FUEL OIL DEFINED: Six classes Fuel oil in the United States 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 (RFO). 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 (RFO) 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. The US nomenclature is used in most of the world. In the United Kingdom the classes comprise 6 commonly used fuels using alphabetical designations, from Class C1 (kerosene) to Class G (heavy fuel oil). There is a Class H designation which is not yet in general use. The characteristics of these oils are specified in British Standard BS2869:1998 - soon to be updated to BS2869:2006. Bunker fuel Bunker fuel is technically any type of fuel oil used aboard ships. It gets its name from the containers on ships and in ports that it is stored in; in the days of steam they were coal bunkers but now they are bunker tanks. The Australian Customs and the Australian Tax Office defines a bunker fuel as the fuel that powers the engine of a ship or aircraft. Bunker A is No. 2 fuel oil, bunker B is No. 4 or No. 5 and bunker C is No. 6. Since No. 6 is the most common, "bunker fuel" is often used as a synonym for No. 6. No. 5 fuel oil is also called navy special fuel oil or just navy special, No. 6 or 5 are also called furnace fuel oil (FFO); the high viscosity requires heating, usually by a reticulated low pressure steam system, before the oil can be pumped from a bunker tank. In the context of shipping, the labelling of bunkers as previously described is rarely used in modern practice.