Fuel oil is a fraction obtained from petroleum distillation, either as a distillate or a residue. Broadly speaking fuel oil is any liquid petroleum product that is burned in a furnace or boiler for the generation of heat or used in an engine for the generation of power, except oils having a flash point of approximately 40 °C (104 °F) and oils burned in cotton or wool-wick burners.
The term fuel oil is also used in a stricter sense to refer only to the heaviest commercial fuel that can be obtained from crude oil i.e. heavier than gasoline and naphtha.
The ASTM classification system, originally based on early refining and combustion engineering practices and nomenclature, identifies 6 fuel oil specifications. Different definitions do however exist.
Increases in the fuel oil number signify an increased boiling point and carbon chain length and heavier viscosity. Price usually decreases as the fuel number increases.
The first standard for fuel oil came in 1982. The latest standard is ISO 8217 from 2005. The ISO standard describe four qualities of distillate fuels and 10 qualities of residual fuels. Over the years the standards have become stricter on environmentally important parameters such as sulfur content.
Some parameters for different grades of marine fuel oils according to ISO 8217 (3. ed 2005):
Bunker fuels are also a type of fuel oil, used aboard ships. Bunker fuel is often used as a synonym for No. 6 fuel oil which is the most common bunker fuel.
Other types of classification used in the maritime field also include :
Marine fuels are often quoted on the international bunker markets with their maximum viscosity (which is set by the ISO 8217 standard) due to the fact that marine engines are designed to use different viscosities of fuel. The unit of viscosity used is the Centistoke and the fuels most frequently quoted are listed below in order of cost, the least expensive first :