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. 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.
Fuel oil is classified into six classes, numbered 1 through 6, according to its boiling point, 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 fuel oil number. Viscosity also increases with 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 and No. 3 fuel oil are variously 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.
Gas oil refers to the process of distillation. The oil is heated, becomes a gas and then condenses. It differentiates distillates from residual oil. No. 1 is similar to kerosene and is the fraction that boils off right after gasoline. No. 2 is the diesel that trucks and some cars run on, leading to the name "road diesel". It is the same thing as heating oil. No. 3 is a distillate fuel oil and is rarely used. No. 4 fuel oil is usually a blend of distillate and residual fuel oils, such as No. 2 and 6; however, sometimes it is just a heavy distillate. No. 4 may be classified as diesel, distillate or residual fuel oil. No. 5 fuel oil and No. 6 fuel oil are called residual fuel oils or heavy fuel oils. As far more No. 6 than No. 5 is produced, the terms heavy fuel oil and residual fuel oil are sometimes used as synonyms for No. 6. They are what remains of the crude oil after gasoline and the distillate fuel oils are extracted through distillation. No. 5 fuel oil is a mixture of No. 6 (about 75-80%) with No. 2. No. 6 may also contain a small amount of No. 2 to get it to meet specifications.
Residual fuel oils are sometimes called light when they have been mixed with distillate fuel oil, while distillate fuel oils are called heavy when they have been mixed with residual fuel oil. Heavy gas oil, for example, is a distillate that contains residual fuel oil. The ready availability of very heavy grades of fuel oil is often due to the success of catalytic cracking of fuel to release more valuable fractions and leave heavy residue.
Oil has many uses; it heats homes and businesses and fuels trucks, ships and some cars. A small amount of electricity is produced by diesel, but it is more polluting and more expensive than natural gas. It is often used as a backup fuel for peaking power plants in case the supply of natural gas is interrupted or as the main fuel for small electrical generators. In Europe the use of diesel is generally restricted to cars (about 40%), SUVs (about 90%), and trucks. The market for home heating using fuel oil, called heating oil, has decreased due to the widespread penetration of natural gas. However, it is very common in some areas, such as the Northeastern United States.