The term marine diesel oil (MDO) generally describes marine fuels that are composed of various blends of distillates (also called marine gasoil) and heavy fuel oil. Unlike diesel fuels on land that are used for cars and trucks, marine diesel oil is not a pure distillate. The different blending ratios of marine diesel oil can be controlled directly by processes in the refinery or by blending ready-made marine fuels. Marine diesel is similar to diesel fuel, but has a higher density. Unlike heavy fuel oil (HFO), marine diesel oil does not have to be heated during storage.

Marine diesel oil is sometimes also used synonymously with the term “intermediate fuel oil” (IFO). In the strict sense, the term marine diesel oil mainly refers to blends with a very small proportion of heavy fuel oil. This type of marine diesel oil is therefore also classified as a distillate in some textbooks, which means it is also categorized as a middle distillate. Intermediate fuel oils, on the other hand, have a higher proportion of heavy fuel oil. Accordingly, IFO types with particularly high proportions of heavy fuel oil are sometimes classified as heavy fuel oils in some textbooks, standards/norms and publications. This results in the following brief descriptions:

• Marine diesel oil in general: a blend of distillates and heavy fuel oil
• Marine diesel oil in a narrow sense: Blend of distillates and heavy fuel oil, but with very low heavy fuel oil content 
• Intermediate fuel oil (IFO): Marine diesel with higher proportions of heavy fuel oil

According to the ISO 8217 "Petroleum Products – Fuel (class F)" international standard, marine diesel oil with a lower proportion of heavy fuel oil include the DMB and RMA 10 fuels. The heavy fuel oil components in DMB marine diesel oil – which is classified as a distillate according to ISO 8217 – come mainly from residues of heavy fuel oil from tanks where DMB is stored. Due to this pollution by heavy fuel oil, its color changes can range from light brown to black. As emission limits for Emission Control Areas (ECAs) become ever stricter, DMB with its relatively high sulfur content of about 2% is being used much less frequently.

According to ISO 8217, marine diesel oil RMA 10 already counts as a residual fuel. It has a similarly low viscosity to DMB, but a higher maximum permissible sulfur content of 3.5%, and also a higher proportion of heavy fuel oil. Therefore, this type of fuel is also usually darker than DMB. Its color spectrum starts from dark brown and likewise ends in black.

Intermediate fuel oils are black due to their higher proportion of heavy fuel oil. In ISO 8217, IFO fuels with RME, RMG and RMK designations and viscosities of 180 mm²/s or 380 mm²/s count as residual fuels. These marine diesel oils are already so viscous that they must be heated so that they can be pumped at all.

Marine diesel oil is sold with different sulfur content levels. For example, IFO 180 and IFO 380 can have a maximum sulfur content of 3.5% according to ISO 8217. They are also sold in a low-sulfur variant, which has a sulfur content of less than 1%. Ships could even enter an Emission Control Area (ECA) with the latter. However, if ship-owners use an engine-fuel combination with a high sulfur content, the emission limit values can also be met by using additional technology (filter systems, scrubbers).

Their different blending ratios make it possible to use marine diesel oil in many different engines. Lighter versions like DMB and RMA 10 are used to power smaller medium- to high-speed marine engines and auxiliary power units, as well as auxiliary engines on very large ships, while the viscous IFO 380 is mainly used in large aggregates. Lightweight and low-sulfur marine diesel oil is sometimes burned in larger engines as well – whenever an area with stricter emission limits is being crossed. Once outside the area, the ship will switch back to a marine fuel with higher emissions.

More information about global maritime regulations and setting emission limit values can be found in the article on Marine Fuels (Bunker Fuels).

Lighter marine gasoil (MGO) and marine diesel oil (MDO) fuels are significantly more expensive than heavy fuel oil (HFO), so that commercial shipping still uses heavy fuel oil for the most part.