IMO Marine Engine Regulations

Background

International Maritime Organization (IMO) is an agency of the United Nations which has been formed to promote maritime safety. It was formally established by an international conference in Geneva in 1948, and became active in 1958 when the IMO Convention entered into force (the original name was the Inter-Governmental Maritime Consultative Organization, or IMCO, but the name was changed in 1982 to IMO). IMO currently groups 176 Member States and 3 Associate Members.

Environmental issues related to international shipping are addressed by the IMO Marine Environment Protection Committee (MEPC), which is responsible for the control and prevention of pollution from ships, including air pollutants and greenhouse gas emissions, as well as water pollution.

Pollutant Emissions. IMO ship pollution rules are contained in the “International Convention on the Prevention of Pollution from Ships”, known as MARPOL 73/78. On 27 September 1997, the MARPOL Convention has been amended by the “1997 Protocol”, which includes Annex VI titled “Regulations for the Prevention of Air Pollution from Ships”. MARPOL Annex VI sets limits on NOx and SOx emissions from ship exhausts, and prohibits deliberate emissions of ozone depleting substances from ships of 400 gross tonnage and above engaged in voyages to ports or offshore terminals under the jurisdiction of states that have ratified Annex VI.

The IMO emission standards are commonly referred to as Tier I...III standards. The Tier I standards were defined in the 1997 version of Annex VI, while the Tier II/III standards were introduced by Annex VI amendments adopted in 2008, as follows:

• 1997 Protocol (Tier I)—The “1997 Protocol” to MARPOL, which includes Annex VI, becomes effective 12 months after being accepted by 15 States with not less than 50% of world merchant shipping tonnage. On 18 May 2004, Samoa deposited its ratification as the 15^th State (joining Bahamas, Bangladesh, Barbados, Denmark, Germany, Greece, Liberia, Marshal Islands, Norway, Panama, Singapore, Spain, Sweden, and Vanuatu). At that date, Annex VI was ratified by States with 54.57% of world merchant shipping tonnage.

  Accordingly, Annex VI entered into force on 19 May 2005. It applies retroactively to new engines greater than 130 kW installed on vessels constructed on or after January 1, 2000, or which undergo a major conversion after that date. The regulation also applies to fixed and floating rigs and to drilling platforms (except for emissions associated directly with exploration and/or handling of sea-bed minerals). In anticipation of the Annex VI ratification, most marine engine manufacturers have been building engines compliant with the above standards since 2000.

• 2008 Amendments (Tier II/III)—Annex VI amendments adopted in October 2008 introduced (1) new fuel quality requirements beginning from July 2010, (2) Tier II and III NOx emission standards for new engines, and (3) Tier I NOx requirements for existing pre-2000 engines.

  The revised Annex VI entered into force on 1 July 2010. By June 2021, Annex VI was ratified by 100 countries (including the Unites States), representing 96.65% of world merchant shipping tonnage.

Greenhouse Gas Emissions. The first measures to reduce GHG emissions from ships focused on ship energy efficiency. They were followed by strategies to reduce GHG emissions from ships, which specify GHG emission reduction targets from international shipping.

• In 2011, the IMO introduced the first mandatory measures to reduce emissions of greenhouse gases. The 2011 Amendments added a new Chapter 4 to Annex VI on “Regulations on energy efficiency for ships”.
• In 2018, the IMO adopted its initial strategy on reduction of GHG emission from ships [3949], which identified “levels of ambition” for the reduction in total GHG emissions from international shipping.
• In 2023, the IMO adopted a revised strategy on reduction of GHG emissions from ships [6527], which specifies strengthened GHG emission reduction targets.

Emission Control Areas

Two sets of emission and fuel quality requirements are defined by Annex VI: (1) global requirements, and (2) more stringent requirements applicable to ships in Emission Control Areas (ECA). An Emission Control Area can be designated for SOx and PM, or NOx, or all three types of emissions from ships, subject to a proposal from a Party to Annex VI.

The existing IMO Emission Control Areas are summarized in Table 1.

The exact requirements for each ECA are specified in Annex VI. While all NOx control ECAs require IMO Tier III NOx compliance, this obligation applies to ships constructed after a certain date. In several ECAs, this date is identical to the effective date of the given ECA requirements, but some ECAs define more complex implementation schedules and requirements.

NOx Emission Standards

The NOx emission limits of Regulation 13 of MARPOL Annex VI apply to each marine diesel engine with a power output of more than 130 kW installed on a ship. A marine diesel engine is defined as any reciprocating internal combustion engine operating on liquid or dual fuel. There are two exceptions: engines used solely for emergencies and engines on a ships operating solely within the waters of the state in which they are flagged. The later exception only applies if these engines are subject to an alternative NOx control measure.

NOx emission limits are set for diesel engines depending on the engine maximum operating speed (n, rpm), as shown in Table 2 and presented graphically in Figure 1. Tier I and Tier II limits are global, while the Tier III standards apply only in NOx Emission Control Areas.

Tier II standards are expected to be met by combustion process optimization. The parameters examined by engine manufacturers include fuel injection timing, pressure, and rate (rate shaping), fuel nozzle flow area, exhaust valve timing, and cylinder compression volume.

Tier III standards are expected to require dedicated NOx emission control technologies such as various forms of water induction into the combustion process (with fuel, scavenging air, or in-cylinder), exhaust gas recirculation, or selective catalytic reduction.

Pre-2000 Engines. Under the 2008 Annex VI amendments, Tier I standards become applicable to existing engines installed on ships built between 1^st January 1990 to 31^st December 1999, with a displacement ≥ 90 liters per cylinder and rated output ≥ 5000 kW, subject to availability of approved engine upgrade kit.

Testing. Engine emissions are tested on various ISO 8178 cycles (E2, E3 cycles for various types of propulsion engines, D2 for constant speed auxiliary engines, C1 for variable speed and load auxiliary engines).

Addition of not-to-exceed (NTE) testing requirements to the Tier III standards is being debated. NTE limits with a multiplier of 1.5 would be applicable to NOx emissions at any individual load point in the E2/E3 cycle.

Engines are tested using distillate diesel fuels, even though residual fuels are usually used in real life operation.

Further technical details pertaining to NOx emissions, such as emission control methods, are included in the mandatory “NOx Technical Code”, which has been adopted under the cover of “Resolution 2”.

Sulfur Content of Fuel

Annex VI regulations include caps on sulfur content of fuel oil as a measure to control SOx emissions and, indirectly, PM emissions (there are no explicit PM emission limits). Special fuel quality provisions exist for SOx Emission Control Areas (SOx ECA or SECA). The sulfur limits and implementation dates are listed in Table 3 and illustrated in Figure 2.

Heavy fuel oil (HFO) is allowed provided it meets the applicable sulfur limit (i.e., there is no mandate to use distillate fuels).

Alternative measures are also allowed (in the SOx ECAs and globally) to reduce sulfur emissions, such as through the use exhaust gas cleaning systems (EGCS), aka scrubbers. For example, in lieu of using the 0.5% S fuel (2020), ships can fit an exhaust gas cleaning system or use any other technological method to limit SOx emissions to ≤ 6 g/kWh (as SO₂).

Greenhouse Gas Emissions

MARPOL Annex VI, Chapter 4 introduces two mandatory mechanisms intended to ensure an energy efficiency standard for ships: (1) the Energy Efficiency Design Index (EEDI), for new ships, and (2) the Ship Energy Efficiency Management Plan (SEEMP) for all ships.

• The EEDI is a performance-based mechanism that requires a certain minimum energy efficiency in new ships. Ship designers and builders are free to choose the technologies to satisfy the EEDI requirements in a specific ship design.
• The SEEMP establishes a mechanism for operators to improve the energy efficiency of ships.

The regulations apply to all ships of 400 gross tonnage and above and enter into force from 1 January 2013. Flexibilities exist in the initial period of up to six and a half years after the entry into force, when the IMO may waive the requirement to comply with the EEDI for certain new ships, such as those that are already under construction.

In April 2018, the IMO adopted an Initial Strategy on the reduction of GHG emissions from ships [3949], which was strengthened by the 2023 Strategy adopted in July 2023 [6527]. The GHG emission reduction targets of the two IMO GHG strategies are summarized in Table 4.

The IMO GHG strategies call for strengthening the EEDI requirements and a number of other measures to reduce emissions, such as operational efficiency measures, further speed reductions, measures to address CH₄ and VOC emissions, alternative low-carbon and zero carbon fuels, as well as market-based measures (MBM).

In April 2025, the IMO approved draft mid-term GHG measures (to be adopted in October 2025) that would regulate the carbon intensity of fuel used by ships with economic measures such as pricing mechanisms.

Other Provisions

Ozone Depleting Substances. Annex VI prohibits deliberate emissions of ozone depleting substances, which include halons and chlorofluorocarbons (CFCs). New installations containing ozone-depleting substances are prohibited on all ships. But new installations containing hydro-chlorofluorocarbons (HCFCs) are permitted until 1 January 2020.

Annex VI also prohibits the incineration on board ships of certain products, such as contaminated packaging materials and polychlorinated biphenyls (PCBs).

Compliance. Compliance with the provisions of Annex VI is determined by periodic inspections and surveys. Upon passing the surveys, the ship is issued an “International Air Pollution Prevention Certificate”, which is valid for up to 5 years. Under the “NOx Technical Code”, the ship operator (not the engine manufacturer) is responsible for in-use compliance.

Special Requirements for the Use or Carriage of Oils in Polar Waters. Effective 1 July 2024, heavy fuel oil (HFO) may no longer be used or carried as domestic fuel in bunker tanks when in Arctic waters. Exceptions include:

• Ships engaged in securing the safety of ships, search and rescue operations, or dedicated to oil spill preparedness and response.
• Ships subject to MARPOL regulations for oil fuel tank protection or the Polar Code until 1 July 2029.
• Ships operating in domestic waters under the jurisdiction of their flag state until 1 July 2029.

For these special requirements, HFO is fuel oil having a density greater than 900 kg/m³ (at 15°C) or a kinematic viscosity greater than 180 mm²/s (at 50°C).