Rudolf Diesel intended the engines that bear his name to run on a wide variety of fuels and in fact his first two engines were fuelled by coal dust but were unsuccessful and Diesel turned instead to oil. Less than 20 years after Diesel patented his engine, the first ships powered by them were brought into service. The fuel used then was a variant of modern MDO with heavier oils coming later.
Modern shipowners enjoy a choice of fuels that their predecessors could only dream of with the available fuel mix today including all the standard types of mineral oils such as HFO, MDO and MGO together with vegetable and animal-based bio-fuels, LNG, ethane, methanol and to a lesser extent battery power. Hydrogen is tipped as the next fuel type – although maybe not in an internal combustion engine – and ammonia is also being touted as a future fuel.
Whatever the future of shipping may hold with regard to fuel and propulsion systems, it is generally accepted that fossil fuels will be the main source of power for the next 30 years at least. Oil fuels exist in several varieties and although it is possible for there to be an infinite number of different composition fuels, in practice the use of standard fuels is the norm. There is an ISO standard for marine fuels which is updated at regular intervals.
Work on the fourth edition of ISO 8217 began in March 2008, about the same time that the IMO requested ISO to prepare a specification for marine fuels to coincide with the implementation of the Revised MARPOL Annex VI on 1st July 2010. The next version appeared in 2012 (5th edition) and the most recent in March 2017 (6th edition).
While these standards exist there is no obligation on freely contracting parties to accept only the latest or indeed any standard whatsoever. Up to and including the 2012 version of ISO 8217, all of the fuels were fossil fuels, The changes in the 2017 version are far-reaching, not least since the main change is the addition of a new set of distillate grades containing bio-fuels.
These new fuels are distinguished as DFA, DFZ and DFB and as can be seen from the table below, they share most characteristics with their fossil fuel counterparts but have an additional entry for fatty acids.
The ISO 8217 standard for distillate fuels lists several grades as can be seen from the tables. Whilst these fuel types will be readily recognisable to bunker professionals and engineers, they may be unfamiliar to other shipping practitioners. For all practical purposes the fuel generally referred to as Marine Diesel Oil (MDO) is listed as DMB in the ISO standard and Marine Gas Oil (MGO) as DMA.
For residual fuels, the terms Heavy Fuel Oil (HFO) and Intermediate Fuel Oil (IFO) are often used interchangeably. This can be confusing as both terms are technical correct. HFO is a residual fuel incurred during the distillation of crude oil. The quality of the residual fuel depends on the quality of the crude oil. To achieve various specifications and quality levels, these residual fuels are blended with lighter fuels such as marine gasoil or marine diesel oil. The resulting blends are also referred to as intermediate fuel oils (IFO) or marine diesel oil. They are classified and named according to their viscosity, IFO 180 and IFO 380, with viscosities of 180 mm²/s and 380 mm²/s, respectively.
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.
The ISO 8217 international standard divides marine fuels into distillate marine fuels and residual marine fuels (RMA). The latter are collectively called heavy fuel oils. An exception is the lowest viscous quality level, RMA 10, which is no longer referred to as an HFO, as its proportion of heavy fuel oil is so small. ISO 8217 stipulates that residual fuels, and therefore all heavy fuel oils, may not contain old oil or lubricating oils.