Marine gas oil (MGO): ISO 8217 distillate grades

Marine gas oil (MGO) is a distillate marine fuel specified under ISO 8217, principally as the grades DMA and DMZ, that is bunkered and burned without preheating and contains no residual component. It is refined from the middle-distillate fraction of crude oil, it is the fuel that most ships burn in their auxiliary engines, and it is the compliance route the majority of ships without abatement equipment take inside the emission control areas designated under MARPOL Annex VI Regulation 14.

That short definition hides a specification that changed materially in May 2024 and a regulatory position that changed four times between 2024 and 2026. ISO 8217:2024, the seventh edition, removed the sulphur ceiling from the distillate table, lifted the FAME cap on the bio-distillate grades and changed the cold-flow method references. The emission control area register grew from five entries to eight. The bunker delivery note gained a flashpoint line. And the retained bunker sample minimum rose from 400 ml to 600 ml. This article states each of those as they stand, with the instrument that sets them.

What marine gas oil is, and what it is not

Marine gas oil is a trade name, not a specification, and the distinction decides contracts. Suppliers sell MGO against ISO 8217 grade DMA or DMZ and, in some markets, against DMB, which is more usually sold as marine diesel oil. The document that fixes what was actually delivered is the bunker delivery note , whose product name entry is required by MARPOL Annex VI Regulation 18.5.1, read with the certificate of quality. A charter party bunker clause or a purchase order that says “MGO” and nothing more has specified almost nothing.

The property that separates distillate from residual bunker is the absence of a residue fraction. Heavy fuel oil and the 0.50 per cent blends sold as very low sulphur fuel oil both carry asphaltenes and, where the refinery route included catalytic cracking, cat fines . Distillate carries neither in any quantity a specification needs to control, which is why ISO 8217 Table 1 has no aluminium plus silicon row and no asphaltene row at all.

What follows from that is a fuel system without heating. A distillate is pumpable and injectable at ambient temperature in most trades, so the steam tracing, the settling tank heating coils and the end heater that a marine fuel oil system needs for residual fuel are not needed. The engineering consequence runs the other way instead, and it is the point this article returns to: distillate more often needs cooling than heating.

ISO 8217:2024 distillate grades

ISO 8217:2024 defines seven distillate categories: the petroleum grades DMX, DMA, DMZ and DMB, and the bio-distillate grades DFA, DFZ and DFB. Clause 1 records that one of the seven, DMX, is for diesel engines used for emergency purposes. The standard is titled “Products from petroleum, synthetic and renewable sources: Fuels (class F): Specifications of marine fuels”, it was published in May 2024, and its Foreword states that this seventh edition cancels and replaces the sixth edition, ISO 8217:2017 .

The edition has four grade tables. Table 1 covers distillate and bio-distillate; Table 2 covers residual fuels at or below 0.50 per cent sulphur; Table 3 covers bio-residual RF grades; Table 4 covers residual fuels above 0.50 per cent sulphur. Tables 2 and 3 are new in this edition, and the former Table 2 was modified and became Table 4.

Table 1: ISO 8217:2024 Table 1 distillate and bio-distillate limits. Values as printed in the seventh edition, published May 2024. The sulphur row is the standard’s own position and is overridden downward by MARPOL Annex VI Regulation 14 inside an emission control area.

Three footnotes carry as much weight as the numbers. The lubricity requirement applies only to fuels with a sulphur content below 500 mg/kg, that is 0.050 per cent by mass. The existent total sediment and water characteristics on DMB and DFB are required only where the sample is not clear and bright. And the pour point rows carry the standard’s own caveat: pour point cannot guarantee operability for all ships in all climates, and the buyer should confirm that the cold flow characteristics suit the ship’s design and intended voyage.

DMA, the ordinary grade

DMA is the grade most stems described as marine gas oil are blended to. It runs 2.000 to 6.000 mm2/s at 40 degrees C, is capped at 890.0 kg/m3 at 15 degrees C, carries a minimum flash point of 60.0 degrees C and a minimum cetane index of 40.0 by ISO 4264, and takes a pour point of minus 6 degrees C in winter and 0 degrees C in summer.

DMZ, the raised viscosity floor

DMZ differs from DMA in one respect that matters and none that does not. Its minimum viscosity is 3.000 mm2/s at 40 degrees C rather than 2.000. Maximum viscosity, maximum density, minimum flash point and minimum cetane index are identical. The purpose of the grade is a guaranteed viscosity floor, so that a ship can hold the engine-inlet minimum without cooling on a fuel that a DMA specification could deliver at 2.000 mm2/s. It is a fuel-pump protection, not a combustion-quality screen.

DMX, the emergency grade

DMX exists to serve the SOLAS permission for a 43 degrees C flash point in emergency service. It is the only distillate grade with a flash point minimum below 60.0 degrees C, runs 1.400 to 5.500 mm2/s at 40 degrees C, carries the highest cetane index minimum in the table at 45.0, and takes its cold-flow control from a maximum cloud point of minus 16 degrees C rather than from a pour point limit, of which it has none.

Its low flash point makes it a segregation and labelling problem in storage, and it is not a general-purpose fuel. WinGD states in its fuel guidance that the DMX grade is not applicable for its two-stroke engines because of its low flash point and viscosity.

DMB, sold as marine diesel oil

DMB is the grade behind the trade name marine diesel oil . It permits a small residual carry-over, which the table controls indirectly: 11.00 mm2/s at 40 degrees C, 900.0 kg/m3, a minimum cetane index of 35.0, and existent total sediment and water characteristics that the purer grades do not carry. Its pour point limits are 0 degrees C in winter and 6 degrees C in summer, so it is a warmer-weather fuel than DMA.

DMB has a long service record in medium-speed four-stroke engines on ferries, coasters and offshore vessels where the engine room was never fitted for residual fuel but the operator wanted a discount to premium distillate. WinGD groups DMB and DFB together as MDO in its fuel tables, separately from DMA, DFA, DMZ and DFZ as MGO.

DFA, DFZ and DFB, the bio-distillate grades

The three DF grades are the bio-distillate counterparts of DMA, DMZ and DMB, and they were introduced in the 2017 sixth edition. Their limits mirror their petroleum equivalents on viscosity, density and flash point, and diverge on three characteristics the 2024 edition added or changed. They carry a minimum cetane number rather than a calculated index. They express oxidation stability as a minimum induction period of 8.0 hours rather than as a maximum of 25 g/m3 of insolubles. And they require the net heat of combustion to be reported.

What the 2024 edition changed for distillate

Five changes, taken from the Foreword’s own list:

• FAME content. The reporting requirement for DF grades changed to allow up to 100 per cent, against the 7.0 per cent by volume ceiling in the 2017 edition.
• Cold flow. The winter and summer distinction was removed for cloud point and cold filter plugging point. It was retained for pour point.
• Net heat of combustion. Reporting was added for the DF grades.
• Cetane number. A minimum was added for the DF grades.
• Oxidation stability. A requirement was added for the DF grades.

Two further changes matter and are not in that list. The sulphur row moved from stated numbers to “statutory requirements”: the 2017 edition had set 1.00 per cent m/m for DMX, DMA, DFA, DMZ and DFZ, and 1.50 per cent m/m for DMB and DFB. And the cold filter plugging point method references changed from IP 309 and IP 612 to EN 116 and EN 16329. A ship’s fuel procedure that still names IP 309 is citing a method the current standard does not reference.

Marine gas oil compared with VLSFO and heavy fuel oil

The choice between distillate and residual fuel is decided on five things: whether the fuel needs heating, what it does to the fuel injection equipment, what it costs, what it emits, and whether the ship carries abatement equipment. The following comparison sets those out against the instruments that fix each row.

Table 2: distillate against residual bunkers on the dimensions that decide a fuel strategy. Grade families from ISO 8217:2024; carbon factor and calorific value rows from IMO resolution MEPC.364(79) section 2.2.1; well-to-tank row from Regulation (EU) 2023/1805 Annex II.

Two rows in that table are read wrongly more often than the rest. The carbon factor row looks like a penalty on distillate and is not, because the calorific value row moves the other way: MGO carries about 3.0 per cent more CO2 per tonne of fuel and about 3.1 per cent less per unit of energy delivered. And the VLSFO carbon factor row cannot be filled in from the marketing name. MEPC.364(79) has no VLSFO row at all: the factor follows the ISO 8217 grade on the bunker delivery note, so an RMG 380 supplied as VLSFO takes the heavy fuel oil value of 3.114, while a distillate-based very low sulphur product supplied as a DM grade takes 3.206.

Sulphur content and emission control area compliance

MARPOL Annex VI Regulation 14 sets two sulphur limits, and marine gas oil is the compliance route that needs no equipment. Regulation 14.1 provides that the sulphur content of fuel oil used or carried for use on board a ship shall not exceed 0.50 per cent m/m. Regulation 14.4 provides that while a ship is operating within an emission control area, the sulphur content of fuel oil used on board shall not exceed 0.10 per cent m/m. Both figures sit in the 2021 Revised MARPOL Annex VI , adopted as resolution MEPC.328(76) on 17 June 2021 and in force from 1 November 2022.

The phrase “or carried for use” in Regulation 14.1 is the carriage prohibition, and it did not arrive with the limit. Resolution MEPC.305(73), adopted 26 October 2018, entered into force on 1 March 2020 and inserted those words into Regulation 14.1 itself, two months after the 0.50 per cent limit took effect on 1 January 2020. A ship without an approved equivalent may therefore not carry non-compliant fuel oil for combustion at all, which is a different and broader duty than not burning it.

An ISO 8217 distillate grade is not automatically compliant. The 2024 edition states no numeric sulphur ceiling, and the 2017 edition’s 1.00 per cent m/m sat ten times above the ECA limit. Compliance is a purchase-specification matter, and its evidence is the sulphur declaration at MARPOL Annex VI Appendix V item 8 read with the supplier’s tick-box declaration at item 10. The Regulation 14 sulphur cap article sets out the enforcement chain in full.

The eight emission control areas

Table 3: SOx and particulate matter emission control areas under MARPOL Annex VI Regulation 14.3, as at 3 September 2026. The “0.10 % applies from” column is a property of Regulation 14.7, not a statement in the designating resolutions.

Seven of the eight are in force. The Canadian Arctic and Norwegian Sea areas entered into force on 1 March 2026 and sit inside the grace period described below, so the 0.10 per cent limit does not yet bite there. The North-East Atlantic ECA is adopted and not in force. The emission control areas article carries the geographic definitions, which are given by coordinates in Appendix VII for five of the areas and by cross-reference to other MARPOL Annexes for the Baltic, the North Sea and the Norwegian Sea.

The Mediterranean Sea ECA is for sulphur oxides and particulate matter only. A ship trading there needs 0.10 per cent fuel or an approved equivalent from 1 May 2025 and needs no NOx Tier III capability for that area.

The twelve-month grace period

Regulation 14.7 provides that for the first 12 months after a designating amendment enters into force, Regulations 14.4, 14.6 and 14.5 so far as it relates to 14.4 do not apply. That single provision generates every “applies from” date in the table above, one year behind the corresponding entry-into-force date. Reading a designating resolution on its own will not produce those dates, because the resolutions do not state them.

The grace period is a sulphur provision and does not extend to NOx. A Tier III obligation applies from the ECA’s entry-into-force date to any ship meeting the build-date test, with no deferral, which is why the two ECA registers have to be read separately.

Flash point and the SOLAS regime

The 60 degrees C minimum flash point for oil fuel is a SOLAS requirement, not a MARPOL one. SOLAS Chapter II-2 Regulation 4.2.1.1 provides that except as otherwise permitted by that paragraph, no oil fuel with a flashpoint of less than 60 degrees C shall be used. MARPOL Annex VI carries no flash point rule of any kind and ends at Regulation 31, so a citation to a MARPOL Annex VI Regulation 43 for flash point points at a provision that does not exist. MARPOL Annex I Regulation 43 is the Antarctic heavy grade oil prohibition, an unrelated rule.

The permitted exceptions are narrow and are all at 43 degrees C. SOLAS II-2/4.2.1.2 permits oil fuel of not less than 43 degrees C flashpoint in emergency generators. Regulations 4.2.1.3 and 4.2.1.4 extend the same floor to emergency fire pump engines and to auxiliary machines outside category A machinery spaces, on conditions covering tank location, temperature measurement on the pump suction, stop valves and joint types. SOLAS II-1/42 and II-1/43 carry the same 43 degrees C closed-cup figure for the emergency generator’s independent fuel oil system. The wider fire-safety framework is at SOLAS Chapter II-2 , and the fuel carriage provisions at SOLAS Chapter VI .

Flash point is measured by the Pensky-Martens closed cup method under ISO 2719, Procedure A for distillate fuels and Procedure B for residual fuels. The flash point of marine fuel article covers the method and its precision.

The documentation regime, which changed twice

Two amendments in quick succession moved flash point from a laboratory result to a document a surveyor reads.

Resolution MEPC.362(79), adopted 16 December 2022 and in force from 1 May 2024, added flashpoint to MARPOL Annex VI Appendix V as item 9. The entry is either the measured flashpoint in degrees C or a statement that the flashpoint has been measured at or above 70 degrees C, and the footnote specifies ISO 2719:2016.

Resolutions MSC.520(106) and MSC.550(108) amend SOLAS Chapter II-2 with effect from 1 January 2026. They add a definition of a confirmed case of flashpoint, being a representative sample analysed by an accredited laboratory reporting the flashpoint as measured to be below 60 degrees C. They require, prior to bunkering, a declaration signed and certified by the oil fuel supplier’s representative that the fuel conforms with the flashpoint requirement, together with the test method used. And they oblige Contracting Governments to notify the IMO of confirmed cases and to take action against non-compliant suppliers.

Low viscosity, lubricity and fuel injection equipment

The binding engineering constraint on distillate is a minimum viscosity at the engine inlet, not a target band. This is the point most often stated backwards, including in older reference material that describes distillate as meeting the 10 to 15 cSt injection requirement at ambient temperature. It does not. DMA at the ISO minimum of 2.000 mm2/s at 40 degrees C sits at the OEM floor before any heat pickup in the fuel system, not in the middle of the preferred band.

MAN Energy Solutions Service Letter SL2014-593, “Guidelines for Operation on Fuels with less than 0.1% Sulphur”, gives a recommended fuel viscosity range at the engine inlet of minimum 2 cSt, normal 10 to 15 cSt, maximum 20 cSt. Its 2019 successor for 0.50 per cent fuels restates the window as 2 to 20 cSt. WinGD requires 2 to 20 mm2/s at the engine inlet, ideally 13 to 17, and permits 3 to 17 for low-viscosity distillate used without heating. WinGD adds a separate table row for distillate: a minimum kinematic viscosity at engine inlet of 2.0 mm2/s, not related to temperature.

What fails below 2 cSt

MAN names three failure modes in the fuel pump, and they are progressive rather than alternative:

• Breakdown of the hydrodynamic oil film, which could result in seizures.
• Insufficient injection pressure, which produces difficulty during start-up and low-load operation.
• Insufficient fuel index margin, which limits acceleration.

WinGD adds a fourth at the system level: leakage in the fuel system, which is the reason it states the 2 mm2/s floor. And the constraint is not confined to the injection pumps. MAN notes that most pumps in the external system, meaning supply pumps, circulating pumps, transfer pumps and the feed pumps for the centrifuge, also need viscosities above 2 cSt to function properly.

MAN’s wear criterion for a fuel pump is expressed as an index increase: a pump is considered worn out for HFO operation when the index increase is 5 to 10 or more under the same conditions as during sea trial. Its practical recommendation is a start check twice a year, to find the individual low-viscosity limit of each engine with its actual worn pumps rather than assuming the design figure.

Why distillate needs a cooler

The equipment that distillate operation adds is a cooler, and in extreme cases a chiller. MAN states that to build in margin for safe and reliable operation and to maintain the required viscosity at engine inlet, installation of coolers will be necessary in those fuel systems which do not have them, and that for very low viscosity distillates a cooler may not be enough given the cooling water available on board, in which case a chiller is a possibility that is not used extensively.

WinGD specifies the arrangement: a direct heat exchanger, tubular or plate, positioned after the high-pressure booster pump and before the viscometer, cooled by low-temperature circuit cooling water at 25 to 36 degrees C. Sea water cooling is not recommended, and chilled water is not required for fuels in accordance with ISO 8217.

The duty is modest. WinGD’s worked example for an 8X52DF at 100 per cent contracted maximum continuous rating, 7,440 kW at a brake specific fuel consumption of 182.3 g/kWh, burning DMA of 2 cSt at 40 degrees C supplied at 45 degrees C and cooled to 28 degrees C to reach 2.5 cSt at the engine inlet, gives 19.7 kW, so a cooler of about 20 kW. On a ship that already carries a residual fuel system with an end heater, the distillate cooler is a small addition; on a distillate-only newbuilding it is designed in from the start. The marine engine fuel injection systems article covers the injection equipment the viscosity window protects.

Lubricity and the wear scar test

Hydrodesulphurisation removes the polar sulphur and nitrogen compounds that give a distillate its natural film-forming ability, so a deeply desulphurised fuel can meet every other limit and still be poor at protecting a fuel pump. ISO 8217:2024 controls it with a maximum corrected wear scar diameter of 520 micrometres at 60 degrees C by ISO 12156-1.

The limit is conditional in a way that is routinely missed. It applies only to fuels with a sulphur content below 500 mg/kg, that is 0.050 per cent by mass, and it cannot be demonstrated at all on a DMB or DFB sample that is not clear and bright. The current test edition made digital camera measurement of the wear scar mandatory.

The field position is less alarming than the specification implies, and it is worth stating because it is a manufacturer’s own record rather than an inference. MAN Energy Solutions states that it does not regard the lubricity of the fuel as a major issue, that it has not heard of or experienced any failure due to fuel lubricity, that its research tests could not provoke a failure due to lack of lubricity, and that it does not usually see the need to use lubricity modifiers. The fuel lubricity and HFRR article covers the test and the debate.

Cetane index and ignition quality

Cetane index is the distillate ignition-quality parameter, and ISO 8217 specifies it by ISO 4264. The index is calculated from density and distillation data; a higher value means a shorter ignition delay and a smoother pressure rise. ISO 8217:2024 requires a minimum of 45.0 for DMX, 40.0 for DMA and DMZ, and 35.0 for DMB.

The bio-distillate grades take a different route. DFA and DFZ carry a minimum cetane number of 40.0 and DFB a minimum of 35.0, measured on an engine or by a derived method rather than calculated. The 2024 edition added that requirement because the calculated index correlation was developed on petroleum distillate and does not hold for an ester blend, so a FAME-containing fuel can carry a plausible calculated index and a poor real ignition quality.

The calculated carbon aromaticity index has no application to distillate. CCAI is a residual-fuel parameter: ISO 8217:2024 carries no CCAI row in Table 1, places it in Tables 2, 3 and 4, and titles the relevant annex ignition characteristics of residual marine fuels. It is calculated from density and viscosity at 50 degrees C, which is not a specified property of any distillate grade, so a CCAI figure quoted for a DMA stem is not merely unhelpful but has no defined input. Any distillate ignition-quality question is answered with cetane index or cetane number, covered further at cetane index and ignition quality .

Cold flow and winter operation

Three temperatures describe how a distillate behaves as paraffin wax crystallises, and ISO 8217:2024 makes only one of them a limit on most grades. Cloud point is where wax first becomes visible. Cold filter plugging point is where the crystals are large enough to block a test filter. Pour point is where the fuel stops flowing under its own weight.

For DMA, DFA, DMZ and DFZ the standard requires cloud point and cold filter plugging point to be reported, not to meet a limit, and sets a pour point maximum of minus 6 degrees C in winter and 0 degrees C in summer. DMB and DFB carry neither cloud point nor CFPP, and take 0 degrees C and 6 degrees C for pour point. DMX has no pour point limit at all, and takes its cold-flow control from a maximum cloud point of minus 16 degrees C.

The standard’s own footnote is the operational warning: pour point cannot guarantee operability for all ships in all climates, and the buyer should confirm that the cold flow characteristics suit the ship’s design and intended voyage. A winter Baltic, Great Lakes or Arctic trade therefore needs cloud point or CFPP written into the purchase specification, because the grade limits alone will not deliver them. The 2024 edition added an informative annex on cold flow characteristics and changed the CFPP methods from IP 309 and IP 612 to EN 116 and EN 16329.

Ships built to an ice class notation and those operating under the Polar Code carry the question into the Polar Water Operational Manual, which must address fuel properties for the anticipated operating temperature range. SOLAS Chapter XIV makes the Code mandatory. Distillate has a structural advantage over residual fuel in cold operation that has nothing to do with wax: a heated HFO bunker becomes unpumpable if the heating fails, and a distillate bunker does not. Further detail is at cold flow properties of marine fuel .

Calorific value and specific fuel consumption

Marine gas oil carries a lower calorific value of 42,700 kJ/kg, against 40,200 for heavy fuel oil, from the table at section 2.2.1 of IMO resolution MEPC.364(79). Regulation (EU) 2023/1805 Annex II states the same figure as 0.0427 MJ/g for ISO 8217 grades DMX to DMB, which is a useful independent confirmation rather than a second source of the number.

The practical consequence is that a ship burns fewer tonnes of distillate than of residual fuel for the same delivered energy, which offsets part of the price premium and which has to be carried into any specific fuel oil consumption comparison. An SFOC figure quoted without its reference calorific value is not comparable across fuels, and the SFOC curves an engine builder publishes are normally referenced to a stated LCV that has to be corrected to the fuel actually burned.

Onboard handling: storage, separation and contamination

Distillate is stored and treated as a separate fuel from residual bunkers, not as a lighter point on the same scale. Most ships carrying both segregate them completely: separate storage, settling, service and day tanks, and separate supply and return pipework. Introducing residual fuel into the distillate system raises viscosity and sulphur and can put the ship out of ECA compliance; introducing distillate into a residual settling tank can destabilise asphaltenes.

Storage needs no heating in most trades, only that the fuel stays above its cloud point. Where trace heating or steam coils are fitted for cold-climate service they must be thermostatically controlled, because overheating drives off light ends and depresses the flash point, which is now a documented and reportable condition under the SOLAS amendments that take effect on 1 January 2026.

Separation

Centrifugal separation remains necessary on distillate even though cat fines and asphaltenes are absent, because free water and sediment are not. The settings do not transfer from residual practice: MAN Energy Solutions gives a separator temperature for distillates of 40 to 50 degrees C, against 98 degrees C or higher for ULSFO and heavy fuel oil. Running a distillate through a separator at HFO temperature wastes heat and risks driving off light ends.

WinGD’s engine-inlet limits are tighter than anything ISO 8217 requires at the point of delivery: a maximum combined aluminium plus silicon of 15 mg/kg and, for MDO, water at 0.20 per cent by volume against 0.30 per cent as bunkered. Those are the numbers the fuel and lube oil purifiers have to deliver, and they are not what the bunker delivery note certifies.

MAN also recommends a fuel pump drain arrangement that keeps distillate and ULSFO pump leakage out of the HFO settling tank, using either two overflow tanks or an additional line from the overflow tank. It is a small piping item that prevents a slow contamination of the residual system.

Microbial contamination

Marine distillate supports microbial growth in a way that residual fuel largely does not, and the growth is a mixed community of bacteria, yeasts and moulds at the fuel and water interface rather than a single organism. The fungus most often named is Amorphotheca resinae, which is the accepted name for the species also encountered as Hormoconis resinae and, in older material, as Cladosporium resinae.

What the growth does is block filters with biomass, emulsify water into the fuel through microbial surfactants, and corrode tank internals with organic acid metabolites. ISO 8217:2024 sets no microbial limit, so the governing documents are test methods and industry guidance rather than the fuel specification: IP 385 by filtration and culture, IP 613 and ASTM D7978 by thixotropic gel culture reporting colony forming units as absent, light, moderate or heavy, immunoassay screening, and the ASTM guide to microbial contamination in fuels and fuel systems together with the Energy Institute guidelines.

Control is water management before it is chemistry. Routine tank draining, effective separation and tight tank venting remove the aqueous phase the community lives in; biocide dosing treats an established population but does not prevent recurrence while free water remains. FAME blending raises the risk, which is one reason a first bio-blend delivery is preceded by tank cleaning. Further detail is at microbial contamination of fuel oil .

Emissions accounting

Marine gas oil carries a carbon conversion factor of 3.206 tonnes of CO2 per tonne of fuel, from the table at section 2.2.1 of IMO resolution MEPC.364(79), the 2022 Guidelines on the method of calculation of the attained EEDI for new ships, adopted at MEPC 79 in December 2022. The row covers ISO 8217 grades DMX through DMB and gives a carbon content of 0.8744 alongside the calorific value.

Table 4: carbon conversion factors and calorific values, from MEPC.364(79) section 2.2.1. The CO2 per unit energy column is derived from the two preceding columns.

Distillate’s higher carbon factor is not evidence of a higher hydrogen content, which is the explanation sometimes offered and which the table refutes: MEPC.364(79) assigns distillate a higher carbon mass fraction than heavy fuel oil, 0.8744 against 0.8493. The higher factor follows directly from that. The carbon conversion factors article carries the full table.

CII, EEXI and EEDI

The Carbon Intensity Indicator applies under MARPOL Annex VI Regulation 28 to twelve ship types of 5,000 GT and above on international voyages, with the first attained CII calculated on calendar year 2023 data. Regulation 28.7 provides that a D rating for three consecutive years, or a single E rating, requires a plan of corrective actions in the SEEMP. The CII guidelines do not carry their own carbon factor table: resolution MEPC.352(78) section 4.1 points at the EEDI guidelines as they may be further amended, and sends the operator to the supplier for a documented factor where a fuel is not covered.

The reduction factors run to 2030 and are settled. Resolution MEPC.400(83), adopted 11 April 2025, replaced the table so that the reduction against the 2019 reference line is 5 per cent in 2023, 7 in 2024, 9 in 2025, 11 in 2026, 13.625 in 2027, 16.250 in 2028, 18.875 in 2029 and 21.500 in 2030. Nothing is set beyond 2030, and a CII corrective action plan is the consequence of falling short.

For a ship weighing distillate against residual fuel on rating, the arithmetic runs both ways and neither direction is large. Per tonne of fuel MGO is 3.0 per cent worse; per unit of delivered energy it is about 3.1 per cent better. CII is computed on mass consumed, so the outcome turns on how much less fuel by mass the ship actually burns, which is a function of the engine and the operating profile rather than of the fuel alone. Slow steaming moves the rating by considerably more than the fuel choice does.

EEXI and EEDI use the same factor. Attained EEXI is Regulation 23 and required EEXI is Regulation 25, both reaching down to 400 GT, with the survey trigger at Regulation 5.4.7 being the first annual, intermediate or renewal survey on or after 1 January 2023. Neither calculation methodology sits in Annex VI itself: they are in recommendatory guidelines that the regulations invoke.

Sulphur oxides, and the scrubber equivalence

Complete oxidation of fuel sulphur gives a mass ratio of SO2 to sulphur of 1.998, so kilograms of SO2 per tonne of fuel is about 20 times the sulphur percentage. A tonne of 0.10 per cent distillate produces about 2.0 kg of SO2; a tonne of 0.50 per cent fuel about 10.0 kg; a tonne of 3.50 per cent heavy fuel oil about 69.9 kg.

That last figure is why an exhaust gas cleaning system is not an equivalence of mass per tonne of fuel. A scrubber ship burning 3.50 per cent fuel puts 35 times as much sulphur into the exhaust as a ship burning 0.10 per cent distillate, and the scrubber’s function is to remove it before discharge. The measure of equivalence is a concentration ratio in the exhaust.

Resolution MEPC.340(77), the 2021 Guidelines for Exhaust Gas Cleaning Systems adopted 26 November 2021, sets that ratio in Table 1 as 4.3 for the 0.10 per cent limit and 21.7 for the 0.50 per cent limit, expressed as parts per million of SO2 divided by percent by volume of CO2. The note under the table restricts its use to petroleum-derived distillate or residual fuel oils. Two things follow that the older literature gets wrong: the ratio is an exhaust gas measurement, not a wash-water criterion, and the guidelines are recommendatory, with the legal basis for the equivalence sitting in MARPOL Annex VI Regulation 4.1. Regulation 4 has no appendix, and Appendix I of Annex VI is the IAPP certificate form.

The guidelines superseded MEPC.259(68), which had carried six rows against the current two, and the discharge water criteria for pH, polycyclic aromatic hydrocarbons, turbidity and nitrate live in section 10 of MEPC.340(77) rather than in the treaty.

Nitrogen oxides

Fuel choice between distillate and residual has secondary influence on NOx compared with engine tuning and aftertreatment, but the tier structure decides what equipment a ship needs.

Table 5: MARPOL Annex VI Regulation 13 NOx limits, where n is the rated engine speed in rpm. Tier III applies only inside a designated NOx emission control area.

Regulation 13.1.1 applies to each marine diesel engine of more than 130 kW, and Regulation 13.1.2 excludes engines used solely for emergencies, engines powering equipment used solely for emergencies, and lifeboat engines. Tier III compliance is achieved through selective catalytic reduction or exhaust gas recirculation, and Regulation 13.5.3 requires the tier and its on or off status to be logged at entry to and exit from a Tier III ECA with date, time and position.

The NOx ECA set is not the same as the sulphur ECA set. The North American and United States Caribbean Sea areas apply Tier III to ships constructed on or after 1 January 2016; the Baltic and North Sea areas to ships constructed on or after 1 January 2021. The Mediterranean Sea ECA is not a NOx ECA at all.

FuelEU Maritime and the EU ETS

Marine gas oil does not meet the FuelEU Maritime greenhouse gas intensity limit on its own, in any year the Regulation has run. This is the most consequential commercial fact about distillate in European trade and it is frequently missed, because the fuel that solves the sulphur problem completely fails the intensity test from the first year.

Regulation (EU) 2023/1805 Annex II gives fossil MGO of ISO 8217 grades DMX to DMB a well-to-tank factor of 14.4 g CO2eq/MJ, a lower calorific value of 0.0427 MJ/g, and tank-to-wake factors of 3.206 g CO2, 0.00005 g CH4 and 0.00018 g N2O per gram of fuel. Applying the global warming potentials that Annex I imports gives a tank-to-wake intensity of about 76.4 g CO2eq/MJ, so the well-to-wake intensity is about 90.8 g CO2eq/MJ.

Article 4(2) sets the reference value at 91.16 g CO2eq/MJ, reduced by 2 per cent from 1 January 2025, 6 per cent from 2030, 14.5 per cent from 2035, 31 per cent from 2040, 62 per cent from 2045 and 80 per cent from 2050. The 2025 limit is therefore 89.34, and a ship on 100 per cent fossil MGO sits about 1.6 per cent above it. Against the 2030 limit of 85.69 the gap widens to about 5.9 per cent. Article 10(4) forbids a company from diverging from the Annex II well-to-tank defaults for fossil fuels, so the deficit cannot be argued away with a better upstream figure.

Two dates on this Regulation are routinely conflated. It entered into force on 12 October 2023, the twentieth day after publication in the Official Journal on 22 September 2023, and it applies from 1 January 2025, except Articles 8 and 9 on monitoring plans, which applied from 31 August 2024. The compliance mechanics, including pooling and the compliance balance , the penalties and the RFNBO multiplier , are at FuelEU Maritime explained and the intensity formula breakdown , with the well-to-wake pathways and the VLSFO and MGO defaults covered separately.

The EU Emissions Trading System applies alongside it and taxes the same combustion on a different basis. Article 3gb of Directive 2003/87/EC as amended by Directive (EU) 2023/959 requires surrender of 40 per cent of verified emissions reported for 2024, 70 per cent for 2025 and 100 per cent for 2026 and each year thereafter. The covered share of a voyage is 100 per cent for intra-EEA voyages and time at berth and 50 per cent for a voyage to or from a port outside the EEA, and methane and nitrous oxide entered the scope on 1 January 2026. There is no relief, exemption or discount for burning distillate: a tonne of MGO costs 3.206 allowances in the covered share. The interaction between the two instruments is at EU ETS and FuelEU double regulation , and the reporting layer at EU MRV and IMO DCS against EU MRV .

The IMO’s own mid-term measure is not yet in the picture. The Net-Zero Framework was approved at MEPC 83 in April 2025 as a draft new chapter of MARPOL Annex VI, and the second extraordinary session of the Marine Environment Protection Committee, held from 14 to 17 October 2025, adjourned without adopting it. MEPC 84, which sat from 27 April to 1 May 2026, did not adopt it either. It has no resolution number and no entry-into-force date, and the current strategic instrument remains the 2023 IMO GHG Strategy , resolution MEPC.377(80). The draft measure is covered at IMO Net-Zero Framework .

Bunkering, the delivery note and the retained sample

A distillate delivery follows the same MARPOL Annex VI Regulation 18 documentary framework as a residual delivery, and the framework changed in three places between 2024 and 2025. The evolution itself is covered at bunkering operations ; this section covers the documents, which are what a port state control officer and an arbitrator read.

The bunker delivery note

Regulation 18.5.1 requires each delivery to be accompanied by a bunker delivery note carrying at least the information in Appendix V, which now runs to ten items: the receiving ship’s name and IMO number; the port; the date on which delivery commenced; the supplier’s name, address and telephone number; the product name; the quantity in metric tonnes; the density at 15 degrees C; the sulphur content; the flashpoint in degrees C, or a statement that the flashpoint has been measured at or above 70 degrees C; and the supplier’s signed declaration with its three tick-boxes for the Regulation 14.1 limit, the Regulation 14.4 limit, or a purchaser-specified limit for use with a Regulation 4 equivalent or a Regulation 3.2 trial.

Item 9 is the newest and, on a distillate stem, the most operationally relevant: it was added by resolution MEPC.362(79) with effect from 1 May 2024. The paragraph numbering itself moved on 1 August 2025, when resolution MEPC.385(81) split the former Regulation 18.5 into 18.5.1 and 18.5.2, the latter covering low-flashpoint and gas fuels. Regulation 18.6 requires the note to be kept on board and readily available for inspection, and retained for three years after delivery; the supplier keeps its own copy for at least three years under Regulation 18.9.3. Detail is at Regulation 18 on bunker delivery notes .

The retained sample

Regulation 18.8.1 requires the bunker delivery note to be accompanied by a representative sample of the fuel oil delivered, taking into account guidelines developed by the Organization, sealed and signed by the supplier’s representative and the master or officer in charge, and retained under the ship’s control until the fuel oil is substantially consumed, but in any case for a period of not less than 12 months from the time of delivery.

The regulation specifies no volume, and the volume that guidelines specify is no longer 400 ml. MSC-MEPC.2/Circ.18, dated 11 July 2024 and approved by MEPC 81 and MSC 108, requires the retained sample to be of sufficient quantity to perform the tests required but not less than 600 ml, with the container filled to 90 per cent plus or minus 5 per cent of capacity and sealed. That circular revoked resolution MEPC.182(59), which had carried the 400 ml figure since 2009, so a shipboard bunkering procedure still specifying 400 ml has been out of date since July 2024.

The circular also fixes terminology that older material blurs. The MARPOL delivered sample is defined at Regulation 2.1.22 and is the sealed retained sample; the onboard sample at Regulation 2.1.24 is a different thing. Sampling is drawn continuously through the delivery at the receiving ship’s inlet bunker manifold, so the primary sample and the retained sample derived from it are distinct objects with distinct roles in a later dispute.

Non-availability and the FONAR

Where a ship cannot obtain compliant fuel, Regulation 18.2.4 requires it to notify its Administration and the competent authority of the relevant port of destination. The standard format is appendix 1 to resolution MEPC.320(74), the 2019 Guidelines for consistent implementation of the 0.50 per cent sulphur limit, adopted 17 May 2019.

A fuel oil non-availability report is not an exemption. The guidelines say so in terms, and add that it is the responsibility of the Party of the destination port, through its competent authority, to scrutinize the information provided and take action as appropriate. The name is also worth getting right, because the wrong expansion changes what the document claims: FONAR is a non-availability report, not a non-compliance report.

The supporting duties are as important as the form. Regulation 18.2.1 requires a record of the actions taken and evidence of attempts to purchase compliant fuel; 18.2.2 requires that the ship not be obliged to deviate from its intended voyage or to delay unduly; and 18.2.3 leaves the destination Party to weigh all the circumstances, including the option of not taking control measures. A copy is kept on board for inspection for at least 36 months. Further detail is at fuel oil non-availability report .

MARPOL Annex VI contains no bunker record book, and Regulation 17 of that Annex is reception facilities, not recording. The recording duties that exist are the fuel oil changeover record under Regulation 14.6, covering the volume in each tank with date, time and position, and the NOx tier status at ECA entry and exit under Regulation 13.5.3, both into whatever logbook or electronic record book the Administration prescribes. The Oil Record Book Part I belongs to MARPOL Annex I Regulation 17, a different Annex entirely.

Fuel switching and ECA transits

A ship burning residual fuel outside an ECA and distillate inside one has to complete the changeover before entry, and log it. Regulation 14.6 requires a written changeover procedure and a record of the volume of low sulphur fuel oil in each tank, together with the date, time and position of completion before entry and of commencement after exit.

The changeover is a thermal and viscometric operation rather than a valve movement. MAN Energy Solutions specifies a maximum temperature ramp of 2 degrees C per minute during switching, and the constraint on the cold side is the engine-inlet viscosity minimum discussed above. WinGD requires viscosity at the engine inlet to stay within range throughout, which on a fast changeover into distillate means the cooler has to pick up as the end heater drops out. Fuel switching operations covers the procedure, the timing calculation and the port state control interface in detail.

Voyage planning follows from it. A ship must arrive at the ECA boundary with enough distillate on board for the ECA segment plus margin, bunkered at a port where 0.10 per cent fuel is available, which constrains the bunkering sequence on a mixed-route voyage more than the price differential does. Voyage planning and routing and voyage estimation carry the commercial side of that calculation.

Biofuel blending into distillate

Distillate is the base fuel for the drop-in biofuel routes, and ISO 8217:2024 handles them through the DF grades rather than by amending the DM grades. The 2024 edition removed the ceiling on FAME content in DFA, DFZ and DFB, allowing blends up to 100 per cent, against the 7.0 per cent by volume cap in the 2017 edition. The DM grades carry a de minimis FAME requirement.

Fatty acid methyl ester must comply with EN 14214, though the marine context disapplies that standard’s own sulphur and cold-flow provisions. What changes in the fuel system is well documented and mostly unwelcome: FAME attacks certain elastomers, notably nitrile rubber seals; it raises the microbial growth risk at the water interface; it oxidises on long storage; and its cold-flow behaviour degrades as blend ratio rises. Those are the reasons the DF grades attract an induction-period oxidation stability requirement and a measured cetane number that the DM grades do not. Detail is at FAME biodiesel in marine fuel .

Hydrotreated vegetable oil is the better-behaved route, because it is a paraffin rather than an ester. ISO 8217:2024 defines HVO at clause 3.14 and notes that it is also called renewable diesel or paraffinic diesel fuel, and its new Clause 9.2 covers marine fuel consisting of 100 per cent paraffinic diesel. EN 15940:2023 specifies two classes, class A with a minimum cetane number of 70 and class B with a minimum of 51, and permits up to 7.0 per cent by volume of FAME in the product. HVO brings better cold flow, better oxidation stability, no elastomer attack and no raised microbial risk, at the cost of a lower density that shortens range for a given tank volume. Detail is at hydrotreated vegetable oil .

Both routes reduce the well-to-wake intensity of the delivered energy, which is what makes them the practical answer to the FuelEU deficit that fossil distillate cannot close. The accounting runs through the FAME and HVO well-to-wake pathways, with the wider picture at biofuels in shipping and alternative marine fuels .

WinGD advises against on-board blending generally, and states that this applies to two or more fuels having the same ISO grade, preferring fresh bunkers into empty tanks. On a distillate the risk is not asphaltene incompatibility, which is a residual-fuel problem, but FAME content, cold-flow degradation and oxidation stability, none of which a compatibility spot test will reveal.

Distillate in service

Auxiliary engines and generator sets

Almost every modern ocean-going ship burns distillate in its auxiliary engines regardless of what the main engine burns. The reasons are operational rather than regulatory: auxiliaries run at light and variable load during port stays where residual fuel produces deposits, they must start reliably from cold, and injector clearances in medium-speed four-stroke designs are tighter than in a two-stroke crosshead engine, which makes them less tolerant of abrasive contamination. Marine auxiliary engines and generators covers the installation.

Emergency generators and lifeboats

Emergency service is where the 43 degrees C flash point permission lives, and it is worth stating plainly what the instruments do and do not require. No instrument mandates DMX. SOLAS II-2/4.2.1.2 and SOLAS II-1/42 and II-1/43 permit fuel of not less than 43 degrees C flashpoint, and LSA Code 4.4.6.1 provides that no engine shall be used for any lifeboat if its fuel has a flashpoint of 43 degrees C or less on the closed cup test. Those are floors, and DMA at 60.0 degrees C satisfies all of them with margin, which is what most fleets actually run.

The cold requirements sit on the starting system rather than on the fuel. SOLAS II-1/44.1 requires emergency generating sets to be capable of being readily started in their cold condition at a temperature of 0 degrees C, with heating arrangements acceptable to the Administration where that is impracticable or lower temperatures are expected, and II-1/44.2 requires stored energy for three consecutive starts. LSA Code 4.4.6.2 requires a lifeboat engine to start at an ambient temperature of minus 15 degrees C within 2 minutes, and 4.4.6.8 requires fuel suitable for the temperature range of the operating area for not less than 24 hours at 6 knots. The appliances are covered at SOLAS Chapter III and lifeboats and survival craft .

Short-sea, ferry and offshore operation

Ships whose routes lie wholly inside an ECA frequently run on distillate exclusively, because the capital and maintenance cost of a residual fuel system buys nothing on that trade. Ro-ro vessels , passenger ships and offshore support vessels fall into that pattern, as do many anchor handling tug supply vessels . The simplification is real: no heating circuits, no residual purifier train, no trace heating, and a shorter changeover procedure. Cruise and passenger operations in the Mediterranean moved substantially onto distillate ahead of 1 May 2025, and shore power under cold ironing removes the at-berth consumption entirely where it is available.

Larger ships that burn residual fuel at sea, including oil tankers , chemical tankers , LNG carriers and general cargo ships , still carry distillate for auxiliaries, for emergency service and for ECA transits, so very few ships carry none at all.

Price, and the compliance-pathway trade-off

Marine gas oil is the most expensive of the conventional bunker fuels because it competes for the middle-distillate pool that road diesel, heating oil and jet fuel also draw from. VLSFO is blended largely from residue and cutter stock with fewer competing outlets, and heavy fuel oil from the bottom of the barrel. The spread between them moves with refining margins, seasonal heating demand and the availability of low-sulphur crude, and it is assessed daily at the major bunker ports, so any quoted figure needs both its port and its date to mean anything. The benchmarks and how a contract cites them are at bunker price indices and benchmarks , and the pass-through mechanism at bunker adjustment factor .

Table 6: ECA compliance pathways and what decides between them. Regulatory bases from MARPOL Annex VI Regulations 4 and 14 and resolution MEPC.340(77); European treatment from Regulation (EU) 2023/1805 and Directive 2003/87/EC as amended.

The distillate against scrubber comparison is the one most often run, and its answer turns on the share of sea time spent inside an ECA and on annual consumption, because the capital cost of a scrubber is recovered only through the fuel price spread. A ship with low ECA exposure recovers it slowly or not at all. Two considerations have shifted the calculation since 2020 and both cut against the scrubber: the discharge water criteria in section 10 of MEPC.340(77) and the growing set of national and port restrictions built on top of them, and the fact that neither pathway improves a FuelEU position. Time charter allocation of these costs is dealt with at bunkers on delivery and redelivery and charter party speed and consumption warranties , and the commercial screening layer at the RightShip GHG rating .

Quality testing and off-specification claims

A distillate quality claim is decided on the sample, the precision of the test and the time bar, and rarely on the chemistry. The first question is which sample is contractual and whether its chain of custody is intact. The second is whether the result is genuinely outside the specification once ISO 4259 reproducibility is applied, because two accredited laboratories testing one fuel are expected to differ by a stated amount, and a result marginally outside a limit frequently fails as a claim for that reason alone. Detail is at ISO 4259 test precision .

The parameters that actually fail on distillate are a short list. Cold-flow properties, flash point and FAME content dominate the off-specification record, which is consistent with the specification’s own structure: cloud point and CFPP are report values rather than limits on DMA and DMZ, flash point is the one parameter carrying a SOLAS prohibition, and the FAME position changed in 2024. Density, viscosity and sulphur are comparatively well behaved because they are what suppliers blend to.

Onboard testing gives early warning and no more. Portable density meters, viscometers and closed-cup flash point testers will identify a grossly off-grade delivery in time to stop a transfer, but they do not replace accredited laboratory certification for any legal purpose. The remedies, the letter of protest and the short time bars in standard supply terms are covered at bunker fuel quality disputes , with the wider quality framework at bunker quality and ISO 8217 . Where the charterer supplied the bunkers, the claim runs through the charter party as well as the supply contract, and off-hire and performance claims is where that interaction sits. Liability for pollution from the fuel itself is a separate regime under the Bunkers Convention 2001 .

Port state control opens the question from the other direction. Under Regulation 18.7 an officer may inspect the bunker delivery notes and take a copy, and the retained sample may be tested under the Appendix VI verification procedure. The inspection regimes are at port state control and the Tokyo MOU .

Limitations

This article states the position at 3 September 2026 and several of its load-bearing figures are on active amendment cycles.

The ISO 8217:2024 Table 1 values are transcribed from the published edition and cross-checked against a second transcription and against the 2017 table for every row the Foreword does not list as changed. ISO does not publish the table openly, and a buyer relying on it contractually should read the standard itself.

Test method edition years are deliberately omitted in most cases. ISO 8217:2024 lists its normative references undated, so the latest edition applies, and printing an edition year for a method the standard does not date would assert something the standard does not. Where an edition is named here, it is because the citing instrument names it, as MARPOL Annex VI Appendix V does for ISO 2719:2016.

Typical commercial values are not given. No sourced statistical series for the density, viscosity, cetane index, cloud point or cold filter plugging point of commercial DMA as actually supplied was available, so this article states specification limits and omits typical values rather than repeating unsourced ranges. The same applies to refinery process conditions, additive treat rates and the effect of cold-flow improvers, none of which could be traced to a primary source.

Prices and spreads are described qualitatively and never quantified, because a bunker price figure without its port and its date is not a figure, and any number printed here would be stale before it was read.

National and port restrictions on scrubber discharge water are not named individually. MARPOL Annex VI itself contains no discharge water criteria; they are recommendatory, in section 10 of MEPC.340(77). A growing set of coastal and port states restricts open-loop discharge, but each such restriction is a national or municipal instrument that has to be checked against its own text and its current status before a voyage, and the list moves.

The IMO Net-Zero Framework is described as adjourned because it is. No date is given for its adoption because none exists, and the draft figures circulating in industry commentary are draft figures.

Instrument status is stated as at 3 September 2026. Seven of the eight designated SOx emission control areas are in force and one is adopted; the CII reduction factors are set through 2030 and not beyond; the EU ETS reaches 100 per cent surrender for emissions reported in 2026; and a Commission proposal to strengthen the maritime ETS was pending at the date of writing and is not law.

Related Articles

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• Very low sulphur fuel oil : the 0.50 per cent residual blends that became the default global bunker after 2020
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• ISO 8217:2024 : the marine fuel specification in full, across all four grade tables
• Bunker quality and ISO 8217 : testing, parameters and the quality assurance chain
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