Fuel oil also known as heavy fuel oil, marine oil or furnace oil is a fraction obtained from petroleum distillation, either as a distillate or a residue. In the MARPOL Marine Convention of 1973, heavy fuel oil is defined either by a density of greater than 900 kg/m³ at 15°C or a kinematic viscosity of more than 180 mm²/s at 50°C. Heavy fuel oils have large percentages of heavy molecules such as long-chain hydrocarbons and aromatics with long-branched side chains. Heavy fuel oil is mainly Marine Fuel, virtually all medium and low-speed marine diesel engines are designed for heavy fuel oil. To achieve various specifications and quality levels, these residual fuels are blended with lighter fuels such as marine gas oil or marine diesel oil. The resulting blends are also referred to as intermediate fuel oils (IFO) or marine diesel oil. The most commonly used types are IFO 180 and IFO 380, with viscosities of 180 mm²/s and 380 mm²/s, respectively. A key differentiator of heavy fuel oils is their sulfur content. Their maximum sulfur content must not exceed 3.5%. The following main classes with regard to the sulfur content can be distinguished:
| Marine Fuel | Max. sulfur content |
| High sulfur fuel oil (HSFO) | 3.5% |
| Low sulfur fuel oil (LSFO) | 1.0% |
| Ultra low sulfur fuel oil (ULSFO) | 0.1% |
Low sulfur fuel oil (LSFO)
Heavy fuel oils are referred to as low sulfur fuel oil (LSFO) if their sulfur content is below 1%. Usually, these are marine fuel types IFO 180 or IFO 380, which have been desulfurized the end of 2014, ships could still travel through Emission Control Areas (ECAs) with this type of marine fuel.
Ultra-low sulfur 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% sulfur 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 sulfur fuel oil (ULSFO) marine fuel, which complies with those limits. Theoretically, heavily desulfurized IFO fuels could also be used here, but in practice, the desulfurization of such heavy fuel oils is too expensive to make economic sense. For this reason, today the term ultra-low sulfur fuel oil usually refers not to desulfurized heavy fuel oils, but to marine gas oil, which is already low in sulfur. It is composed exclusively of distillates and has a sulfur content of under 0.1%. This marine fuel is also known as ultra-low marine gas oil. 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-sulfur fuel oil (HSFO)
The alternative to using marine fuels with such low sulfur content in ECAs is the use of scrubbers. This technology involves injecting water into the exhaust stream to reduce sulfur and other emissions. However, refiting 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 ship-owners. On the other hand, a scrubber allows higher-sulfur marine fuels to be used. In this context, such heavy fuel oils are designated as high-sulfur fuel oils (HSFO), which have a maximum sulfur content of 3.5% as permitied under ISO 8217.