Fuel oil

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 and oils burned in cotton or wool-wick burners. In this sense, diesel is a type of fuel oil. Fuel oil is made of long hydrocarbon chains, particularly alkanes, cycloalkanes and aromatics. 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, heavier than gasoline and naphtha.

Six classes
Fuel oil in the United States is classified into six classes, according to its boiling temperature, composition and purpose. The boiling point, ranging from 175 to 600 °C, and carbon chain length, 20 to 70  atoms, of the fuel increases with number. Viscosity also increases with fuel oil number and the heaviest oil has to be heated to get it to flow. Price usually decreases as the fuel number increases. 

No. 1 fuel oil, No. 2 fuel oil (heating oil), and No. 3 fuel oil are referred to as distillate fuel oils, diesel fuel oils, light fuel oils, gasoil or just distillate. For example, No. 2 fuel oil, No. 2 distillate and No. 2 diesel fuel oil are almost the same thing. Diesel is different in that it also has a cetane number limit which describes the ignition quality of the fuel. Distillate fuel oils are distilled from crude oil.

Related pages
Diesel
Gasoline
Kerosene
Naphtha

Sources
Bunkerworld - media site for the marine fuels industry
National Park Service - Fuel Oil 
How Stuff Works - Oil Refining
The True Cost of #6 Oil
Burning #6 Fuel Oil 
Identity and Analysis of Total Petroleum Hydrocarbons
Poten & Partners: Advisors to the Fuel Oil Industry
Classification of Fuel Oils
Marine Fuel Oil (ISO 8217) 

Category:Fossil fuels
Category:Oils