Fuel oil

An oil tanker taking on fuel, or "bunkering."

Fuel oil (also known as heavy oil, marine fuel or furnace oil) is a fraction obtained from petroleum distillation, either as a distillate or a residue. Broadly speaking, fuel oil is any liquid fuel 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 42 °C (108 °F) and oils burned in cotton or wool-wick burners. 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, i.e., heavier than gasoline and naphtha.


United States

Although the following trends generally hold true, different organizations may have different numerical specifications for the six fuel grades. The boiling point and carbon chain length of the fuel increases with fuel oil number. Viscosity also increases with number, and the heaviest oil must be heated for it to flow. Price usually decreases as the fuel number increases. [1]

Number 1 fuel oil is a volatile distillate oil intended for vaporizing pot-type burners. [2] It is the kerosene refinery cut that boils off immediately after the heavy naphtha cut used for gasoline. Former names include: coal oil, stove oil and range oil. [1]

Number 2 fuel oil is a distillate home heating oil. [2] This fuel is sometimes known as Bunker A. Trucks and some cars use similar diesel fuel with a cetane number limit describing the ignition quality of the fuel. Both are typically obtained from the light gas oil cut. Gas oil refers to the original use of this fraction in the late 19th and early 20th centuries – the gas oil cut was used as an enriching agent for carburetted water gas manufacture. [1]

Number 3 fuel oil was a distillate oil for burners requiring low-viscosity fuel. ASTM merged this grade into the number 2 specification, and the term has been rarely used since the mid-20th century. [2]

Number 4 fuel oil is a commercial heating oil for burner installations not equipped with preheaters. [2] It may be obtained from the heavy gas oil cut. [1]

Number 5 fuel oil is a residual-type industrial heating oil requiring preheating to 77–104 °C (171–219 °F) for proper atomization at the burners. [2] This fuel is sometimes known as Bunker B. It may be obtained from the heavy gas oil cut, [1] or it may be a blend of residual oil with enough number 2 oil to adjust viscosity until it can be pumped without preheating. [2]

Number 6 fuel oil is a high-viscosity residual oil requiring preheating to 104–127 °C (219–261 °F). Residual means the material remaining after the more valuable cuts of crude oil have boiled off. The residue may contain various undesirable impurities, including 2% water and 0.5% mineral soil. This fuel may be known as residual fuel oil (RFO), by the Navy specification of Bunker C, or by the Pacific Specification of PS-400. [2]

United Kingdom

The British Standard BS 2869, Fuel Oils for Agricultural, Domestic and Industrial Engines, specifies the following fuel oil classes:

Fuel Oil Classes per BS 2869
Class Type Min. kinematic viscosity Max. kinematic viscosity Min. flash point Max. sulphur content Alias
C1 Distillate 43 °C 0.040 % (m/m) Paraffin
C2 Distillate 1.000 mm²/s at 40 °C 2.000 mm²/s at 40 °C 38 °C 0.100 % (m/m) Kerosene, 28-second oil
A2 Distillate 2.000 mm²/s at 40 °C 5.000 mm²/s at 40 °C > 55 °C 0.001 % (m/m) low-sulphur gas oil, ULSD
D Distillate 2.000 mm²/s at 40 °C 5.000 mm²/s at 40 °C > 55 °C 0.100 % (m/m) Gas oil, red diesel, 35-second oil
E Residual 8.200 mm²/s at 100 °C 66 °C 1.000 % (m/m) Light fuel oil, LFO, 250-second oil
F Residual 8.201 mm²/s at 100 °C 20.000 mm²/s at 100 °C 66 °C 1.000 % (m/m) Medium fuel oil, MFO, 1000-second oil
G Residual 20.010 mm²/s at 100 °C 40.000 mm²/s at 100 °C 66 °C 1.000 % (m/m) Heavy fuel oil, HFO, 3500-second oil
H Residual 40.010 mm²/s at 100 °C 56.000 mm²/s at 100 °C 66 °C 1.000 % (m/m)

Class C1 and C2 fuels are kerosene-type fuels. C1 is for use in flueless appliances (e.g. lamps). C2 is for vaporising or atomising burners in appliances connected to flues.

Class A2 fuel is suitable for mobile, off-road applications that are required to use a sulphur-free fuel. Class D fuel is similar to Class A2 and is suitable for use in stationary applications, such as domestic, commercial, and industrial heating. The BS 2869 standard permits Class A2 and Class D fuel to contain up to 7% (V/V) biodiesel ( fatty acid methyl ester, FAME), provided the FAME content meets the requirements of the BS EN 14214 standard.

Classes E to H are residual oils for atomizing burners serving boilers or, with the exception of Class H, certain types of larger combustion engines. Classes F to H invariably require heating prior to use; Class E fuel may require preheating, depending on ambient conditions.


Mazut is a residual fuel oil often derived from Russian petroleum sources and is either blended with lighter petroleum fractions or burned directly in specialized boilers and furnaces. It is also used as a petrochemical feedstock. In the Russian practice, though, "mazut" is an umbrella term roughly synonymous with the fuel oil in general, that covers most of the types mentioned above, except US grades 1 and 2/3, for which separate terms exist ( kerosene and diesel fuel/solar oil respectively — Russian practice doesn't differentiate between diesel fuel and heating oil). This is further separated in two grades, "naval mazut" being analogous to US grades 4 and 5, and "furnace mazut", a heaviest residual fraction of the crude, almost exactly corresponding to US Number 6 fuel oil and further graded by viscosity and sulphur content.

Other Languages
العربية: زيت الوقود
català: Fueloil
Deutsch: Schweröl
español: Fueloil
euskara: Fuel olio
français: Fioul
Gaeilge: Ola (breosla)
galego: Fuel
hrvatski: Loživa ulja
Bahasa Indonesia: Minyak bakar
lietuvių: Fuel-oil
Nederlands: Stookolie
日本語: 重油
norsk: Tungolje
norsk nynorsk: Semidiesel
português: Óleo combustível
Simple English: Fuel oil
slovenščina: Kurilno olje
српски / srpski: Ulje za loženje
srpskohrvatski / српскохрватски: Loživa ulja
svenska: Tjockolja
தமிழ்: எரிநெய்
Türkçe: Fuel oil
Yorùbá: Fuel oil