Marine gas oil

Marine gas oil (MGO) is a family of distillate marine fuels refined entirely or predominantly from the middle-distillate fractions of crude oil, spanning the ISO 8217:2024 grades DMA, DMB, DMX, and DMZ. Unlike the heavy residual bunkers that power most ocean-going vessels, MGO requires no preheating before combustion, contains very low concentrations of catalytic fines and heavy metals, and consistently meets the 0.10% m/m sulphur limit mandatory inside Emission Control Areas (ECAs) designated under MARPOL Annex VI. Those properties have made distillate the default compliance fuel for ships without exhaust gas cleaning systems (scrubbers) or alternative-fuel propulsion in the North Sea, Baltic Sea, North American, US Caribbean, and Mediterranean ECAs. Beyond ECA operations, MGO is the standard fuel for all auxiliary engines and emergency generators on the majority of modern fleets, and it is the only grade approved for lifeboat and rescue craft engines under SOLAS requirements. ShipCalculators.com provides calculators for every aspect of distillate fuel management, from grade compliance checks to bunker delivery note verification and voyage emissions accounting; see the ShipCalculators.com calculator catalogue for the full list.

Contents

Origins and refinery production

Crude oil distillation and distillate recovery

Marine gas oil derives from the atmospheric gas oil (AGO) and light vacuum gas oil (LVGO) fractions separated during the refinery distillation of crude oil. In an atmospheric distillation column operating at roughly 360 °C, crude vapours condense at successively lower temperatures as they rise through the trays. Gas oil side-draws are collected at column temperatures of approximately 250 to 360 °C, corresponding to hydrocarbon chain lengths of roughly C14 to C25. The boiling range overlaps substantially with diesel fuel for road transport and heating oil, and the same refinery streams often serve all three markets depending on sulphur content requirements and seasonal demand.

After atmospheric distillation, the raw distillate fraction undergoes hydrotreatment - catalytic reaction with hydrogen under moderate pressure (30 to 80 bar) at 300 to 380 °C over a cobalt-molybdenum or nickel-molybdenum catalyst. Hydrotreatment removes sulphur through hydrodesulphurisation (HDS), nitrogen through hydrodenitrogenation (HDN), and oxygen through hydrodeoxygenation (HDO), and saturates some aromatic rings to improve cetane quality and reduce particulate-forming tendencies. The severity of hydrotreatment and the nature of the feedstock crude are the primary determinants of the sulphur content achievable in the finished marine distillate. Deeply hydrotreated DMA from sweet North Sea crudes routinely achieves sulphur levels of 0.001 to 0.005% m/m; material blended from mildly treated Middle East fractions may sit at 0.08 to 0.09% m/m while still meeting the 0.10% ECA limit.

Unlike heavy fuel oil, marine distillate contains negligible quantities of catalytic fines (aluminium plus silicon below 15 mg/kg, compared with the 60 mg/kg maximum for residual grades in ISO 8217:2024) because the FCC unit generates cat fines only in the heavier vacuum gas oil and residue fractions, which are not part of the distillate stream. The practical consequence is that distillate fuel does not require the intensive centrifugal purification necessary for HFO and is far less likely to cause abrasive wear of fuel injection equipment. ISO 8217:2024 does not specify an Al + Si limit for distillate grades, reflecting the fact that contamination is not a foreseeable production risk, though bunker sampling should still check for inadvertent contamination during the supply chain.

Marine diesel oil and the DMB grade

Marine diesel oil (MDO), standardised as the ISO 8217 grade DMB, occupies an intermediate position between pure distillate and residual fuel. DMB permits a small residual content - typically introduced by blending gas oil with a fraction of light residue or cycle oil from a fluid catalytic cracker. The blend enables a price reduction relative to pure DMA while retaining sufficient fluidity to eliminate the need for continuous preheating. ISO 8217:2024 does not set an explicit limit on residual content in DMB but does specify maximum density, viscosity, and cold-flow properties that effectively constrain how much heavy residual fraction can be incorporated without degrading the grade.

The presence of residual material in DMB introduces some of the quality risks associated with heavy fuel: marginally elevated cat-fines risk, higher potential for asphaltene deposition if the residual component is incompatible with the base distillate, and a higher density that may affect fuel system settings calibrated for pure DMA. Despite these caveats, DMB has a long history of reliable use in medium-speed diesel engines on ferries, coasters, offshore support vessels, and bulk carriers where the engine room infrastructure for HFO heating is not fitted but the operator wants to reduce fuel costs relative to premium DMA.

ISO 8217:2024 distillate grades

ISO 8217 is the primary international standard governing the quality of marine fuels delivered as bunkers. The fourth edition (2024) superseded the 2017 edition and introduced refinements to several distillate parameters. Four distillate grades are defined.

DMA

DMA (distillate, marine, grade A) is the predominant ECA compliance fuel and the standard specification for auxiliary diesel engines worldwide. ISO 8217:2024 specifies DMA with a kinematic viscosity of 2.000 to 6.000 cSt at 40 °C, a density at 15 °C not exceeding 890.0 kg/m³, a sulphur content not exceeding 1.00% m/m (in practice ≤0.10% m/m when used as an ECA fuel), a flash point not below 60.0 °C per the Pensky-Martens closed-cup method, a cetane index of at least 40, and a cloud point that the purchase specification may tighten for cold-climate use. The minimum flash point of 60 °C is mandated by MARPOL Annex VI Regulation 43 for all bunker fuels used or carried on board ships except for fuels used in emergency generators and lifesaving appliances. Typical commercial DMA supplied in northern European ports in 2024 has density around 840 to 855 kg/m³ at 15 °C and viscosity of 2.5 to 4.5 cSt at 40 °C.

DMB

DMB (distillate, marine, grade B) allows a wider viscosity range of 2.000 to 11.000 cSt at 40 °C and a higher maximum density of 900.0 kg/m³ at 15 °C. The flash point minimum is the same 60 °C. No maximum sulphur content is stated in the base ISO 8217:2024 standard for DMB beyond the 1.00% m/m that applies universally, but the 0.10% ECA and 0.50% global MARPOL limits still apply as regulatory overrides. DMB does not carry a minimum cetane index in ISO 8217:2024, which reflects its intended use in medium-speed engines with fuel systems designed to accommodate heavier distillate.

DMX

DMX (distillate, marine, grade X) is the only distillate grade with a minimum viscosity of 1.400 cSt at 40 °C and an upper limit of 5.500 cSt. Its defining characteristic is a maximum pour point of −6 °C and a relaxed minimum flash point of 43.0 °C - an explicit exception to the 60 °C minimum expressly permitted under MARPOL Annex VI Regulation 43 for fuels used in emergency generators and lifesaving appliances. DMX is the required fuel for lifeboat engines and emergency fire pump engines in most fleet technical specifications because it must remain liquid and pumpable at sub-zero temperatures without cold-flow additives. The lower flash point requires careful segregation and labelling in storage. The lifeboat fuel quantity calculator accounts for DMX-specific properties when sizing emergency fuel reserves.

DMZ

DMZ (distillate, marine, grade Z) was introduced in the 2010 revision of ISO 8217. It carries a minimum flash point of 60.0 °C, a viscosity range of 3.000 to 11.000 cSt at 40 °C, a maximum density of 890.0 kg/m³, and no explicit cetane requirement. DMZ was designed to provide a grade intermediate between DMA and DMB that includes a cetane-like quality screen via density and viscosity limits without mandating a cetane test. In commercial practice DMZ is less common than DMA or DMB; most purchasers specify DMA for high-quality distillate supply and DMB where a wider cut is acceptable.

Physical and chemical properties

Density

The density of marine distillate at 15 °C typically ranges from 820 to 890 kg/m³ for DMA and DMZ. Commercial DMA delivered in European ports commonly measures 840 to 855 kg/m³; DMB material blended with a light residual fraction may reach 875 to 895 kg/m³. The practical significance of density includes calculating the mass of fuel from measured tank volumes during bunkering, verifying that the grade supplied is consistent with the ordered specification, and applying the volume correction factor (VCF) when converting observed volumes at ambient temperature to volumes at the reference temperature of 15 °C. The density-temperature correction calculator implements ASTM D1250 Table 54B corrections for distillate fuels.

The mass of CO2 produced per tonne of marine distillate burned is fixed by carbon content, not density. ISO 9001 and the IPCC define the emission factor for distillate marine fuel as 3.206 t CO2 per tonne of fuel. This value, known as the carbon factor Cf, is used in all CII, EEDI, and EEXI calculations for ships operating on distillate. The CO2 from fuel mass calculator applies this factor directly.

Kinematic viscosity

Distillate fuels are considerably less viscous than heavy residual bunkers. DMA viscosity at 40 °C is 2 to 6 cSt, compared with 180 cSt or 380 cSt at 50 °C for IFO grades. The lower viscosity has several important consequences:

Atomisation in fuel injectors is governed by the same 10 to 15 cSt injection viscosity requirement as HFO engines, but marine distillate already meets this target at ambient temperature. No fuel heating is required, which eliminates steam or electrical trace heating circuits and the associated energy cost, but it also means the injector plungers and needle valves rely entirely on the fuel’s inherent lubricity for hydrodynamic film formation. Lubricity is a known quality concern for deeply desulphurised distillates because the polar sulphur compounds that provided natural lubricity in less-refined products are largely removed by hydrodesulphurisation. ISO 8217:2024 imposes a maximum wear scar diameter (WSD) of 520 micrometres at 60 °C per ISO 12156-1 for distillate grades, and modern additive treatment packages routinely raise lubricity to well within this limit.

At the cold end of the operating envelope, distillate viscosity rises as wax crystals begin to form on cooling. The viscosity-temperature calculator and the viscosity index calculator support the prediction of pumpability across the full operating temperature range.

Cetane index and ignition quality

Cetane index is the measure of auto-ignition quality in diesel fuel, analogous to octane number for gasoline but inverted in direction: a higher cetane index means shorter ignition delay and smoother combustion. ISO 8217:2024 specifies a minimum cetane index (CI) of 40 for DMA. The cetane index is calculated from the measured density at 15 °C and the mid-boiling point temperature, following methods such as ASTM D4737 or ISO 4264. The cetane index calculator (ASTM D4737 method) and the cetane index calculator (ASTM D976 method) implement these two approaches; the estimated cetane calculator provides a rapid estimate from density and viscosity when distillation data are not available.

Low cetane index in distillate leads to rough ignition, elevated NOx emissions, and in extreme cases misfire. Highly aromatic distillate cuts - for example, light cycle oil (LCO) from FCC units used to blend down viscosity of heavier grades - can have cetane indices below 30 and are incompatible with DMA specifications without cetane improver additives. Cetane improvers such as 2-ethylhexyl nitrate (EHN) are added at concentrations of 500 to 2,000 ppm to raise calculated cetane equivalents; their use is permitted under ISO 8217 provided the final product meets the grade specification.

Flash point

The flash point of a fuel is the lowest temperature at which its vapour, when mixed with air, ignites transiently on application of an ignition source, as measured by the closed-cup method. For DMA, DMB, and DMZ the MARPOL minimum is 60 °C; for DMX it is 43 °C. Flash point matters primarily for fire safety classification and storage segregation. A fuel lot below 60 °C flash point would need to be treated as a low-flash-point fuel under the vessel’s fire prevention procedures. The flash point compliance checker evaluates whether a measured flash point satisfies the applicable MARPOL or class requirement for a given use case.

Lower calorific value

The lower calorific value (LCV) - also called net calorific value (NCV) - of marine distillate is approximately 42.7 MJ/kg for DMA, compared with approximately 40.0 to 40.5 MJ/kg for HFO and 40.2 MJ/kg for VLSFO. The higher energy content per kilogram reflects the lower carbon-to-hydrogen ratio of distillate relative to residual fuels, which contain larger proportions of aromatic and polynuclear aromatic hydrocarbons with lower hydrogen content. For a given installed power output, a ship burning distillate consumes fewer tonnes of fuel to produce the same energy than it would burning HFO. This partially offsets the price premium of distillate. LCV differences must be accounted for in specific fuel oil consumption (SFOC) conversions and in CII calculations; the SFOC to CII calculator accepts a user-specified LCV to handle distillate correctly.

Cold-flow properties: pour point, cloud point, and CFPP

Distillate fuels contain straight-chain paraffin (n-alkane) molecules that begin to crystallise as wax as the fuel cools below the cloud point - typically −5 to +5 °C for untreated commercial DMA. As cooling continues, the wax crystals interlock to form a gel at the pour point, typically a few degrees below cloud point, at which the fuel ceases to flow under its own weight. The cold filter plugging point (CFPP), defined in EN 116, measures the temperature at which wax crystals are large enough to block a 45-micrometre test filter within a specified time; CFPP correlates better with actual operability problems in pipework and filter screens than pour point alone.

In tropical and sub-tropical bunker ports the cloud point of standard DMA is rarely a concern. Operations in the Baltic Sea in winter, the Arctic, Antarctic waters, the Great Lakes, and other high-latitude routes require attention to cold-flow properties. The polar fuel margin calculator models fuel system temperature margins in polar conditions. Cold-flow improvers - polymeric additives that modify wax crystal habit and reduce inter-crystal bonding - can lower the effective CFPP by five to 15 °C without significantly affecting pour point or cloud point; separate pour point depressants lower the pour point by a similar margin. Arctic or sub-Arctic operators typically specify maximum cloud point or CFPP values in their bunker purchase specifications and apply standard cold-flow additive packages or source Arctic-grade distillate with a naturally lower wax content derived from paraffinic-lean crude types.

ISO 8217:2024 does not specify a cloud point for distillate grades, though it allows purchase-contract parties to agree on a cloud point limit. The standard sets maximum pour points of −6 °C for DMA and DMX, a practical floor that still requires additive treatment for reliable filtration at Arctic temperatures. The ISO 8217 grade compliance checker cross-checks measured fuel properties against grade limits including pour point.

Sulphur content and ECA compliance

MARPOL Annex VI global and ECA limits

MARPOL Annex VI Regulation 14 establishes a two-tier sulphur limit for marine fuels. The global cap, which entered force on 1 January 2020 under what is commonly known as IMO 2020, limits sulphur to 0.50% m/m in all ocean areas. Inside designated Emission Control Areas the limit is 0.10% m/m. Standard DMA, when purchased from compliant suppliers, has a sulphur content well below 0.10% and therefore satisfies both limits simultaneously in all operating areas. The IMO 2020 regulation is examined in depth at IMO 2020 sulphur cap; the MARPOL convention overview sets the broader regulatory context.

The current ECAs for sulphur under MARPOL Annex VI are:

the North Sea ECA (designated 2006, effective 2007, reduced to 0.10% m/m from 1 January 2015);
the Baltic Sea ECA (same dates as North Sea);
the North American ECA, covering waters within 200 nautical miles of the United States and Canadian coastlines (designated 2010, 0.10% m/m from 1 January 2015);
the US Caribbean Sea ECA (designated 2011, 0.10% m/m from 1 January 2015); and
the Mediterranean Sea ECA (designated by MEPC 80 in July 2023, 0.10% m/m limit effective 1 May 2025).

Ships entering any ECA must either be burning 0.10% m/m sulphur fuel or have an equivalent compliance method in operation, typically an exhaust gas cleaning system (scrubber) as described at exhaust gas cleaning system. Ships without scrubbers must switch to low-sulphur distillate before or upon entry. The FONAR (fuel oil non-availability report) sulphur calculator supports documentation of unavailability in port-specific scenarios.

The SOx from sulphur content calculator quantifies sulphur dioxide emitted per tonne of fuel burned, which feeds into port-level air quality assessments and reporting under the EU Monitoring, Reporting and Verification (MRV) framework.

Verification and the MARPOL sample

MARPOL Annex VI Regulation 18 requires a Fuel Oil Non-Compliance Report (FONAR) in the event of non-availability and mandates that each bunker delivery be accompanied by a Bunker Delivery Note (BDN) stating, among other parameters, the declared sulphur content. The regulation also requires that a sealed primary sample (MARPOL sample) be retained on board for 12 months from the date of delivery as evidence of the fuel quality at the point of supply. Regulation 18.8.1 specifies the sample volume at approximately 400 ml minimum. The bunker delivery note calculator automates BDN completion and flags discrepancies between declared density and measured density.

Port state control officers may open the MARPOL sample for testing, and flag states may require ships to demonstrate sulphur compliance through fuel oil testing carried out at approved laboratories. If the test result of the MARPOL sample exceeds 0.10% m/m in an ECA or 0.50% m/m globally, the ship may be liable for enforcement action under the flag state’s national implementing legislation. The bunker sampling procedure calculator provides the procedural framework for correct continuous drip sampling during bunkering to ensure the sample is representative of the delivery.

Onboard handling and fuel system considerations

Storage and segregation

Marine distillate is stored in dedicated service and settling tanks separate from any residual fuel system. Most modern vessels carrying both HFO and MGO segregate the two fuels entirely - separate storage tanks, separate service tanks, separate day tanks, separate supply and return pipework - to prevent cross-contamination. Accidental introduction of HFO into the distillate system raises viscosity and sulphur content, potentially causing ECA non-compliance; introduction of distillate into HFO settler reduces heating efficiency and may destabilise asphaltenes.

Because distillate does not require heating, the storage tanks need only maintain fuel above the cloud point. In warm-climate operations no active heating is needed. In cold-climate or arctic operations, low-level trace heating or insulation is fitted to storage and service tanks to prevent wax crystallisation. Steam heating coils, where fitted, must be thermostatically controlled to prevent overheating that could drive off light fractions and depress the flash point below the MARPOL 60 °C minimum.

Tank surfaces in contact with distillate must be compatible with the solvent properties of light hydrocarbons. Zinc and zinc-based paints are susceptible to attack by acidic species in hydrotreated distillate. Epoxy tank coatings approved for diesel service are standard. MARPOL Annex I (oil tankers) and class society rules also specify minimum steel thicknesses and tank location requirements for fuel tanks.

Fuel transfer, purification, and day tanks

The lower density and viscosity of marine distillate make it easier to pump at ambient temperatures than HFO. Transfer pumps sized for distillate service are often smaller than equivalent HFO transfer pumps. Gravity-based transfer (settling tank to day tank) is straightforward in all but the coldest conditions.

Centrifugal purification (centrifuge-type separator) is less critical for distillate than for HFO because cat fines, asphaltenes, and heavy-metal contamination are absent in clean DMA. Nevertheless, centrifugal separation of free water and sediment remains good practice. Water contamination in distillate fuels can arise from condensation inside tanks, from seawater ingress at fittings, or from supplier-side contamination. Water reduces effective calorific value and, in combination with sulphur compounds, promotes microbial growth (see below).

Microbial contamination

Unlike HFO, which is too hostile an environment for microbial survival, marine distillate is susceptible to contamination by hydrocarbon-degrading bacteria and fungi, most notably Hormoconis resinae (previously Cladosporium resinae). These microorganisms colonise the water-fuel interface in storage tanks and produce acidic metabolites, biomass (slime), and surfactants that emulsify water into the fuel. The resulting contamination blocks filters, corrodes tank internals, and degrades the flash point of the fuel. The bunker microbial contamination assessment tool identifies risk factors based on tank condition, water bottom volume, and storage duration.

Prevention relies on eliminating free water - routine tank draining, effective separation, and tight closure of tank vents - and periodic treatment with biocide additives. ISO 8217:2024 does not set a microbial contamination limit, but CIMAC, the International Council on Combustion Engines, has issued guidance on assessment and treatment.

Compatibility with biofuel blends

Marine distillate serves as the base fuel for drop-in biofuel blends such as fatty acid methyl ester (FAME) blends (B7, B20, B30) and hydrotreated vegetable oil (HVO) blends. The compatibility of FAME blends with the existing distillate fuel system requires assessment: FAME has higher microbial growth risk than pure DMA, attacks certain elastomers (nitrile rubber seals), and may degrade on long-term storage through oxidation. HVO is generally more compatible because its molecular structure closely resembles that of fossil distillate. The biofuel blend property calculator computes blended cetane index, density, and heating value for any FAME or HVO proportion in DMA; the biofuel-bunker compatibility checker assesses blend compatibility risks. The broader context for sustainable marine fuels is at biofuels in shipping.

Emissions and regulatory accounting

CO2 emission factor

The carbon dioxide emission factor (Cf) for marine distillate (DMA, DMB, DMX, DMZ) under IMO MARPOL Annex VI and the Fourth IMO GHG Strategy is 3.206 t CO2 per tonne of fuel. This is slightly higher than the 3.114 t CO2/t for HFO, reflecting the somewhat higher hydrogen-to-carbon ratio of distillate. The paradox - higher hydrogen content leading to higher CO2 factor - is resolved by noting that distillate carbon content is also high (approximately 86.2% by mass) and that the factor accounts for complete oxidation of all carbon. Despite the marginally higher Cf, the higher LCV of distillate means the CO2 emitted per unit of energy delivered (in t CO2/GJ) is lower: approximately 74.1 g CO2/MJ for distillate versus approximately 77.4 g CO2/MJ for HFO.

CII, EEXI, and EEDI calculations

The CII attained calculator and the CII required calculator both accept fuel type as an input, applying the Cf of 3.206 for distillate grades in the annual carbon intensity ratio calculation. Ships that operate predominantly on distillate - for example, ferries burning DMA in an ECA for the entirety of their routes - may see a CII rating effect compared with an equivalent ship burning VLSFO (with Cf = 3.151 t CO2/t), because the higher Cf of distillate marginally increases the computed CII numerator. The effect is small but non-negligible for vessels optimising their rating boundary.

For EEXI and EEDI calculations the emission factor is embedded in the formula via the same Cf parameter. The EEXI attained calculator and EEDI attained calculator implement the current MARPOL Annex VI calculation methodology. Both are covered in detail at what is EEXI and what is EEDI, respectively.

SOx emissions and scrubber equivalence

A ship burning 0.10% m/m sulphur DMA emits approximately 2 kg SO2 per tonne of fuel. The same ship, if fitted with an open-loop scrubber burning 3.50% sulphur HFO, emits approximately the same mass of SO2 per tonne of fuel to the atmosphere if the scrubber achieves the required outlet concentration threshold of 4.3 g SO2 per g CO2 specified in MARPOL Annex VI Regulation 4, Appendix 1 as a wash-water discharge standard (emission ratio method). The scrubber equivalency calculation is managed through the exhaust gas cleaning system overview. Port states with closed-loop or hybrid scrubber requirements (notably certain California ports and Singapore’s Port of Singapore Authority regulations on wash-water discharge) may require switch to distillate regardless of scrubber type.

FuelEU Maritime

Under Regulation (EU) 2023/1805 (FuelEU Maritime), which entered into force on 1 January 2025, the greenhouse gas intensity of marine fuels on a well-to-wake (WtW) basis must decrease progressively against a 2020 baseline. Marine distillate contributes to GHG intensity calculations through its well-to-tank (WtT) upstream emission factor (approximately 13.5 g CO2eq/MJ WtT, slightly higher than HFO at approximately 13.0 g CO2eq/MJ) and its tank-to-wake (TtW) combustion factor. The WtW emission factor for MGO calculator computes the full well-to-wake emission intensity for distillate fuel in g CO2eq/MJ, consistent with the FuelEU delegated regulation. The FuelEU framework for penalties and pooling is at FuelEU Maritime explained.

Economic aspects

Price premium over heavy fuel oil

Marine gas oil commands a persistent price premium over HFO because it is produced from the higher-value middle-distillate fraction of crude and because demand from road and aviation sectors competes directly with marine demand for the same refinery streams. Historically the premium of DMA over IFO 380 at major bunker ports (Rotterdam, Singapore, Fujairah) has ranged from approximately US$80 to US$250 per tonne in ordinary conditions, rising sharply in periods of refinery tightness or regulatory transition.

Following the IMO 2020 sulphur cap, the DMA-HFO spread in Rotterdam widened from approximately US$100-130/t in 2019 to US$200-300/t in 2020 as demand for 0.50% compliant VLSFO blends compressed the relative differential between VLSFO and DMA while pushing VLSFO well above IFO 380 prices. By 2022 to 2024, the Rotterdam DMA-VLSFO spread settled in the range of approximately US$100-200/t, with DMA also commanding a premium over VLSFO because of its guaranteed ECA compliance. The alternative fuel premium calculator allows operators to compare the incremental cost of distillate compliance versus scrubber operation over a given voyage.

Scrubber versus distillate economics

For ships trading heavily in ECAs, the choice between investing in an exhaust gas cleaning system (EGCS) and burning compliant low-sulphur distillate is the central fuel strategy decision. EGCS capital costs for a large container ship or tanker have ranged from approximately US$2 million to US$10 million depending on vessel size and system type, with payback periods of two to five years at spreads of US$150-250/t and fuel consumption of 20,000 to 60,000 t/year. Ships trading mixed routes where ECA time represents less than 30 to 40% of total sea time often find distillate compliance more economic than EGCS installation. The engine bunker economics calculator and the voyage bunker fuelling plan support these trade-off analyses for specific voyage profiles.

The charter bunker adjustment calculator models time-charter fuel cost adjustments when vessels switch between distillate and heavy fuel grades in response to market price movements or ECA scheduling.

Voyage bunker planning

Operators planning voyages involving ECA transits must sequence distillate bunkering at ports where 0.10% sulphur DMA is available and manage tank volume such that the ECA entry tank contains sufficient distillate to cover the ECA segment plus a reasonable margin. For vessels with combined HFO and distillate service systems, a changeover period of two to four hours is typically required to flush residual HFO from the fuel system. The changeover must be completed before ECA entry. Logbooks must record the changeover time, tank volumes, and fuel qualities, and this information must be available on request to port state control. The voyage CO2 and fuel calculator computes ECA segment fuel consumption for voyage planning, accounting for the distillate grade’s LCV.

MGO in specific vessel types and operations

Auxiliary engines

The overwhelming majority of modern oceangoing ships - regardless of main engine fuel type - burn marine distillate in their auxiliary diesel generator sets. Auxiliary engines are invariably medium-speed four-stroke designs from manufacturers including MAN Energy Solutions, Wärtsilä, Caterpillar (MaK), and Bergen. The key reasons for universal distillate use in auxiliaries include: the engines are often under light or variable load during port stays where cold combustion would cause fuel deposits from HFO; the engines must start reliably from cold, which requires a low-viscosity fuel; and the injector clearances in medium-speed designs are tighter than in two-stroke crosshead engines, making them more sensitive to abrasion from cat fines. Some vessels rated for HFO-capable auxiliary engines burn HFO in port during sustained port operations, but the trend in newbuildings is towards distillate-only auxiliaries.

Ferries and short-sea vessels

Short-sea ferries, ro-ro vessels, and coastal cargo ships frequently operate exclusively on distillate because their routes are often entirely within ECAs (Baltic, North Sea, Mediterranean), their relatively small fuel consumption does not justify the capital and maintenance cost of an HFO system, and their schedules require frequent cold starts and rapid load changes for which distillate is better suited. The simplification in fuel systems - absence of heating circuits, heavy-oil purifiers, and trace heating - reduces crew workload and maintenance cost significantly. See ro-ro vessel for an overview of the vessel type.

Ice-class and polar operations

Ships designed to IACS Unified Requirements for Polar Class (PC1 through PC7) and those operating under the IMO Polar Code (polar code overview) must address cold-flow properties of all fuels and lubricants as a safety requirement. For distillate fuel, the principal concern is wax crystallisation blocking filters and fuel pipework in the fuel system when ambient temperatures fall below the cloud point of the stored fuel. The Polar Code requires that the vessel’s PWOM (Polar Water Operational Manual) addresses fuel properties and fuel treatment for the anticipated operating temperature range.

MGO with cold-flow additives or purpose-formulated Arctic distillate is the preferred fuel for ice-class vessels because HFO presents additional cold-weather risks: heated HFO storage tanks demand continuous operation of heating systems, and a heating failure can result in the entire bunker becoming solid and unpumpable. Distillate avoids this failure mode in vessels with adequate cold-flow additives. The polar fuel margin calculator computes the temperature safety margin between ambient conditions and the cold-flow limits of the grade in use.

Emergency generators and lifeboats

DMX is the mandated grade for emergency diesel generator fuel systems on most classes of vessel. SOLAS Chapter II-1 requires emergency power sources to be independent of the ship’s main fuel system, instantly available, and capable of operation at temperatures down to 0 °C (or lower for special-purpose vessels). DMX’s low pour point (maximum −6 °C in ISO 8217:2024), relatively low viscosity at cold temperatures, and moderate flash point of 43 °C address these operational requirements. Lifeboat engines and rescue boat engines also require a non-gelling fuel; the lifeboat fuel quantity calculator sizes DMX fuel reserves for required survival durations.

Inland and short-sea vessels

Inland waterway vessels in Europe operating under European inland waterway regulations, and vessels subject to CCNR (Central Commission for the Navigation of the Rhine) requirements, use distillate fuels with parameters largely consistent with ISO 8217 DMA. Some EU port areas impose local particulate and NOx requirements that effectively mandate low-sulphur distillate even before national ECA obligations apply.

Fuel quality testing and ISO 8217 compliance

Key test parameters

Shipboard and laboratory fuel quality assessment for distillate grades covers the following measured properties, cross-referenced to their ISO 8217:2024 limits:

Kinematic viscosity at 40 °C (ASTM D445 or ISO 3104) must be within the range 2.000 to 6.000 cSt (DMA) or 2.000 to 11.000 cSt (DMB). Viscosity outside range indicates off-grade supply or contamination.

Density at 15 °C (ASTM D4052 or ISO 12185) must not exceed 890.0 kg/m³ (DMA, DMZ) or 900.0 kg/m³ (DMB). Elevated density suggests blending with heavier fractions.

Flash point (ISO 2719 Pensky-Martens, or equivalent) must meet the 60 °C minimum (43 °C for DMX). Flash point below limit triggers fire-safety non-compliance.

Sulphur content (ISO 8754 X-ray fluorescence or combustion methods) must not exceed 0.10% m/m for ECA use. Shipboard portable XRF instruments provide rapid indicative results during bunkering.

Cetane index (calculated per ASTM D4737 from density and distillation data) must be at least 40 for DMA. The cetane index calculator covers the standard method.

Pour point (ISO 3016) must not exceed −6 °C for DMA and DMX.

Acid number (ASTM D664): ISO 8217:2024 imposes a maximum of 0.5 mg KOH/g for distillate grades, intended to flag highly acidic material arising from improper processing.

Oxidation stability (ISO 12205): a maximum of 25 g/m³ insoluble by-products, signalling susceptibility to sediment formation on storage.

Total sediment by hot filtration (ISO 10307-1): maximum 0.10% m/m for DMB.

The ISO 8217 compliance checker accepts measured results for all key parameters and returns a grade-pass or grade-fail verdict for DMA, DMB, DMX, or DMZ.

CCAI and combustibility

The calculated carbon aromaticity index (CCAI) is primarily relevant for residual fuel ignition quality but is occasionally applied to check borderline distillate batches where cetane index calculation is unavailable. CCAI correlates inversely with ignition quality: a lower CCAI indicates better ignition. Typical clean DMA has a CCAI below 830, compared with 850 to 870 for IFO 180 and above 870 for highly aromatic residual grades. The CCAI calculator computes CCAI from density and viscosity.

Laboratory versus onboard testing

Full ISO 8217 compliance testing requires accredited shore-based laboratories with gas chromatography, distillation apparatus, cold-flow testing equipment, and X-ray fluorescence or UV fluorescence sulphur analysers. Onboard, crews can perform rapid indicative testing using portable density meters (digital oscillating-tube type), portable viscometers, and portable closed-cup flash point testers. The results provide early warning of off-spec supply but cannot replace laboratory certification for legal compliance purposes. MARPOL Regulation 18 MARPOL sample testing in a flag-state-approved laboratory provides the legally binding quality reference.

Bunker compatibility

When blending distillate from different suppliers, or when a vessel switches between DMA from different origins, compatibility issues are unlikely compared with residual fuels, because distillate does not contain asphaltenes that can precipitate on mixing. However, co-mingling FAME-containing distillate with non-FAME distillate can affect blend cetane, oxidation stability, and cold-flow properties. The bunker compatibility spot test calculator supports assessment of blending risks.

Environmental performance and alternative fuels

Comparison with VLSFO and HFO

Marine distillate produces lower particulate matter (PM) and SOx per tonne burned than any residual fuel, whether HFO or VLSFO. PM emissions from DMA combustion are dominated by organic carbon and a small contribution from lubricating oil carry-over, rather than the sulphate-nucleated secondary particulate that characterises HFO combustion. Black carbon emissions from distillate engines are lower per unit energy than from slow-speed two-stroke engines burning residual fuel, a distinction relevant to Arctic shipping where black carbon deposition on sea ice has an amplified climate warming effect.

NOx emissions from diesel engines on distillate are broadly comparable to those on HFO for the same engine load and speed. The relevant regulatory framework is MARPOL Annex VI Regulation 13, which establishes Tier I, II, and III NOx limits. In NOx-TECAs (NOx Tier III Emission Control Areas, presently the North American ECA and US Caribbean ECA), ships constructed on or after 1 January 2016 must meet the Tier III limit of approximately 2.0 g/kWh (for engines with n < 130 rpm), achieved through selective catalytic reduction (SCR) or exhaust gas recirculation (EGR). The fuel type, whether distillate or HFO, has secondary influence on NOx compared with the fundamental engine tuning and aftertreatment strategy. See selective catalytic reduction for the SCR system overview.

Biofuel drop-in options

Marine distillate’s properties - low sulphur, moderate viscosity, clean combustion - make it an effective carrier for drop-in biofuel blends. The principal options are:

Fatty acid methyl ester (FAME), produced by transesterification of vegetable oils or animal fats with methanol. FAME has a higher cloud point than fossil DMA (cloud point often +5 to +15 °C for B100), higher microbial growth susceptibility, lower oxidative stability, and higher NOx emissions, but high cetane and zero fossil carbon content. Blends up to B7 are typically covered by ISO 8217 distillate specifications; higher blends (B20, B30) require separate agreed specifications. See biofuels in shipping for full coverage.

Hydrotreated vegetable oil (HVO), also marketed as renewable diesel, is chemically similar to fossil paraffin-type distillate. HVO blends are compatible with existing distillate fuel systems at all blend ratios, have better cold-flow properties than FAME, higher cetane index (70 to 90), and lower PM and NOx emissions than equivalent fossil DMA. The HVO renewable diesel fuel summary calculator provides key properties of HVO at 100% and blended.

Under the FuelEU Maritime regulation and IMO CII corrective plans, biofuel blends in DMA can reduce the effective GHG intensity of the fuel on a lifecycle basis, enabling operators to improve their CII rating or FuelEU compliance balance without hardware changes. The fuel blend WtW emission calculator computes the blended well-to-wake emission factor for any ratio of DMA and biofuel.

LNG and other alternative fuels

Liquefied natural gas (LNG) is the primary alternative to distillate for ECA compliance on vessels built for that purpose. LNG produces negligible SOx, very low PM, and, with appropriate engine tuning, meets NOx Tier III limits. However, LNG requires purpose-built fuel containment and supply systems. The comparison of distillate versus LNG is covered at LNG as a marine fuel and LNG fuel system. Methanol, ammonia, and hydrogen are emerging alternatives covered at methanol as a marine fuel, ammonia as a marine fuel, and in the broader context of decarbonisation policy at what is CII.

For operators considering the carbon cost of distillate against alternative compliance strategies over a vessel’s service life, the lifecycle fuel total cost of ownership calculator and the CII year-on-year improvement calculator provide multi-year scenario analysis.

Bunker procurement and delivery

Bunkering procedure for distillate

Bunkering of marine distillate follows the same MARPOL Annex VI Regulation 18 procedural framework as HFO: a pre-delivery survey confirming tank capacity and initial quantities, continuous drip sampling of the fuel as it flows through the bunker delivery hose to form the representative MARPOL sample, measurement of delivered volume and temperature for density correction to mass, and completion and signing of the Bunker Delivery Note. The bunker density-temperature correction calculator computes the mass of distillate delivered from volume and temperature measurements, which is the quantity that appears on the BDN and in MARPOL Annex VI Appendix V.

Because distillate does not require heating, bunkering at ambient temperature is straightforward. Transfer rates for DMA are higher per unit time than for viscous HFO (lower pipe friction loss), so operators must verify that the receiving vessel’s overflow prevention procedures are effective and that the receiving tanks’ high-level alarms are tested before bunkering commences. The bunker wedge formula calculator models ullage and sounding relationships for irregular tank geometries.

BDN declared sulphur and ship-to-ship transfer

Where distillate is supplied ship-to-ship at an anchorage rather than from a shore terminal, sampling and BDN procedures are unchanged in principle but physical logistics - hose connection, spill response, emergency shutdown - require specific operational planning. The offshore bunker transfer calculator covers these logistics. The bunker record book must document all distillate transfers, loadings, and consumption in accordance with MARPOL Annex VI Regulation 17.

Supplier quality assurance and FONAR

When a vessel is unable to obtain compliant 0.10% m/m fuel at a port, the master must report the non-availability in writing using a FONAR and make every reasonable effort to obtain compliant fuel at the next available port. The FONAR sulphur non-availability calculator structures the FONAR data required by MARPOL Regulation 18 for submission to the flag state and port state.

References

International Maritime Organization. MARPOL Annex VI and NTC 2008 with Guidelines for Implementation, 2013 consolidated edition and amendments. IMO, London.
International Organisation for Standardisation. ISO 8217:2024 Petroleum and related products - Fuels (class F) - Specifications of marine fuels. ISO, Geneva, 2024.
MEPC.1/Circ.881 - Guidance on best practice for fuel oil providers and users. IMO, 2019.
MEPC 80/17/Add.1 - Amendments to MARPOL Annex VI (Mediterranean ECA designation). IMO, July 2023.
International Maritime Organization. Fourth IMO GHG Strategy. Resolution MEPC.377(80), 2023.
Regulation (EU) 2023/1805 of the European Parliament and of the Council on the use of renewable and low-carbon fuels in maritime transport (FuelEU Maritime).
CIMAC. Guideline for the Operation of Marine Engines on Low Sulphur Distillate Fuels. CIMAC WG7, 2013.
ASTM International. ASTM D4737-10 Standard Test Method for Calculated Cetane Index by Four Variable Equation. ASTM, West Conshohocken.
ASTM International. ASTM D1250-04 Guide for Use of the Petroleum Measurement Tables (Tables 54B). ASTM, West Conshohocken.
Woodyard, D. Pounder’s Marine Diesel Engines and Gas Turbines, 9th edition. Butterworth-Heinemann, Oxford, 2009.
International Council on Combustion Engines (CIMAC). Guideline on Microbial Contamination in Marine Distillate Fuels. CIMAC, 2019.