The term marine dieseloil(MDO) generally describesmarine fuelsthat are composed of various blends of distillates (also calledmarine gasoil) andheavy fuel oil. Unlikediesel fuelson land that are used for cars and trucks, marine dieseloilis not apuredistillate. The different blending ratios of marine dieseloilcan be controlled directly by processes in the refinery or by blending ready-made marinefuels. Marine diesel is similar to dieselfuel, but has a higher density. Unlikeheavyfueloil(HFO), marine dieseloildoes not have to be heated during storage.
Marine dieseloilis sometimes also used synonymously with the term “intermediatefueloil” (IFO). In the strict sense, the term marine dieseloilmainly refers to blends with a very small proportion ofheavyfueloil. This type of marine dieseloilis therefore also classified as a distillate in some textbooks, which means it is also categorized as amiddle distillate. Intermediatefueloils, on the other hand, have a higher proportion ofheavyfueloil. Accordingly, IFO types with particularly high proportions ofheavyfueloilare sometimes classified asheavyfueloils in some textbooks, standards/norms and publications. This results in the following brief descriptions:
- Marine dieseloilin general: a blend of distillates andheavyfueloil
- Marine dieseloilin a narrow sense: Blend of distillates andheavyfueloil, but with very lowheavyfueloilcontent
- Intermediatefueloil(IFO): Marine diesel with higher proportions ofheavyfueloil
According to the ISO 8217 "Petroleum Products –Fuel(class F)" international standard, marine dieseloilwith a lower proportion ofheavyfueloilinclude the DMB and RMA 10fuels. Theheavyfueloilcomponents in DMB marine dieseloil– which is classified as a distillate according to ISO 8217 – come mainly from residues ofheavyfueloilfrom tanks where DMB is stored. Due to this pollution byheavyfueloil, 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 dieseloilRMA 10 already counts as a residualfuel. It has a similarly low viscosity to DMB, but a higher maximum permissible sulfur content of 3.5%, and also a higher proportion ofheavyfueloil. Therefore, this type offuelis also usually darker than DMB. Its color spectrum starts from dark brown and likewise ends in black.
Intermediatefueloils are black due to their higher proportion ofheavyfueloil. In ISO 8217, IFOfuels with RME, RMG and RMK designations and viscosities of180 mm²/sor380 mm²/scount as residualfuels. These marine dieseloils are already so viscous that they must be heated so that they can be pumped at all.
Marine dieseloilis 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-fuelcombination 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 dieseloilin 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 dieseloilis 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 marinefuelwith higher emissions.
More information about global maritime regulations and setting emission limit values can be found in the article onMarine Fuels (Bunker Fuels).
Lighter marine gasoil(MGO) and marine dieseloil(MDO)fuels are significantly more expensive thanheavyfueloil(HFO), so that commercial shipping still usesheavyfueloilfor the most part.
