Leonetti & Matarazzo

Oil & Gas

Fuel Oil

Fuel oil (also known as heavy oil, marine fuel or furnace oil) is a fraction obtained from petroleum distillation. 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 can be defined as Fuel Oil up to a flash point of approximately 42 °C (108 °F). 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. Small molecules like those in propane, naphtha, gasoline for cars, and jet fuel have relatively low boiling points, and they are removed at the start of the fractional distillation process. Heavier petroleum products like diesel fuel and lubricating oil are much less volatile and distil out more slowly, while bunker oil is literally the bottom of the barrel; in oil distilling, the only things denser than bunker fuel are carbon black feedstock and bituminous residue (asphalt), which is used for paving roads and sealing roofs.

General Classification

United States

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.

  • Number 1 fuel oil is a volatile distillate oil intended for vaporizing pot-type burners. 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.
  • Number 2 fuel oil is a distillate home heating oil. 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.
  • 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.
  • Number 4 fuel oil is a commercial heating oil for burner installations not equipped with preheaters. It may be obtained from the heavy gas oil cut.
  • 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. This fuel is sometimes known as Bunker B. It may be obtained from the heavy gas oil cut,[4] or it may be a blend of residual oil with enough number 2 oil to adjust viscosity until it can be pumped without preheating.
  • 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.


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.

Maritime Fuel Classification

In the maritime field another type of classification is used for fuel oils:

  • MGO (Marine gas oil) - roughly equivalent to No. 2 fuel oil, made from distillate only.
  • MDO (Marine diesel oil) - A blend of heavy gasoil that may contain very small amounts of black refinery feed stocks, but has a low viscosity up to 12 cSt so it need not be heated for use in internal combustion engines.
  • IFO (Intermediate fuel oil) - A blend of gasoil and heavy fuel oil, with less gasoil than marine diesel oil.
  • MFO (Marine fuel oil) - same as HFO (just another "naming").
  • HFO (Heavy fuel oil) - Pure or nearly pure residual oil, roughly equivalent to No. 6 fuel oil.

Standards and Classification

CCAI and CII are two indexes which describe the ignition quality of residual fuel oil, and CCAI is especially often calculated for marine fuels. Despite this, marine fuels are still quoted on the international bunker markets with their maximum viscosity (which is set by the ISO 8217 standard - see below) due to the fact that marine engines are designed to use different viscosities of fuel. The unit of viscosity used is the centistoke (cSt) and the fuels most frequently quoted are listed below in order of cost, the least expensive first.

  • IFO 380 - Intermediate fuel oil with a maximum viscosity of 380 centistokes (< 3.5% sulphur).
  • IFO 180 - Intermediate fuel oil with a maximum viscosity of 180 centistokes (< 3.5% sulphur).
  • LS 380 - Low-sulphur (< 1.0%) intermediate fuel oil with a maximum viscosity of 380 centistokes.
  • LS 180 - Low-sulphur (< 1.0%) intermediate fuel oil with a maximum viscosity of 180 centistokes.
  • MDO - Marine diesel oil.
  • MGO - Marine gasoil.
  • LSMGO - Low-sulphur (< 0.1%) Marine Gas Oil - The fuel is to be used in EU Ports and Anchorages. EU Sulphur directive 2005/33/EC
  • ULSMGO - Ultra-Low-Sulphur Marine Gas Oil - referred to as Ultra-Low-Sulphur Diesel (sulphur 0.0015% max) in the US and Auto Gas Oil (sulphur 0.001% max) in the EU. Maximum sulphur allowable in US territories and territorial waters (inland, marine and automotive) and in the EU for inland use.

Residual Bunker Oils (ISO 8217)

Residual Fuel Kin. Viscosity [mm²/s] at 50°C Density [g/cm³] at 15°C
RMA 30 < 30 < 0.960
RMB 30 < 30 < 0.975
RMD 80 < 80 < 0.980
RME 180 < 180 < 0.991
RMF 180 < 180 < 0.991
RMG 380 < 380 < 0.991
RMH 380 < 380 < 0.991
RMH 700 < 700 < 0.991
RMK 380 < 380 < 1.010
RMK 700 < 700 < 1.010

A key differentiator of heavy fuel oils is their sulphur content. According to ISO 8217, their maximum sulphur content must not exceed 3.5%. The following main classes with regard to the sulphur content can be distinguished:

Marine fuel Max. sulphur content
High sulphur fuel oil (HSFO) 3.5%
Low sulphur fuel oil (LSFO) 1.0%
Ultra low sulphur fuel oil (ULSFO) 0.1%

Low sulphur fuel oil (LSFO)
Heavy fuel oils are referred to as low sulphur fuel oil (LSFO) if their sulphur content is below 1%. Usually these are marine fuel types IFO 180 or IFO 380, which have been desulphurised. Until the end of 2014, ships could still travel through Emission Control Areas (ECAs) with this type of marine fuel.

Ultra-low sulphur fuel oil (ULSFO)
Since January 1, 2015, in accordance with Annex VI of the MARPOL Conventions, ship emissions must contain no more than 0.1% sulphur in such protected areas (ECAs). Due to these tightened restrictions, LSFOs no longer play an appreciable role in these areas and have been virtually replaced with the ultra-low sulphur fuel oil (ULSFO) marine fuel, which complies with those limits. Theoretically, heavily desulphurised IFO fuels could also be used here, but in practice the desulphurization of such heavy fuel oils is too expensive to make economic sense. For this reason, today the term ultra-low sulphur fuel oil usually refers not to desulphurised heavy fuel oils, but to marine gasoil, which is already low in sulphur. It is composed exclusively of distillates and has a sulphur content of under 0.1%. This marine fuel is also known as ultra-low marine gasoil. ULSFO is used in medium- to high-speed diesel engines. When converting from LSFO to ULSFO, it must be ensured that the engine technology is compatible with ULSFO.

High-sulphur fuel oil (HSFO)
The alternative to using marine fuels with such low sulphur content in ECAs is the use of scrubbers. This technology involves injecting water into the exhaust stream to reduce sulphur and other emissions. However, refitting a ship with this technology costs several million euros and means that the ship is docked for a period of time, which leads to a loss in revenue and income for the shipowners. On the other hand, a scrubber allows higher-sulphur marine fuels to be used. In this context, such heavy fuel oils are designated as high-sulphur fuel oils (HSFO), which have a maximum sulphur content of 3.5% as permitted under ISO 8217.