines burn ultra-low-suflur diesel, which contains no more than 15 parts per million of sulfur. This change allowed diesel engines to use their catalytic converters and traps to significantly reduce the emission of nitrogen oxide and soot. The device couldn't be used before without being clogged by the relatively high sulfur content of available diesel fuel.
  
  

Grades No. 1D and No. 2D

• Diesel fuels have varying densities, which affect how the fuel is combusted within an engine. Diesel comes in two main grades: 1D and 2D. Grade 1D, the lighter of the two, allows the engine to start more easily in cold weather, while 2D is preferred for high-speed engines.
  
  

Biodiesel

• In the wake of recent spikes in the price of oil, there has been increased interest in the production of fuels derived from sources other than petroleum, including biodiesel, which is produced from biological matter, commonly vegetable oil and animal fats. The performance of biodiesel in engines is comparable to that of petroleum-derived diesel, and biodiesel has been shown to produce fewer emissions.
  
  

Gas-To-Liquid Diesel

• Gas-to-liquid (GTL) diesels are produced using the Fischer-Tropsch process of converting gases into liquid fuels. The gas used can be natural gas, or it can be derived from certain types of coal and biomass. GTL diesel has been shown to produce lower tailpipe emissions than regular diesel. GTL fuels typically suffer from a slight loss of fuel economy, but diesel engines are already significantly more efficient than engines that burn gasoline.
  
  

Blended Diesel

• Diesel fuel is often composed of a blend of different types of diesel fuel. Recently, biodiesel has begun to be regularly blended in with regular, petroleum-derived diesel. One fuel, called B-20, is composed of 20 percent biodiesel and 80 percent regular diesel.