NOx Tier I, II and III: MARPOL Annex VI Reg.13

Background and history

Health impact of marine NOx

Nitrogen oxides (NOx, comprising nitric oxide NO and nitrogen dioxide NO2) emitted from marine diesel engines contribute substantially to air pollution in coastal regions, port cities, and along busy shipping lanes. The health and ecological consequences are specific and documented at scale.

NO2 and the secondary fine particulate matter (PM2.5) formed from NOx in the atmosphere are associated with increased mortality from chronic obstructive pulmonary disease, asthma exacerbation, ischaemic heart disease and stroke. The 2020 European Environment Agency assessment attributed approximately 50,000 premature deaths per year in Europe to ship-source air pollution, with NOx and the secondary PM2.5 it forms responsible for approximately 30% of that toll (roughly 15,000 deaths annually).

NOx is a precursor to tropospheric ozone, a greenhouse gas and respiratory irritant. Ship-source NOx contributes to ozone formation in the marine boundary layer with effects extending hundreds of kilometres downwind of shipping lanes. NOx is also oxidised in the atmosphere to nitric acid (HNO3), contributing to acid deposition that damages forests, freshwater ecosystems and built infrastructure. In semi-enclosed seas such as the Baltic and the North Sea, nitrogen deposition contributes to eutrophication. The 2018 IMO impact assessment for the Tier III amendments concluded ship-source NOx accounted for 5% to 15% of total NOx emissions in major coastal regions of Europe, North America and East Asia.

Pre-Annex VI: state of marine engine NOx

Before Annex VI entered force in 2005, marine diesel engines were not subject to any binding international NOx limit. The typical NOx emission rate of a slow-speed two-stroke engine in the 1990s was 18 to 22 g/kWh; medium-speed four-stroke engines ran 12 to 16 g/kWh. Engine manufacturers had developed combustion techniques (timing retardation, water injection, charge-air cooling) capable of 30% to 50% NOx reduction, but these were applied only where regional rules required them.

The 1997 Annex VI Protocol was adopted with Tier I limits calibrated to approximately match the then-typical state of the art for combustion-only NOx control. The Tier I limit of 17 g/kWh at low rated speeds sat roughly 5% to 15% below the 1990s slow-speed two-stroke baseline, requiring engine manufacturers to apply low-NOx combustion techniques as a standard production requirement rather than an option.

The 1997 Protocol and Tier I

The 1997 Annex VI Protocol introduced Regulation 13 with Tier I limits applicable to engines installed on or after 1 January 2000, with effect from the Protocol’s entry into force on 19 May 2005. The Protocol simultaneously adopted the original NOx Technical Code as Annex VI Appendix II. Tier I was implemented through low-NOx combustion techniques on new engines: retarded fuel injection timing, increased compression ratio, optimised injector spray patterns and improved charge-air cooling.

An additional pre-2000 retrofit obligation exists under Reg.13.7 (as renumbered in the 2008 amendments): engines of more than 5,000 kW output and a per-cylinder swept volume of 90 litres or more, installed on ships built on or after 1 January 1990 but before 1 January 2000, are required to meet Tier I limits if an approved method for such engines exists. This provision applies to the largest slow-speed two-stroke engines of that era and has driven retrofits on a limited number of older vessels where flag states have activated it.

2008 amendments and the three-tier framework

The 2008 amendments to Annex VI, adopted by Resolution MEPC.176(58) on 10 October 2008 and in force from 1 July 2010, substantially restructured Regulation 13 and introduced Tier II and Tier III. Tier II (applicable globally to engines installed on or after 1 January 2011) was calibrated to the state of the art for advanced combustion techniques without after-treatment: electronic fuel injection, Miller cycle, two-stage turbocharging. Tier III (applicable from the NECA effective date) was technology-forcing: the 75% to 80% NOx reduction required by Tier III over Tier II cannot be achieved by combustion optimisation alone and requires either after-treatment or LNG operation in the Otto cycle.

The 2008 amendments also adopted the NOx Technical Code 2008 (NTC 2008) by Resolution MEPC.177(58), replacing the 1997 Technical Code with a substantially updated procedure for engine certification and on-board verification. The NTC 2008 was further updated by Resolution MEPC.272(69) of April 2016, which clarified the procedure for engine modifications and the parameter check method, and introduced guidance on dual-fuel and gas-fuelled engine certification.

NOx formation in marine diesel engines

Thermal NOx: the Zeldovich mechanism

The dominant NOx formation mechanism in marine diesel engines is the thermal NOx mechanism described by Yakov Zeldovich in 1946. The mechanism comprises three principal reactions:

1. O + N2 = NO + N
2. N + O2 = NO + O
3. N + OH = NO + H

The first reaction is rate-limiting, with an activation energy of approximately 318 kJ/mol. The thermal NOx formation rate accordingly has a strong Arrhenius-type dependence on combustion temperature: at 1,500 K the rate is approximately 1 ppm/s; at 2,000 K approximately 100 ppm/s; at 2,500 K approximately 10,000 ppm/s. In a typical marine diesel combustion chamber with peak temperatures in the 2,000 K to 2,400 K range, the dominant NOx formation occurs in the highest-temperature regions of the flame front.

The thermal NOx Zeldovich calculator implements the rate-of-formation calculation for an arbitrary temperature, residence time and oxygen partial pressure.

Combustion temperature: the principal control lever

Reducing peak combustion temperature is the main variable for combustion-only NOx control. The standard techniques used in Tier I and Tier II engines are:

• Retarded fuel injection timing: delays the start of combustion, lowering peak temperature at the cost of a small fuel-consumption penalty.
• Increased compression ratio: improves thermal efficiency and lowers peak temperature for the same injection timing.
• Miller cycle: closes the intake valve early, reducing the effective compression ratio and the temperature of the trapped charge before ignition.
• Charge-air cooling: lowers inlet air temperature and therefore in-cylinder charge temperature before combustion.
• Exhaust gas recirculation (EGR): re-introduces a portion of exhaust gas into the combustion chamber, raising the heat capacity of the charge and reducing peak temperature.
• Water injection or water-fuel emulsion: absorbs heat through evaporation, lowering peak temperature.

Each technique in isolation reduces NOx by 5% to 30%; combinations of several techniques can reach 50% to 80% reduction relative to an unoptimised baseline. Reaching Tier III (approximately 80% NOx reduction from Tier II) requires either after-treatment via SCR, high-EGR-rate combustion at 25% to 45% EGR, or LNG operation in the Otto cycle.

Prompt NOx and fuel NOx: minor contributions

Beyond thermal NOx, two additional mechanisms contribute small fractions to the total:

Prompt NOx forms in the fuel-rich zone at the base of the diffusion flame through a mechanism first described by Fenimore involving CH radicals reacting with N2. In marine diesel engines this accounts for less than 5% of total NOx in most operating conditions.

Fuel NOx forms from nitrogen chemically bound in the fuel itself, oxidising during combustion. Residual fuels (heavy fuel oil, VLSFO) contain 0.2% to 0.5% nitrogen by mass; distillate fuels (MGO) contain 0.01% to 0.1%. Fuel NOx at typical HFO nitrogen levels contributes 5% to 15% of total engine-out NOx. This fraction is not amenable to combustion temperature control and must be addressed by after-treatment if minimising total NOx is the objective.

Fuel composition and secondary effects

The fuel composition has a secondary effect on total NOx. Low-sulphur fuels (MGO, VLSFO) typically produce slightly less NOx than high-sulphur HFO due to differences in flame stoichiometry; the effect is small, typically less than 5%. Higher-cetane fuels ignite faster and tend to have lower peak combustion temperatures, giving a 5% to 10% NOx reduction. Higher aromatic content can increase NOx slightly. Biofuels containing 10% to 11% oxygen by mass can increase NOx marginally through the prompt-NOx mechanism. None of these effects is large enough to affect Tier I, II or III compliance classification on its own.

Tier I, II and III limits

Limit equations and the rated-speed dependency

The NOx limit at each tier is a function of the engine’s rated rotational speed n in rpm, with three piecewise constant or power-law segments. The rationale for the speed dependency is that slow-speed two-stroke engines (typically n 70 to 120 rpm) achieve higher per-cycle thermal efficiency and lower cycle-average temperatures than high-speed four-stroke engines, justifying a correspondingly more lenient absolute limit.

MARPOL Annex VI Regulation 13 specifies the following limits, verified against the 2008 amendment text adopted by MEPC.176(58):

The NOx Tier III limit calculator and Tier II limit calculator implement these equations directly. The NOx Tier compliance check calculator integrates installation date, NECA status and EIAPP-certified NOx into a single pass/fail determination.

The exponents differ between tiers: Tier I and Tier III both use $n^{-0.2}$ in the mid-speed range, while Tier II uses the steeper $n^{-0.23}$. The different exponent for Tier II was chosen to produce a flatter speed-curve relative to Tier I, placing proportionally heavier reduction obligations on medium-speed engines.

Representative limit values at key rated speeds

Worked examples at representative rated speeds, all values in g/kWh:

Tier III limits are 75% to 80% lower than the corresponding Tier II limit. At 80 rpm (n < 130 bracket) the reduction is exactly 76.4% (3.4 vs 14.4 g/kWh). At 500 rpm using the formula, Tier II = $44 \cdot 500^{-0.23}$ = 11.2 g/kWh and Tier III = $9 \cdot 500^{-0.2}$ = 2.6 g/kWh, a reduction of 76.8%.

Engine installation date rules

The applicable tier for each engine is determined by the engine’s installation date on the ship, not the date of manufacture. An engine manufactured in 2020 but not installed until 2024 is treated as a 2024 installation for tier-determination purposes.

Pre-2000 engines are not subject to Annex VI NOx limits, except under the Reg.13.7 retrofit provision (engines above 5,000 kW output and 90 L/cyl swept volume on ships built 1990 to 1999, where an approved method exists). If such an engine undergoes a “major conversion” (defined in Reg.13.2, as amended by MEPC.231(65), as a substantial modification, replacement of a major component, or an increase in the maximum continuous rating by more than 10%) on or after 1 January 2000, it becomes subject to the applicable tier as of the conversion date.

Engines installed 1 January 2000 to 31 December 2010: Tier I applies globally.

Engines installed 1 January 2011 to 31 December 2015 (or up to the NECA effective date for engines that will operate in a NECA): Tier II applies globally.

Engines installed on or after 1 January 2016: Tier II applies globally. Tier III applies additionally when operating in a NECA with an effective date of 1 January 2016 (North American NECA and US Caribbean Sea NECA). For the Baltic Sea NECA and North Sea NECA, Tier III applies to engines installed on or after 1 January 2021. An engine installed between 1 January 2016 and 31 December 2020 that operates in the Baltic or North Sea NECA is required to meet Tier II, not Tier III, because it predates those NECAs’ effective dates.

NOx Technical Code 2008

Pre-installation type-approval

Before an engine can be installed on a ship subject to Annex VI, it must complete a pre-installation type-approval test on the manufacturer’s test bed. The test operates the engine over the prescribed test cycle for its intended use, measures the NOx emission rate at each mode using calibrated instrumentation (typically chemiluminescent NOx analysers), calculates the cycle-weighted average NOx emission rate, compares the measured value against the applicable tier limit, and documents the engine’s adjustable components, their settings and operating parameters in an EIAPP Technical File.

If the engine passes, the engine manufacturer issues the Engine International Air Pollution Prevention (EIAPP) Certificate documenting the certified NOx emission rate, the test cycle used, and the engine’s adjustable parameters envelope. The certificate accompanies the engine throughout its operational life. If the engine moves between ships, the certificate moves with it.

The survey calculator and the IAPP certificate calculator implement the related survey and certification cycle for Annex VI compliance management.

Test cycles by engine type

The NTC 2008 prescribes different test cycles depending on the engine’s intended use:

E2 test cycle: constant-speed propulsion engines (e.g. fixed-pitch propeller driven by a constant-speed generator). 4 modes at constant speed and varying load: 100%, 75%, 50%, 25%.

E3 test cycle: variable-speed propulsion engines, the standard case for most marine main engines. 4 modes at varying speed and load along the propeller law curve: 100%, 75%, 50%, 25%. The weighting factors are 0.20, 0.50, 0.15 and 0.15 respectively; the 75% mode is the dominant contributor to the certified weighted average because it represents the typical deep-sea cruising point.

D2 test cycle: constant-speed auxiliary engines, e.g. generator sets. 5 modes at constant speed and varying load: 100%, 75%, 50%, 25%, 10%.

C1 test cycle: variable-speed, variable-load auxiliary engines. 8 modes covering the full operating envelope.

D1 test cycle: variable-speed power-generation engines (variable-speed generator drives). 3 modes at varying speed and load.

The choice of test cycle is determined at type-approval and recorded on the EIAPP Certificate. Switching the engine to a different application (e.g. from propulsion to generator duty) requires a new certification under the applicable cycle.

On-board parameter check method

The parameter check method is the standard verification used at each Annex VI annual or intermediate survey. It verifies that the engine’s adjustable components (fuel rack, injectors, valve timing, charge air cooler) are set to values within the EIAPP-documented envelope, and that operating parameters (compression pressure, exhaust temperature, fuel consumption per kWh) are within the permitted ranges. If all parameters are within the envelope, the engine is presumed compliant with its certified NOx emission rate without direct NOx measurement.

The parameter check is fast, typically 2 to 4 hours per engine, and cost-effective. Its limitation is that it relies on the manufacturer’s parameter envelope being well-correlated with actual NOx output. If any parameter is out of envelope, or if the engine has been modified outside the EIAPP, the simplified measurement method is required.

On-board simplified measurement method

The simplified measurement method involves direct measurement of NOx emissions using portable instrumentation. It operates the engine at representative test-cycle modes, measures NOx using a portable analyser meeting NTC 2008 calibration requirements, calculates the cycle-weighted average, and compares against the EIAPP-certified value.

The simplified measurement method is more rigorous but considerably more resource-intensive than the parameter check: typically 1 to 2 days per engine and approximately USD 10,000 to USD 25,000 per engine due to instrumentation rental and surveyor time. It’s used when the parameter check identifies an issue or when the engine has been substantially modified outside the original EIAPP envelope.

EIAPP Certificate content

The EIAPP Certificate records engine make, model and serial number; the cycle-weighted average certified NOx emission rate (g/kWh); the applicable tier; the test cycle used (E2, E3, D2, C1 or D1); the reference fuel used in the test (typically ISO 8217 DMA distillate or a reference fuel for dual-fuel engines); the adjustable components and their permitted settings; and the operating parameters and their permitted ranges.

If the engine is modified beyond the EIAPP envelope (uprating, dual-fuel conversion, after-treatment addition for Tier III), a new EIAPP Certificate must be issued by the engine manufacturer or an authorised service organisation, following re-testing under the NTC 2008. This is the mechanism by which a Tier II engine gains Tier III certification after SCR or EGR retrofit: the retrofitted engine is re-tested with the after-treatment system in service, and the EIAPP is reissued at the Tier III-compliant emission rate.

Dual-fuel and gas-engine certification

Resolution MEPC.272(69) introduced guidance on certifying dual-fuel engines (engines capable of operating on both liquid fuel and gas fuel) and dedicated gas-fuelled engines under the NTC 2008. For dual-fuel engines, the NOx certification covers both the diesel mode and the gas (Otto cycle) mode separately. The EIAPP Certificate shows the certified NOx for each mode.

In gas (Otto cycle) mode, the NOx emission rate is typically 1.0 to 2.5 g/kWh for LNG dual-fuel engines, well below the Tier III limit of 2.0 to 3.4 g/kWh depending on rated speed. The diesel mode of the same engine may be certified at Tier II. The engine must operate in gas mode whenever it is in a NECA if it holds only Tier II certification in diesel mode.

NOx Emission Control Areas

Geographic and chronological coverage

The current NOx Emission Control Areas (NECAs) and their Tier III effective dates are established by specific MEPC resolutions:

The North American NECA covers US and Canadian coastal waters out to 200 nautical miles, including Hawaii. The US Caribbean Sea NECA covers waters around Puerto Rico and the US Virgin Islands. The Baltic Sea NECA covers the entire Baltic Sea including the Gulfs of Bothnia, Finland and Riga. The North Sea NECA covers the North Sea south of 62 degrees N and the English Channel west to 5 degrees W.

A critical distinction applies to the Mediterranean Sea: the Mediterranean SOx ECA, effective 1 May 2025, is a sulphur and particulate matter emission control area only. It requires fuel oil with a maximum 0.10% sulphur content but does NOT impose any Tier III NOx requirement. A Mediterranean NECA for NOx has been under discussion at the IMO but had not been formally adopted as of mid-2026. The Norwegian Sea NECA has similarly been under early negotiation.

The emission control areas article covers the geographic boundaries and the separate SOx ECA framework in detail.

Compliance requirements for NECA operation

For an engine installed on or after the relevant NECA effective date, the ship must ensure Tier III operation whenever the engine is operating in any designated NECA. The EIAPP Certificate must show Tier III compliance and the SEEMP Part I (SEEMP I, II and III) must include the procedures for ensuring Tier III operation in NECAs, covering SCR urea management, EGR control actuation, or LNG gas-mode selection for dual-fuel ships.

The practical compliance requirement for SCR-fitted ships is that the SCR system must be operational whenever the ship is in a NECA. Urea consumption records and SCR control-system logs are the primary evidence for port state control inspections. For EGR-fitted ships, EGR control system logs and exhaust analyser records serve the same purpose. For dual-fuel ships, fuel consumption records showing gas-mode operation during NECA transit are required.

Port state control verification

NECA compliance is verified by port state control inspectors at port calls following operation in a NECA. The principal verification mechanisms are:

EIAPP Certificate review, confirming the engine holds Tier III certification. On-board parameter check, verifying the SCR or EGR or after-treatment system was operational during NECA transit through urea consumption records, control system logs, or exhaust analyser records. Fuel consumption records for LNG dual-fuel ships, confirming gas-mode operation during NECA transit. Bridge logs and AIS data cross-check, confirming the after-treatment system was active when the ship was within the NECA boundary.

A failure at any of these checks may result in a deficiency notice and, for systemic non-compliance, vessel detention. The PSC NOx calculator implements the inspection-targeting risk-score calculation used by Paris MOU and Tokyo MOU for NOx-related inspection selection.

Tier III compliance technologies

Selective catalytic reduction

Selective catalytic reduction is the dominant Tier III compliance technology for slow-speed two-stroke and medium-speed four-stroke marine engines. The principle is the catalysed reduction of NOx by ammonia (or, in marine practice, urea hydrolysed to ammonia) over a vanadium-tungsten-titanium oxide catalyst bed:

4 NO + 4 NH3 + O2 -> 4 N2 + 6 H2O

NO + NO2 + 2 NH3 -> 2 N2 + 3 H2O

The reaction proceeds efficiently at 280°C to 450°C. For slow-speed two-stroke main engines, the SCR reactor is typically located upstream of the turbocharger turbine where exhaust temperatures are in the 300°C to 420°C window; for four-stroke engines, the SCR is located in a duct section with thermal management to maintain the temperature window. Typical SCR characteristics are:

NOx reduction efficiency of 80% to 95%, sufficient to bring a Tier II-certified engine to Tier III compliance when the SCR is in service. Urea consumption of approximately 5% to 10% of fuel consumption by mass, depending on the NOx reduction required and the SCR design. The SCR urea consumption calculator implements the consumption-rate calculation.

Capital cost for retrofit runs USD 1.5 million to USD 4 million per engine; new-build integration runs USD 1 million to USD 2.5 million. Operating cost is dominated by urea solution cost (approximately USD 350 to USD 500 per tonne of AdBlue-grade 32.5% urea solution). Pressure drop across the catalyst bed is typically 20 to 50 mbar, producing a small fuel-consumption penalty of approximately 0.5% to 1.0%. The principal SCR vendors serving the marine market are MAN Energy Solutions (for MAN-branded engines), Wartsila, Yara Marine Technologies, Caterpillar Marine, Hitachi Zosen and several regional specialists.

Exhaust gas recirculation

EGR is the alternative Tier III compliance technology used primarily on slow-speed two-stroke engines where operators prefer to avoid urea logistics. The principle is the re-introduction of a portion of the exhaust gas into the combustion chamber, increasing the heat capacity of the charge and lowering peak combustion temperature.

Typical EGR characteristics: NOx reduction efficiency of 50% to 80%, typically sufficient for Tier III compliance when combined with combustion optimisation; the EGR rate required for Tier III is usually 25% to 45% by mass of the cylinder charge, computed by the EGR rate calculator. Capital cost for retrofit runs USD 1 million to USD 2 million per engine. Operating cost is lower than SCR (no urea required) but comes with a fuel-consumption penalty of 1% to 3% at high EGR rates due to reduced combustion efficiency.

EGR is particularly associated with MAN B&W slow-speed two-stroke engines, where MAN Energy Solutions offers an integrated EGR system from the engine manufacturer. Maintenance requirements are higher than SCR due to EGR cooler fouling and corrosion from recirculated exhaust containing acid condensate. The EGR retrofit on two-stroke engines article covers the installation-specific design considerations.

LNG dual-fuel in Otto cycle

LNG dual-fuel engines operating in gas mode (Otto cycle) achieve Tier III compliance without after-treatment. The Otto cycle in dual-fuel operation has lower peak combustion temperatures than the diesel cycle (typically 100 to 200 K lower), bringing the thermal NOx formation rate well below the Tier III limit. Measured NOx from LNG dual-fuel engines in gas mode typically runs 1.0 to 2.5 g/kWh for medium-speed four-stroke engines and below 1.5 g/kWh for slow-speed two-stroke engines operating in dual-fuel mode.

The LNG/dual-fuel pathway simultaneously provides Tier III NOx compliance, compliance with the 0.10% sulphur ECA limit (LNG contains negligible sulphur), and a pathway toward future GHG compliance through transition to bio-LNG. The principal limitation for Tier III compliance is methane slip in the Otto cycle, ranging from 1% to 5% of fuel by mass. The methane slip is converted to CO2-equivalent using GWP100 = 28 in the IMO Net-Zero Framework accounting; see the methane slip CO2-equivalent calculator and the CH4 slip calculator.

In-cylinder combustion optimisation

In-cylinder combustion optimisation is the primary Tier II compliance pathway and a complement to SCR or EGR for Tier III. The principal techniques are high-pressure fuel injection (1,500 to 2,500 bar injection pressure) producing finer fuel atomisation and faster, lower-temperature combustion; variable injection timing optimised for each operating point; variable valve timing implementing Miller cycle through early inlet valve closure; two-stage turbocharging improving charge-air cooling between stages; and improved combustion chamber geometry through optimised piston bowl and injector spray patterns.

Modern engine designs combine these techniques to achieve Tier II compliance without after-treatment. For Tier III, after-treatment is generally required in addition because combustion optimisation alone cannot deliver 75% to 80% reduction from the Tier II baseline.

National and regional NOx instruments

Norway NOx Fund

The Norway NOx Fund has operated since 2008 under the Norwegian government as the principal national NOx market instrument in maritime regulation. Ships operating in Norwegian waters choose between paying the full NOx tax (NOK 24.45 per kg NOx in 2025, approximately USD 2.30 per kg) or joining the Fund at a lower per-kg contribution (NOK 12.50 per kg, approximately USD 1.20 per kg) and gaining access to NOx-reduction project grants.

Ships that join the Fund and implement NOx-reduction measures (SCR retrofit, LNG dual-fuel conversion, EGR retrofit) receive grants covering 30% to 80% of project cost. The Norway NOx Fund levy calculator implements the levy and grant calculation. The Fund is credited with driving approximately 90% of Norwegian-flagged ships to implement NOx-reduction measures and with a 50% reduction in ship-source NOx emissions in Norwegian waters since 2008.

Other regional schemes

The California Air Resources Board at-berth rule requires shore power or equivalent for at-berth ships, indirectly reducing port-area NOx emissions, but does not directly apply the MARPOL tier structure.

The Environmental Ship Index (ESI) is a voluntary index recognised by approximately 50 ports worldwide, granting port-fee discounts to ships scoring above defined thresholds for SOx, NOx and CO2. The ESI score calculator implements the standard scoring formula. The Singapore Green Ship Programme, the Port of Rotterdam Environmental Ship Differentiation scheme, and the Swedish Maritime Administration differentiated fairway dues all reference NOx Tier certification as a criterion for discounts.

Reg.13.7 retrofit provision (pre-2000 large engines)

Regulation 13.7 is specific to a limited class of large, older engines. It covers engines with output above 5,000 kW and per-cylinder swept volume of 90 litres or more, installed on ships built on or after 1 January 1990 but before 1 January 2000. Where an approved method (certified to Tier I limits under the NTC 2008 framework) exists for a specific engine type, the flag state is required to apply it.

In practice, Reg.13.7 applies to a small number of large slow-speed two-stroke engine families from MAN B&W, Wartsila and Sulzer in the 90,000 to 120,000 kW range installed on VLCCs and large bulk carriers built in the 1990s. MAN Energy Solutions and Wartsila have each developed and IMO-approved retrofit kits for their relevant engine families. A ship owner with a 1995-built VLCC carrying a MAN B&W 12K98MC engine (swept volume approximately 1,556 litres per cylinder, MCR approximately 68,000 kW) is subject to this provision if the flag state activates it and an approved method exists for that specific engine series.

Compliance under Reg.13.7 follows the same NTC 2008 certification process as any other Tier I engine: the retrofit method is type-approved, the engine is re-tested on a test bed or in-service under the simplified measurement method, and a new EIAPP Certificate is issued.

Future regulatory outlook

The principal regulatory developments expected through 2028 are:

MEPC 84 (October 2025): possible adoption of a Mediterranean NECA and a Norwegian Sea NECA. Both were under active MEPC negotiation as of mid-2026. A Mediterranean NECA would require Tier III compliance for engines installed after the designation effective date, creating a new demand for SCR and EGR retrofits on Mediterranean-trading vessels.

MEPC 86 and beyond: any newly designated NECAs would typically enter force 12 to 18 months after adoption, with a further phase-in period before Tier III becomes mandatory for newly installed engines.

Post-2028 Tier IV discussion: a “Tier IV” level further reducing the NOx limit is under early discussion at the IMO Sub-Committee on Pollution Prevention and Response (PPR). No formal proposal had been adopted as of 2026. Any Tier IV would likely be technology-forcing at a level beyond current SCR/EGR capability, potentially requiring combined after-treatment systems or fundamentally different combustion architectures.

The current Tier III is achievable with mature technology: SCR with 90%+ efficiency is commercially proven, EGR at 30% to 40% recirculation rates is in widespread service, and LNG dual-fuel new buildings deliver intrinsic Tier III compliance. The regulatory trajectory from Tier I through Tier III represents a total NOx reduction of approximately 80% from the pre-2000 baseline at slow speeds; any further tier would push toward near-zero NOx in designated areas.

Limitations

MARPOL Annex VI Regulation 13 sets NOx limits only for marine diesel engines. Gas turbines on ships are subject to a separate provision (Reg.13.11) requiring flag-state approval against a generally equivalent standard, but the three-tier limit table does not apply directly. Boilers, incinerators and other combustion equipment on board are not subject to Reg.13.

The tier framework applies to engines installed on or after the relevant dates. The global marine fleet includes thousands of pre-2000 engines that carry no NOx obligation and thousands more Tier I engines that are legal to operate globally at 17 g/kWh indefinitely. The expected NOx reduction from Tier III is therefore realised only over the multi-decade fleet-replacement cycle, not immediately upon the NECA effective dates.

The EIAPP Certificate certifies the engine’s type-approval NOx at test-bed conditions. In-service NOx can deviate from the certified value due to engine wear, fuel quality variations, operating patterns outside the tested load range, and EGR or SCR system degradation. The parameter check method detects only deviations from the certified settings, not actual emissions; a well-maintained engine that has drifted within the parameter envelope may still emit more NOx than its EIAPP value suggests.

The simplified measurement method is the only on-board means of direct NOx verification, but its cost (USD 10,000 to USD 25,000 per engine) limits deployment to cases where the parameter check raises a flag. Continuous emissions monitoring systems (CEMS) capable of in-service real-time NOx measurement are not yet mandatory under Annex VI, though several flag states and classification societies recommend them as best practice for Tier III vessels.

The Mediterranean SOx ECA (effective 1 May 2025) is sometimes confused with a NECA. It is not. It imposes no Tier III NOx requirement. Mediterranean-trading vessels must meet the 0.10% sulphur limit on fuel oil in that ECA, but their Tier I or Tier II NOx certification is unaffected by the designation.

The NOx limits are weighted averages across the prescribed test cycle. An engine that exceeds the limit at the 100% load mode but meets it at partial load may still pass the cycle-weighted average test. The practical consequence is that some engines, particularly those optimised for fuel economy at partial load, may produce NOx spikes at full load that exceed the tier limit but average down to a compliant cycle value. This is a known limitation of the type-approval regime.

Related calculators

• MARPOL Annex VI/13: NOx emissions calculator
• MARPOL Annex VI: NOx Tier II Limit calculator
• MARPOL Annex VI: NOx Tier III Limit calculator
• NOx Tier compliance check calculator
• Thermal NOx Zeldovich calculator
• SCR urea consumption calculator
• EGR rate for Tier III compliance calculator
• Norway NOx Fund levy calculator
• MARPOL Annex VI/5: survey and certification calculator
• MARPOL Annex VI/6: IAPP certificate calculator
• LNG methane slip GHG calculator
• CH4 methane slip calculator
• ESI Environmental Ship Index calculator
• MARPOL Annex VI/10: port state control NOx calculator
• ECA fuel-cost premium calculator
• SOx from fuel sulphur calculator
• PM10/PM2.5 particulate calculator
• Black carbon calculator
• SOx scrubber NaOH dosing calculator
• MARPOL Annex VI/14: sulphur emissions calculator
• MARPOL Annex VI/18: fuel oil quality calculator
• BDN reconciliation/ROB check calculator
• Bunker quality dispute calculator
• FONAR fuel oil non-availability calculator
• Engine cube-law fuel calculator
• Engine brake thermal efficiency calculator
• Engine CO2 per kWh calculator
• LNG well-to-wake calculator
• GFI attained calculator
• GFI compliance calculator
• SEEMP combined operational measures calculator
• MARPOL Annex VI/22: SEEMP Part I calculator
• MARPOL Annex VI/26: SEEMP revised calculator
• EPL required MCR reduction calculator
• EU MRV emissions calculator
• EU ETS annual allowance cost calculator
• FuelEU Maritime GHG penalty calculator

See also

• MARPOL Annex VI
• Emission Control Areas
• Selective catalytic reduction
• EGR retrofit on two-stroke engines
• LNG as marine fuel
• LNG fuel system
• Tier III compliant two-stroke engines
• Port state control
• SEEMP I, II and III
• IMO Net-Zero Framework
• IMO GHG Strategy
• What is CII
• What is EEDI
• What is EEXI
• EEXI EPL and ShaPoLi
• Slow steaming and CII
• Exhaust gas cleaning system
• Methanol as marine fuel
• Ammonia as marine fuel
• Heavy fuel oil
• Marine gas oil
• Biofuels in shipping
• Marine diesel engine
• Specific fuel oil consumption
• EU ETS for shipping
• FuelEU Maritime explained
• IMO DCS vs EU MRV
• Cold ironing and shore power
• MARPOL Convention
• SOLAS Convention
• Classification society
• Flag state and flag of convenience

References

1. IMO MEPC. Resolution MEPC.176(58): 2008 Amendments to MARPOL Annex VI (revised Regulation 13, three-tier NOx framework). IMO, 10 October 2008.
2. IMO MEPC. Resolution MEPC.177(58): NOx Technical Code 2008. IMO, 10 October 2008.
3. IMO MEPC. Resolution MEPC.272(69): 2016 Amendments to the NOx Technical Code 2008. IMO, 22 April 2016.
4. IMO MEPC. Resolution MEPC.286(71): Designation of Baltic Sea and North Sea as NOx Emission Control Areas. IMO, 7 July 2017.
5. IMO MEPC. Resolution MEPC.231(65): Amendments to MARPOL Annex VI Regulation 13.2 (major conversions). IMO, 17 May 2013.
6. IMO. MARPOL Consolidated Edition 2022, Annex VI Regulation 13. IMO, London, 2022.
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