MARPOL Annex VI Reg 13: NOx emission limits

Background: Annex VI 1997 + 2008 amendments under MEPC.176(58)

Air pollution from ships was outside the scope of the original 1973 MARPOL Convention and remained unregulated internationally until the 1997 Protocol added Annex VI as a separate annex covering air pollution and ozone-depleting substances. Adopted at an International Conference of the Parties to MARPOL convened in London in September 1997, the 1997 Protocol entered into force on 19 May 2005, eight years after adoption, once the requirement of fifteen states representing at least 50% of world merchant gross tonnage was met by Samoa’s accession on 18 May 2004.

The original 1997 text of Regulation 13 set out Tier I NOx limits for marine diesel engines installed on ships with a keel-laying date on or after 1 January 2000, with a retroactive effect from the date of entry into force of the Protocol. The 1997 text also referenced the original Technical Code on Control of Emissions of Nitrogen Oxides from Marine Diesel Engines, which prescribed the engine-certification methodology in much the same form as the current NTC 2008 but without the engine-family architecture.

By the late 2000s, several factors converged to motivate a major revision of Regulation 13. Health-impact studies had concluded that ship-source PM2.5 and the secondary nitrate aerosol formed from ship-source NOx were contributing to between 60,000 and 90,000 premature deaths per year worldwide, with disproportionate burden in port cities and coastal communities. Combustion-only NOx control on Tier I engines had reached approximately the limit of what could be achieved without after-treatment, and the marginal cost of further reduction through combustion alone was rising rapidly. After-treatment technologies, particularly selective catalytic reduction, had matured in the heavy-duty road-diesel sector and were ready for marinisation.

The Marine Environment Protection Committee adopted the 2008 amendments to Annex VI by Resolution MEPC.176(58) on 10 October 2008, with entry into force on 1 July 2010. The 2008 amendments completely rewrote Regulations 13 and 14, introduced Tier II and Tier III, established the NECA designation framework, made the NOx Technical Code 2008 mandatory under Resolution MEPC.177(58), and tightened the bunker delivery note (BDN) requirements of Regulation 18. The 2008 amendments are the structural backbone of the modern Regulation 13 regime and remain in force, supplemented by Resolution MEPC.272(69) of April 2016, which clarified the engine-modification procedure and the on-board parameter check method.

The North American ECA, designated jointly for SOx, NOx and PM by Resolution MEPC.190(60) of 26 March 2010, was the first to apply the Tier III regime, taking effect on 1 January 2016 for engines installed on ships with a keel-laying date on or after that date. The US Caribbean ECA followed on the same date under Resolution MEPC.202(62). The Baltic and North Sea NECAs were designated by Resolution MEPC.286(71) of 7 July 2017, with Tier III effective from 1 January 2021 for engines installed on ships with a keel-laying date on or after that date.

Reg 13 scope: marine diesel engines >130 kW post-2000 keel-laying

The scope of Regulation 13 is defined in paragraph 13.1 by reference to a power threshold, an engine type and an installation date. The detailed scope rules are as follows.

Power threshold of 130 kW

Regulation 13 applies only to marine diesel engines with a maximum continuous rated power output greater than 130 kW. The threshold excludes auxiliary engines used solely for emergency purposes such as the emergency generator, the emergency fire-pump engine and the emergency lifeboat engine, but only if those engines operate solely in the emergency role. A power output of 130 kW is in practice the threshold below which engine-certification testing is impractical and the contribution to total ship-source NOx is small. Most main-propulsion engines of even small commercial vessels exceed 130 kW; the threshold mainly excludes outboard engines, small auxiliary generators on yachts and small inland-waterway craft.

Marine diesel engine definition

Regulation 13 defines a marine diesel engine as any reciprocating internal combustion engine operating on liquid or dual fuel to which the regulations apply. The definition includes:

• Slow-speed two-stroke engines for main propulsion (typical rated speed 60-120 rpm), as offered by MAN Energy Solutions and Winterthur Gas + Diesel.
• Medium-speed four-stroke engines for main propulsion or auxiliary generation (typical rated speed 400-900 rpm), as offered by Wartsila, MAN, Caterpillar, Yanmar and Hyundai Himsen.
• High-speed four-stroke engines for auxiliary generation, lifeboat engines and small craft (typical rated speed 1,200-2,400 rpm).
• Dual-fuel engines where one of the fuels is liquid (fuel oil, MGO, methanol or ammonia) and the other is gaseous (LNG primarily). The pure gas-mode operation of dual-fuel engines is also captured because the engine is still classified as a marine diesel engine under Annex VI.

The definition excludes pure gas-only engines (no liquid fuel pilot) under the original Annex VI text, but the 2014 amendments by Resolution MEPC.258(67) extended Regulation 13 to gas-fuelled engines installed on ships subject to the IGF Code, with the same Tier limits applying. In practice, all main-propulsion engines on commercial ships fall within scope, and the exception is now mainly notional.

Installation date and keel-laying date

The applicable Tier depends on the date of installation of the engine, which is taken as the date the engine is fitted on the ship. For new-build ships the installation date coincides with the keel-laying date or the engine-fitting milestone of the building schedule. For replacement engines fitted to existing ships, the installation date is the actual fitting date, with the consequence that a 2024 replacement engine fitted to a 2010-built ship is subject to Tier II (or Tier III if NECA-designated) rather than the Tier I regime that applied at the original keel-laying date.

The Tier I retroactive provision under Regulation 13.7 applies to engines of 5,000 kW or more, with a per-cylinder displacement of 90 litres or more, installed on ships built between 1 January 1990 and 1 January 2000. Such engines must be brought into compliance with Tier I limits at the first renewal survey after a flag-state-approved technical file establishes that an approved low-NOx kit is commercially available for the engine. The retroactive provision is in practice closed: nearly all engines in this category have been retrofitted or scrapped.

Identical-replacement and major-conversion rules

Regulation 13.2 establishes the identical replacement rule: an engine identical to the engine being replaced may keep the original Tier of the engine being replaced, provided the replacement is necessary because the original is beyond economical repair. Regulation 13.2.2 establishes the non-identical replacement rule: a non-identical replacement is treated as a new installation and must meet the Tier in force at the date of fitting.

A major conversion of an engine, as defined in Regulation 13.2.1 and elaborated in NTC 2008 paragraph 1.3.1, is a conversion that substantially alters the engine performance or NOx emissions, including changes to fuel-injection timing, charge-air cooling, turbocharger matching or cylinder-head design beyond the certified component family. A major conversion triggers re-certification at the Tier in force at the date of conversion.

Tier I limits (17.0 / 45·n^(-0.2) / 9.8 g/kWh)

Tier I applies to every marine diesel engine of more than 130 kW installed on a ship with a keel-laying date on or after 1 January 2000, and to specified retroactive cases between 1990 and 2000. The Tier I limit is a piecewise function of the rated engine speed n in rpm, expressed in grams of NOx per kilowatt-hour of mechanical work delivered:

The piecewise structure reflects the inverse relationship between rated speed and the time available for thermal NOx formation in the combustion chamber. Slow-speed two-stroke engines, with long combustion durations and high in-cylinder temperatures, form more NOx per unit work and have a higher allowable limit. High-speed engines, with short combustion durations, form less NOx per unit work and have a stricter limit.

Worked examples at canonical engine speeds illustrate the curve:

• A slow-speed two-stroke main engine at 80 rpm: 17.0 g/kWh (the n < 130 rpm plateau).
• A medium-speed four-stroke main engine at 500 rpm: 45 · 500^(−0.2) = 45 · 0.288 = 12.96 g/kWh.
• A medium-speed auxiliary generator at 720 rpm: 45 · 720^(−0.2) = 45 · 0.270 = 12.13 g/kWh.
• A medium-speed auxiliary at 900 rpm: 45 · 900^(−0.2) = 45 · 0.258 = 11.62 g/kWh.
• A high-speed engine at 1,800 rpm: 45 · 1800^(−0.2) = 45 · 0.220 = 9.92 g/kWh, very close to the 9.8 g/kWh plateau.
• A high-speed engine at 2,400 rpm: 9.8 g/kWh (the n ≥ 2,000 rpm plateau).

Tier I represented a roughly 5% to 15% reduction below the typical 1990s NOx baseline of 18 to 22 g/kWh for slow-speed engines and was achievable through combustion-only techniques: retarded fuel-injection timing (typically 2 to 4 degrees crank-angle late of the optimum thermal-efficiency point), increased compression ratio, optimised injector spray pattern with smaller droplet diameter, improved charge-air cooling to reduce peak in-cylinder temperature and refined combustion-chamber geometry. Engine manufacturers absorbed Tier I into their standard engine designs from 2000 onwards.

The Tier I regime remains relevant in 2026 because a substantial population of ships built between 2000 and 2010 carry Tier I engines and continue in service. PSC inspection of a Tier I ship verifies the EIAPP Certificate, the Technical File and the absence of unapproved engine modifications.

Tier II limits (14.4 / 44·n^(-0.23) / 7.7 g/kWh)

Tier II applies globally to every marine diesel engine of more than 130 kW installed on a ship with a keel-laying date on or after 1 January 2011. The Tier II limit is:

The Tier II limit represents a roughly 20% reduction from Tier I across the rated-speed range. Worked examples:

• 80 rpm: 14.4 g/kWh (Tier I was 17.0; reduction 15.3%).
• 500 rpm: 44 · 500^(−0.23) = 44 · 0.234 = 10.30 g/kWh (Tier I was 12.96; reduction 20.5%).
• 720 rpm: 44 · 720^(−0.23) = 44 · 0.217 = 9.55 g/kWh (Tier I was 12.13; reduction 21.3%).
• 900 rpm: 44 · 900^(−0.23) = 44 · 0.207 = 9.11 g/kWh (Tier I was 11.62; reduction 21.6%).
• 1,800 rpm: 44 · 1800^(−0.23) = 44 · 0.171 = 7.52 g/kWh (Tier I was 9.92; reduction 24.2%).
• 2,400 rpm: 7.7 g/kWh (Tier I was 9.8; reduction 21.4%).

Tier II is achieved through advanced combustion-only techniques without after-treatment in the great majority of installations. The principal techniques are electronic fuel injection (replacing the cam-driven mechanical injection of Tier I engines), two-stage turbocharging with intercooling between stages, Miller-cycle valve timing (early intake valve closing for late-cycle expansion cooling), variable-geometry turbocharging and refined combustion-chamber geometry. Common-rail injection systems with peak rail pressures above 1,500 bar enable smaller droplet diameter and more uniform fuel-air mixing, reducing local peak temperatures and the thermal NOx that scales with the Zeldovich mechanism.

The marginal cost of Tier II compliance over a Tier I baseline is approximately 3% to 6% of new-engine list price for slow-speed two-stroke engines and 4% to 8% for medium-speed four-stroke engines. Specific fuel consumption typically rises by 1 to 2 g/kWh because the combustion is shifted toward the cooler, lower-NOx region of the trade-off curve. Engine manufacturers recover part of the SFOC penalty through waste-heat recovery and improved turbocharging efficiency.

Tier III limits (3.4 / 9·n^(-0.2) / 2.0 g/kWh, NECA only)

Tier III applies only to engines operating in a designated Nitrogen Emission Control Area (NECA) and only to engines installed on ships with a keel-laying date on or after the effective date of the relevant NECA. The Tier III limit is:

The Tier III limit represents a roughly 80% reduction from Tier I across the rated-speed range. Worked examples:

• 80 rpm: 3.4 g/kWh (Tier I was 17.0; reduction 80.0%).
• 500 rpm: 9 · 500^(−0.2) = 9 · 0.288 = 2.59 g/kWh (Tier I was 12.96; reduction 80.0%).
• 720 rpm: 9 · 720^(−0.2) = 2.43 g/kWh (Tier I was 12.13; reduction 80.0%).
• 900 rpm: 9 · 900^(−0.2) = 2.32 g/kWh (Tier I was 11.62; reduction 80.0%).
• 1,800 rpm: 9 · 1800^(−0.2) = 1.98 g/kWh, just below the 2.0 g/kWh plateau.
• 2,400 rpm: 2.0 g/kWh.

The Tier III plateau at n ≥ 2,000 rpm of 2.0 g/kWh is below the n < 130 rpm plateau of 3.4 g/kWh, the inverse of the Tier I and Tier II curve relationships. The reason is that the Tier III curve is set near the practical achievable limit of after-treatment regardless of rated speed; the 80% reduction figure quoted at every rated speed is achieved by SCR or LNG operation rather than by exploiting combustion-rate scaling.

The dual-Tier nature of Tier III is critical to understanding the regulation. A Tier III engine is certified to meet Tier III when operating in NECA mode and certified to meet Tier II when operating outside NECA. Most SCR-equipped engines run in bypass mode outside NECA (SCR offline, NOx at Tier II level) and in active mode inside NECA (SCR engaged, NOx at Tier III level). The ship’s NTC 2008 Technical File documents both operating modes and the parameters for each.

NECA scope: N-AM ECA (2016), US Caribbean ECA (2016), Baltic NECA (2021), North Sea NECA (2021)

As of 2026 the four designated NECAs are:

North American NECA (1 January 2016)

The North American ECA was designated jointly for SOx, NOx and PM by Resolution MEPC.190(60) of 26 March 2010. The geographic scope covers the Atlantic and Pacific coastlines of the United States and Canada (including Hawaii) out to 200 nautical miles, the Bering Sea coastline of Alaska, and the Great Lakes via the St Lawrence Seaway. The NECA dimension applies Tier III to engines installed on ships with a keel-laying date on or after 1 January 2016. The North American NECA is the largest by sea-area among the four.

US Caribbean NECA (1 January 2016)

The US Caribbean ECA was designated by Resolution MEPC.202(62) of 15 July 2011. The scope covers the waters around Puerto Rico and the US Virgin Islands out to 200 nautical miles. The Tier III effective date matches the North American NECA at 1 January 2016.

Baltic NECA (1 January 2021)

The Baltic NECA was designated by Resolution MEPC.286(71) of 7 July 2017. The scope is the Baltic Sea proper plus the Skagerrak and Kattegat, abutting the North Sea NECA at the Skagerrak entrance. Tier III applies to engines installed on ships with a keel-laying date on or after 1 January 2021. The Baltic NECA overlays the existing Baltic SECA (in force since 2006), creating a joint sulphur+NOx control area.

North Sea NECA (1 January 2021)

The North Sea NECA was designated by the same Resolution MEPC.286(71). The scope covers the North Sea, the English Channel and the Strait of Dover, abutting the Baltic NECA at the Skagerrak boundary. Tier III applies on the same date as the Baltic NECA. The North Sea NECA overlays the existing North Sea SECA (in force since August 2007).

Mediterranean is not a NECA

The Mediterranean SECA, in force from 1 May 2025 under Resolution MEPC.361(79), is not a NECA. It is a sulphur and particulate-matter ECA only; it imposes the 0.10% sulphur cap but carries no Tier III NOx obligation. The Mediterranean coastal states have signalled intent to seek NECA designation in the late 2020s, with a target effective date no earlier than 2030 or 2031 if a proposal is adopted in 2027. Keeping this distinction sharp matters at PSC and in bunker planning: a ship trading purely in the Mediterranean in 2026 must comply with the SECA sulphur limit but faces no Tier III requirement.

NTC 2008: engine-family certification methodology

The NOx Technical Code 2008 (NTC 2008), made mandatory under Regulation 13 by Resolution MEPC.177(58) of 10 October 2008, prescribes the methodology by which a marine diesel engine demonstrates compliance with the applicable Tier limit. NTC 2008 has eight chapters and a series of appendices and is the operative technical document for engine certification.

Pre-installation type-approval test

The pre-installation type-approval test is conducted on the manufacturer’s test bed before the engine is fitted on the ship. The test prescribes a test cycle appropriate to the engine type:

• E2 cycle for constant-speed main propulsion engines (controllable-pitch propeller, electric drive): four steady-state modes at 100%, 75%, 50% and 25% of rated power, all at rated speed.
• E3 cycle for variable-speed propeller-law main engines (fixed-pitch propeller): four steady-state modes following the propeller curve at 100%, 75%, 50% and 25% of rated power with the corresponding propeller-law speed.
• D2 cycle for constant-speed auxiliary engines (generator sets): five steady-state modes at 100%, 75%, 50%, 25% and 10% of rated power, all at rated speed.
• C1 cycle and D1 cycle for variable-speed and constant-speed off-road engines, used for some marine auxiliary applications.

The NOx emission rate is measured at each mode by gas-analyser sampling of the exhaust, and the weighted-average is computed using mode-specific weighting factors prescribed in NTC 2008 Appendix III. The weighted average is then compared to the applicable Tier limit at the rated speed.

On-board verification

NTC 2008 prescribes two on-board verification methods used at periodic surveys:

• Parameter check method: the surveyor verifies that the engine’s as-fitted configuration matches the certified configuration recorded in the Technical File. Parameters checked include fuel-injection timing, injector type and number, charge-air cooling, turbocharger model, valve timing and combustion-chamber components. If all parameters match, the engine is presumed to meet the certified NOx level.
• Simplified measurement method: a portable gas-analyser is used to measure NOx at a selection of operating modes, and the result is compared to the certified NOx level. The simplified method is used when parameter check is inconclusive or when the engine has been modified outside the certified configuration.

The third option, direct measurement and monitoring, is the full re-test on the certified test cycle, used when the engine has undergone a major conversion.

NTC 2008 amendments

Resolution MEPC.272(69) of 22 April 2016 amended NTC 2008 to clarify the procedure for engine modifications, the parameter check method and the dual-fuel engine certification procedure. Two subsequent packages extended coverage further: MEPC 71 (Resolution MEPC.286(71), July 2017) carried the Baltic and North Sea NECA designations alongside NTC 2008 gas-fuel provisions, and MEPC 74 (Resolution MEPC.314(74), May 2019) revised the dual-fuel test-cycle weighting to address split-cycle gas-injection engines. The most recent substantive package, Resolution MEPC.343(78) of June 2022, addressed ECDIS-anchored NECA boundary determination methodology, allowing NECA-entry and NECA-exit timestamps to be derived from integrated bridge-system position records rather than a separate NOx Record Book log where the flag state permits the substitution.

Parent-engine vs member-engine concept

The engine-family concept is a central efficiency feature of NTC 2008: it reduces the certification burden for engine manufacturers by allowing a single certification test on a representative engine to cover an entire family of similar engines, while preserving regulatory rigour through a defined family-eligibility test.

Engine-family definition

NTC 2008 paragraph 4.4 defines an engine family as a group of engines that share design parameters relevant to NOx emissions to a degree that the worst-case engine (the parent) is representative of the entire family. The shared parameters include:

• Combustion-chamber design and geometry
• Fuel-injection system architecture
• Inlet- and exhaust-valve timing
• Turbocharger and charge-air cooler architecture
• Combustion mode (compression-ignition, spark-ignition, dual-fuel)

The bore, stroke, swept-volume per cylinder, number of cylinders and rated speed may vary within an engine family within prescribed bounds, allowing a manufacturer to certify a multi-cylinder family from one test.

Parent engine

The parent engine is the engine within the family judged most likely to fail the NOx limit, identified by an a priori screening that considers per-cylinder displacement, rated power per cylinder and rated speed. The parent is subjected to the full test-cycle measurement, and the resulting weighted-average NOx is recorded as the certified value for the family.

Member engines

A member engine is any engine within the family other than the parent. A member is certified by reference to the parent: the as-built member is required to share the family-defining parameters, and the parent’s certified NOx is presumed to apply. NTC 2008 paragraph 4.4.6 requires that if the survey of a member engine by the parameter check method reveals divergence from the family, the member must be re-tested by the simplified measurement method or by full direct measurement.

In-line and V engine relationships

For multi-cylinder engines, the in-line and V configurations of the same cylinder family are typically certified together. The 6-cylinder in-line, 8-cylinder in-line, V-12 and V-16 versions of a Wartsila 31 medium-speed engine, for example, are members of a single engine family with one parent test covering all configurations. The V configurations have shorter intake and exhaust path lengths and may require slightly different turbocharger matching, but the combustion-chamber geometry and fuel-injection timing are identical.

EIAPP certificate issuance and renewal

The Engine International Air Pollution Prevention (EIAPP) Certificate is the engine-level certificate issued under NTC 2008 evidencing compliance of a specific engine with the applicable Tier limit. Each certified engine carries one EIAPP Certificate.

Issuance

The EIAPP Certificate is issued by the flag-state administration or by a recognised classification society (DNV, Lloyd’s Register, ABS, BV, NK, RINA, KR, CCS, RS, IRS, PRS, ClassNK or another IACS member acting on behalf of the flag) at the conclusion of the pre-installation type-approval test. The certificate specifies:

• Engine manufacturer, model and serial number
• Rated power and rated speed
• Test cycle used (E2, E3, D2, C1, D1)
• Weighted-average NOx measured (g/kWh)
• Applicable Tier limit at the rated speed (g/kWh)
• Engine family identification and parent-engine cross-reference if applicable
• Tier III dual-mode certification entries if applicable

Technical File

The NOx Technical File is the engine-specific document accompanying the EIAPP Certificate. It records the as-certified engine configuration, the parameter list used in the parameter check method, the NOx-relevant components and their identification numbers, and the operating instructions and adjustment limits. The Technical File is required to be on board the ship at all times and is presented to the surveyor at periodic surveys.

Renewal

The EIAPP Certificate has no expiry date but is voided by:

• Major conversion of the engine, triggering re-certification under Regulation 13.2.1
• Replacement of an engine component outside the certified family, triggering re-certification under NTC 2008 paragraph 1.3
• Failure of the parameter check method or simplified measurement method at survey, requiring corrective action and re-survey

A successful renewal of the IAPP Certificate at the ship level (typically every five years) verifies the validity of all on-board EIAPP Certificates and the consistency of the Technical Files.

Tier III compliance: SCR (Selective Catalytic Reduction)

Selective catalytic reduction (SCR) is the most common Tier III compliance pathway, used in approximately 80% of Tier III installations on Tier III-certified ships built since 2016. The technology has matured from heavy-duty road-diesel applications and is offered by Wartsila (NOR), MAN Energy Solutions, Yara Marine, Hug Engineering, Caterpillar (Cat-SCR) and several other vendors.

Working principle

A urea solution (typically 32.5% urea by mass in deionised water, marketed as AdBlue or DEF in road-diesel contexts and as NOx Reducer or AUS-32 in marine contexts) is injected into the exhaust stream upstream of a vanadium-titania catalyst. The urea decomposes thermally to ammonia and isocyanic acid, and the ammonia reacts with NO and NO2 on the catalyst surface to form nitrogen and water:

• 4 NO + 4 NH3 + O2 → 4 N2 + 6 H2O (the principal reaction)
• 6 NO2 + 8 NH3 → 7 N2 + 12 H2O
• NO + NO2 + 2 NH3 → 2 N2 + 3 H2O (the fast reaction at intermediate exhaust temperatures)

The vanadium-titania catalyst operates in the temperature window of approximately 300 to 450 °C. Below 300 °C the urea decomposition is incomplete and ammonia slip rises; above 450 °C the ammonia oxidises directly to NOx and the catalyst loses selectivity.

NOx reduction efficiency

A correctly sized and operated marine SCR system achieves approximately 80% to 90% NOx reduction at steady-state high-load operation, sufficient to bring a Tier II engine into Tier III compliance. The reduction efficiency falls at low-load operation due to lower exhaust temperatures and at very high load due to peak-temperature ammonia oxidation. Modern systems with active urea-injection control and exhaust-temperature management maintain the 80% reduction across the load range from approximately 25% to 100% MCR.

Two-stroke vs four-stroke arrangement

For two-stroke main engines, SCR is most commonly arranged as a high-pressure SCR (HP-SCR) upstream of the turbocharger to operate in the higher-temperature exhaust window before turbocharger expansion. Wartsila and MAN HP-SCR designs typically place the SCR reactor in the exhaust gas receiver. For four-stroke main and auxiliary engines, low-pressure SCR (LP-SCR) downstream of the turbocharger is the standard arrangement, placed in the exhaust uptake before the silencer.

Tier III compliance: EGR (Exhaust Gas Recirculation)

Exhaust gas recirculation (EGR) is the second most common Tier III pathway, used principally on MAN ME-GI two-stroke and MAN ME-B dual-mode installations. EGR achieves NOx reduction by recirculating a fraction of the exhaust gas into the intake charge, displacing oxygen and reducing peak combustion temperatures and the thermal NOx that scales with the Zeldovich mechanism.

Working principle

A fraction (typically 25% to 35% in Tier III mode) of the exhaust gas is extracted from the exhaust receiver, cooled in an EGR cooler, scrubbed in a wet scrubber to remove sulphur oxides and particulate matter, dehumidified, and re-introduced into the intake charge upstream of the turbocharger or the intake manifold. The recirculated exhaust has lower oxygen content and higher specific heat than fresh charge air, lowering the peak combustion temperature and the thermal NOx. Typical NOx reduction at the engine exhaust is approximately 75% to 80%, sufficient for Tier III.

MAN ME-GI Tier III mode

The MAN ME-GI dual-fuel two-stroke main engine, introduced in 2014, was the first commercially deployed marine engine to use EGR for Tier III compliance. In gas mode the engine operates in the diesel cycle on LNG with a small pilot of MGO; the EGR loop is engaged during NECA operation to bring NOx below 3.4 g/kWh. In oil mode the engine operates as a conventional diesel; EGR is engaged in NECA. Outside NECA, the EGR is bypassed and the engine operates at Tier II level.

Footprint and complexity

EGR has a smaller deck-space footprint than SCR (no urea tank) but a higher complexity in the wet-scrubber loop (closed-loop wash-water management, scrubber sludge disposal). The closed-loop wash-water from the EGR scrubber is regulated under MEPC.184(59) and must be retained on board for shore disposal. Operating cost is comparable to SCR at the system level, with the urea-purchase cost replaced by scrubber consumables and sludge-disposal cost.

Tier III compliance: LNG / dual-fuel

LNG dual-fuel combustion in the Otto cycle offers an inherent Tier III pathway for Otto-cycle engines without after-treatment. The Otto-cycle dual-fuel engine, exemplified by the Wartsila DF series (W34DF, W31DF, W46DF), operates in lean-burn premixed combustion at lower peak combustion temperatures than the diesel cycle, producing approximately 80% lower NOx at the engine-out level.

Otto cycle vs diesel cycle dual-fuel

The Wartsila DF series and the Rolls-Royce Bergen B33:45L use the Otto cycle in gas mode: gas is admitted with air through the intake valves, compressed, and ignited by a small pilot of MGO at the end of the compression stroke. The premixed lean charge limits the local peak temperature and the thermal NOx. Engine-out NOx in gas mode is approximately 1.5 to 2.5 g/kWh, well below the Tier III limit of 2.0 to 3.4 g/kWh.

The MAN ME-GI uses the diesel cycle in gas mode: gas is injected at high pressure (~300 bar) directly into the cylinder near the end of the compression stroke and ignites by compression. The diesel-cycle dual-fuel achieves the high specific power and efficiency of the diesel cycle but the higher peak combustion temperature, so engine-out NOx is at Tier II level rather than Tier III; the ME-GI uses EGR for Tier III compliance as described above.

Methane slip

A material drawback of Otto-cycle dual-fuel for the climate footprint is methane slip: a small fraction (typically 0.5% to 3% of fuel mass) of the gas charge passes through the engine unburnt and is emitted as methane, a powerful short-lived greenhouse gas. Methane slip is reported under both IMO Carbon Intensity Indicator requirements and the EU FuelEU Maritime Regulation (EU 2023/1805, applied from 1 January 2025), but it is not currently subject to a NOx-equivalent IMO limit under Regulation 13. The per-fuel WTW LNG Otto vs diesel comparison discusses the trade-off in detail.

Ammonia and methanol dual-fuel

Newer dual-fuel options include ammonia and methanol. Ammonia combustion produces low NOx in lean-burn operation but with N2O (nitrous oxide) and unburnt-NH3 slip concerns; the regulatory regime is still being developed under IMO IGC Code. Methanol combustion produces low NOx without the methane-slip concern, and the MAN LGIM (Liquid Gas Injection Methanol) engine and the Wartsila MethanolPac achieve Tier III in NECA without after-treatment.

Tier III compliance: water-emulsion fuel (legacy)

Water-emulsion fuel is a legacy NOx-reduction technique in which water is mechanically emulsified into the fuel oil at typically 10% to 25% water-by-fuel ratio. The emulsion atomises in the injector, and the water vaporisation in the cylinder cools the local flame and reduces thermal NOx. Typical NOx reduction is 25% to 40%, far below the 80% required for Tier III, which is why the technique is classified as a legacy approach and is not used as a primary Tier III pathway.

Water emulsion is used in some Tier II compliance scenarios where the engine is on the borderline and emulsion is sufficient to bring the certified NOx below the limit. The technique is also used in retrofitting older Tier I engines for voluntary NOx reduction in port (as required by the Norway NOx Fund or the Vancouver Fraser Port voluntary programme). For Tier III compliance, water emulsion is combined with SCR or EGR as a layered approach in a small number of installations.

The principal disadvantage of water emulsion is the specific fuel consumption penalty: the energy required to vaporise the water in the cylinder is debited from the work output, increasing SFOC by typically 1% to 3% per 10% water content. Water emulsion is also incompatible with high-pressure common-rail injection at very high pressures because of cavitation in the injector tip.

AdBlue / urea consumption (~5% of fuel mass)

A correctly sized SCR system on a Tier II base engine consuming AUS-32 (32.5% urea solution) for Tier III compliance consumes approximately:

That is, the urea-solution mass flow is about 5% of the fuel mass flow. The 5% figure derives from the stoichiometry of the SCR reaction (1 mole NH3 per mole NO, 17 g NH3 per 30 g NO) and the urea-decomposition factor (1 mole urea yields 2 moles NH3, 60 g urea per 34 g NH3) combined with the 32.5%-urea concentration of AUS-32 and the typical 12 g/kWh Tier II NOx baseline reduced to 2 g/kWh Tier III. Worked example for a 14,000 kW main engine at 80% load (11,200 kW shaft power) burning 180 g/kWh fuel:

• Fuel mass flow: 11,200 · 180 / 1000 = 2,016 kg/h
• Urea solution mass flow: 0.05 · 2,016 = 100.8 kg/h
• AUS-32 density 1.087 kg/L, so volumetric flow approximately 92.7 L/h
• Over a 7-day NECA voyage: 100.8 · 24 · 7 = 16,934 kg AUS-32, or 15.6 m³

The 5% figure is a useful first-order rule of thumb; the actual ratio varies between 3% and 7% depending on the engine NOx baseline, the SCR conversion efficiency target, the ammonia-slip allowance and the AUS-32 concentration. The SCR urea consumption calculator implements the detailed stoichiometry.

Bunker availability

AUS-32 is bunkered at most major NECA ports (Rotterdam, Antwerp, Hamburg, Gothenburg, Copenhagen, Helsinki, Long Beach, New York, Vancouver, Halifax). Bunker price is approximately USD 350 to 600 per tonne in 2025-2026, well above road-grade AdBlue but reflecting the marine-grade purity, the marine logistics and the reduced supply competition in port-bunker quantities. Marine AUS-32 is typically supplied by truck or barge in 5 m³ to 30 m³ lots; very large NECA-deployed ships (LNG carriers, large container ships) take 50 m³ to 100 m³ lots once or twice per round-voyage.

Storage on board

AUS-32 storage tanks must be of stainless steel or polyethylene (urea is corrosive to carbon steel), with temperature management to prevent freezing below −11 °C and decomposition above approximately 35 °C. The typical container ship Tier III installation has a 50 to 200 m³ urea tank, sized to allow approximately one or two NECA round-voyages between refills.

SCR retrofit cost: USD 1.5-2.5m typical container ship

The capital cost of a Tier III SCR retrofit on an existing Tier II ship is approximately USD 1.5 million to 2.5 million for a typical 6,000 to 10,000 TEU container ship as of the 2024-2026 market. The cost components are:

• SCR reactor and catalyst: USD 400,000 to 700,000, depending on size and catalyst grade
• Urea storage and dosing system: USD 200,000 to 350,000, including stainless-steel tank, dosing pumps, atomisation injectors and control system
• Exhaust integration and ducting: USD 250,000 to 450,000, including new exhaust spool pieces, expansion bellows, insulation and exhaust uptake reinforcement
• Engine-room modifications: USD 150,000 to 300,000, including additional foundations, piping and fire-protection adjustments
• Class survey, commissioning and EIAPP re-certification: USD 150,000 to 300,000
• Yard time and lay-up: USD 200,000 to 400,000 (typically 14 to 21 days off-hire)
• Engineering and project management: USD 150,000 to 300,000

For a smaller feeder container ship or a bulk carrier, the cost falls to USD 0.8 to 1.5 million; for a very large container ship or a VLCC, the cost rises to USD 3.0 to 4.5 million per main engine. Auxiliary-engine SCR retrofit is typically USD 200,000 to 400,000 per generator set, with most ships requiring SCR on three or four auxiliary generators in addition to the main engine.

The operating cost of SCR is approximately USD 5 to 12 per MWh of shaft work in NECA at 2025-2026 urea prices, dominated by AUS-32 consumption. For a typical container ship spending 15% of voyage time in NECA at 50% MCR average, the additional operating cost is in the order of USD 0.5 to 1.5 million per year, recovered through ECA-surcharge clauses in liner-trade contracts.

The retrofit business case for older Tier II ships is often marginal. Tier III is required only for ships built on or after 1 January 2016 (North America / US Caribbean) or 1 January 2021 (Baltic / North Sea), so the retrofit population is principally pre-2016 ships voluntarily upgrading to access NECA cargoes or to comply with charter-party clauses requiring Tier III. The 2030 review of Tier III scope is the principal driver of any future retrofit wave.

Class society implementation: DNV, LR, ABS, BV, NK, RINA, KR, CCS, RS, IRS

Engine certification under NTC 2008 is implemented in practice by the classification societies acting on behalf of the flag-state administration. The principal societies and their NOx-certification scopes are:

• DNV: largest by certified tonnage in the European trades; full coverage of slow-speed two-stroke (MAN B&W, WinGD), medium-speed four-stroke (MAN, Wartsila, Yanmar, Caterpillar), high-speed and dual-fuel families; pioneer of the Tier III SCR-on-test-bed certification
• Lloyd’s Register (LR): full coverage; particularly strong in the LNG carrier, Singapore-flag and UK-flag fleets; the Tier III HP-SCR implementation on Wartsila X-DF and MAN ME-GA was developed jointly with LR
• American Bureau of Shipping (ABS): largest in the Liberia-flag and US-flag fleets; full coverage of all engine types
• Bureau Veritas (BV): full coverage; particularly strong in French, French overseas and African flag fleets
• Nippon Kaiji Kyokai (NK): dominant in the Japan-flag and Panama-flag fleets; full coverage of Mitsui MAN B&W, Hitachi MAN B&W and Yanmar engines
• RINA: Italian, Mediterranean, Black Sea and some African flag fleets; full coverage
• Korean Register (KR): dominant in the Korea-flag and Marshall-Islands fleets with Korean shipbuilding; full coverage of Hyundai Himsen, HSD MAN B&W, Doosan Wartsila and STX engines
• China Classification Society (CCS): dominant in the China-flag fleet and large in the Hong Kong fleet; full coverage of CSSC MAN B&W, CSSC Winterthur Gas + Diesel, Shanghai Diesel and Anqing CSSC engines
• Russian Maritime Register of Shipping (RS): Russia-flag fleet; full coverage with some restrictions on advanced after-treatment certifications post-2022 sanctions
• Indian Register of Shipping (IRS): India-flag fleet and growing in non-IACS markets; full coverage

Each society maintains an engine type-approval register with the certified engine families, parent engines, member engines and Tier statuses. The cross-recognition agreements among IACS members allow an engine certified by one society to be installed on a ship classed with another society subject to the receiving society’s acceptance of the certification documentation.

The PSC inspection at port reads the EIAPP Certificate as the primary evidence of certification compliance and the Technical File as the secondary evidence. The class society annual survey verifies the parameter check method or simplified measurement method as applicable.

PSC inspection: NTC 2008 certificate + simplified measurement parameter check

Port State Control under the Paris MOU and the Tokyo, Caribbean, Mediterranean, Black Sea, Indian Ocean and Riyadh MOUs verifies Regulation 13 compliance through documentary inspection and selective on-board parameter checks. The principal PSC focus areas are:

Documentary verification

• IAPP Certificate (ship-level) and its NOx supplement, recording the Tier statuses of all engines on board and the NECA certification status if applicable
• EIAPP Certificate for each engine of more than 130 kW
• NOx Technical File for each engine, with the parameter list and the operating instructions
• NOx Record Book (under Regulation 14 and 13 cross-link), recording NECA-mode entry/exit timestamps for Tier III dual-mode engines
• Bunker Delivery Notes under Regulation 18 (for the fuel-quality cross-check; not directly Reg 13 but read together)

On-board verification

• Parameter check: PSC officer or accompanying class surveyor verifies that the as-fitted engine matches the Technical File parameters for fuel-injection timing, injectors, turbocharger model and charge-air cooling
• Simplified measurement (rare in PSC, typically only at flag-state surveys): portable gas-analyser measurement of NOx at one or two operating modes, compared to the certified value
• SCR / EGR system verification: visual inspection of urea tank level, urea-injection pump operation, catalyst housing seals; for EGR, scrubber loop and wash-water sample analysis
• NECA-mode entry/exit verification: cross-check of the NOx Record Book entries against the AIS track to verify that the SCR or EGR was active during NECA transits

Common PSC findings

The most frequent Reg 13 findings at PSC are:

• Missing or expired EIAPP Certificate (typically after a major engine conversion not re-certified)
• Technical File not on board or incomplete (typically a missing parameter list or operating instruction)
• Parameter divergence (typically replacement of an injector with an unapproved part)
• Tier III system not engaged in NECA (typically a SCR bypass left open, leading to excess NOx in the NECA transit)
• NOx Record Book gaps or inconsistencies with AIS

A finding of non-compliance with Reg 13 is typically classified as a detainable deficiency under the Paris MOU New Inspection Regime if the engine is identified as a primary contributor to the ship’s emissions and a clear breach is found. Less serious findings result in a “rectify before departure” or “rectify within 14 days” notation.

Relationship to Reg 14 sulphur (cross-link)

Regulation 13 and Regulation 14 (sulphur) are the two principal pollutant-control regulations of Annex VI Chapter 3 and are operationally distinct but functionally parallel:

• Different pollutant: Reg 13 controls NOx (a combustion-product, formed in-cylinder, addressable by combustion modification or after-treatment); Reg 14 controls SOx (a fuel-quality product, fixed by the sulphur content of the fuel oil burnt, addressable by fuel switch or scrubber)
• Different compliance pathway: Reg 13 addresses the engine and its after-treatment; Reg 14 addresses the fuel oil bunkered and consumed
• Different regulatory architecture: Reg 13 is an engine-certification regime with a Technical File and an EIAPP Certificate; Reg 14 is a fuel-quality regime with a bunker delivery note and a Fuel Oil Quality Certificate
• Different geographic scope: Reg 13 has Tier I and Tier II globally and Tier III in NECAs only; Reg 14 has the global cap (0.50% from 2020) and the SECA cap (0.10% from 2015)

The two regulations interact at the level of ECA designation. Most ECAs are designated for both sulphur and NOx control, applying both Reg 14 SECA limits and Reg 13 Tier III. The North American ECA, the US Caribbean ECA, the Baltic ECA and the North Sea ECA are joint SECA + NECA. The Mediterranean SECA is a sulphur-only ECA as of 2026; NECA designation is under negotiation. ECAs are not required to be joint (a coastal state can apply for sulphur-only or NOx-only designation), but the operational reality is that the same coastal states that designate for one pollutant typically designate for the other.

The two regulations also interact in the EGR scrubber loop: the EGR water-scrubber for NOx Tier III compliance also removes a fraction of the SOx and PM, providing partial Reg 14 compliance benefit when burning higher-sulphur fuels (compliant fuels are mandatory in SECA, but the EGR scrubber does provide an emission-reduction co-benefit on PM and SOx).

Relationship to Reg 18 BDN (cross-link)

Regulation 18 of Annex VI governs fuel oil quality and the bunker delivery note (BDN). The BDN is the document accompanying every bunker transaction recording the fuel oil supplied, including the sulphur content, the density, the viscosity and the fuel-grade designation under ISO 8217.

The Reg 13 to Reg 18 cross-link is operationally important because:

• The NOx-certification of the engine under Reg 13 assumes that the fuel oil is within the ISO 8217 quality band specified in the Technical File. Severe degradations of fuel quality (very high asphaltene content, very high cat-fines, water contamination) can shift the engine NOx outside the certified band.
• The bunker delivery note under Reg 18 is the documentary basis for the assumption above. A BDN showing fuel within the ISO 8217 range is presumptive evidence that the engine remains within its NOx-certified envelope.
• A mismatch between the BDN and the actual fuel quality (typically discovered through an independent fuel-oil sample analysis) can give rise to a Reg 14 finding (sulphur-cap breach if the BDN shows compliant sulphur but the sample shows higher), a Reg 18 finding (BDN inaccuracy) and indirectly a Reg 13 concern (the engine’s NOx envelope assumed compliant fuel).

The Reg 18 BDN calculator implements the BDN structure and the sulphur-cross-check logic.

IMO 2030 review of Tier III scope expansion

The IMO Marine Environment Protection Committee adopted at MEPC 80 in July 2023 a 2030 review of the Tier III scope, with a working group commissioned to examine the case for expansion to additional NECAs in the late 2020s. The review covers:

Mediterranean NECA proposal

The Mediterranean coastal states (France, Italy, Spain, Greece, Cyprus, Malta, Tunisia, Algeria, Morocco) have signalled, through a 2024 joint declaration under the Barcelona Convention, intent to seek IMO designation of the Mediterranean Sea as a NECA in the late 2020s, with a likely target effective date of 1 January 2030 or 2031. The proposal is supported by the European Commission and the European Parliament and is technically advanced; the principal blocker is the alignment of all Mediterranean coastal states (including Algeria, Tunisia, Morocco and the eastern Mediterranean states), which has been incomplete as of 2026. A companion NECA designation would be separate from the existing Mediterranean SECA and would require its own MEPC resolution.

Other potential NECAs

• Norwegian Sea / Norwegian coast: Norway has been studying the case for a Norwegian Sea NECA since 2019. The proposal would extend the existing North Sea NECA up the Norwegian coast to the Russian border. A draft submission is under preparation as of 2026.
• South Korean coastal NECA: South Korea has indicated intent to study a coastal NECA covering the Yellow Sea and the East Sea (Sea of Japan) but has not yet made a formal proposal at IMO.
• Australian / Tasman NECA: Australia studied a Tasman NECA in 2018 but did not pursue. The 2030 review may revive the proposal.
• Caribbean NECA expansion: The eastern Caribbean states (CARICOM) have discussed extending the US Caribbean ECA to the wider Caribbean but have not yet made a formal proposal.

Tier IV concept

The 2030 review also covers the case for a Tier IV NOx limit beyond Tier III, potentially in the order of 1.0 g/kWh for slow-speed engines (a further 70% reduction from Tier III). The technical case for Tier IV rests on advanced two-stage SCR with downstream ammonia-slip catalyst, and the regulatory case rests on the convergence of NOx limits with the road-diesel sector. As of 2026 the Tier IV proposal has not advanced beyond preliminary IMO-secretariat work and is unlikely to be adopted before 2032.

The three tiers as speed-dependent limits

Each Tier limit is a single piecewise function of the rated engine speed n in rpm, sharing the same shape across all three tiers. The general form is $E_{\text{NOx},T}(n) = a_T$ for $n < 130$ rpm, $b_T \cdot n^{-c_T}$ for $130 \leq n < 2{,}000$ rpm, and $d_T$ for $n \geq 2{,}000$ rpm, in g/kWh. The four coefficients per tier are the two flat plateaus and the prefactor-plus-exponent of the medium-speed band: Tier I uses $a_1 = 17.0$, $b_1 = 45$, $c_1 = 0.20$, $d_1 = 9.8$; Tier II uses $a_2 = 14.4$, $b_2 = 44$, $c_2 = 0.23$, $d_2 = 7.7$; Tier III uses $a_3 = 3.4$, $b_3 = 9$, $c_3 = 0.20$, $d_3 = 2.0$. The 130 rpm and 2,000 rpm breakpoints are fixed across all tiers, so only the four anchors move. The NOx Tier I/II/III overview article presents the summary table; this article carries the full regulatory detail.

The exponent on n is not arbitrary. The medium-speed dependence comes from the Zeldovich thermal-NOx mechanism, in which the formation rate scales with in-cylinder temperature T, oxygen partial pressure $p_{O_2}$ and combustion residence time $\tau$. The leading rate term is $\frac{d[\text{NO}]}{dt} \approx 2 k_1 [O][N_2]$ with $k_1 \propto \exp(-E_a / RT)$ and an activation energy $E_a \approx 318$ kJ/mol. Residence time scales inversely with engine speed because faster engines finish combustion sooner, so the integrated NOx per unit work falls roughly as $\tau \propto n^{-1}$. The empirical fit to historical marine-engine NOx data gave the $n^{-0.20}$ slope for Tier I and Tier III and the steeper $n^{-0.23}$ slope for Tier II, with prefactors of 45, 44 and 9. The plateau above 2,000 rpm reflects the near-flat NOx-versus-speed relationship at high speed, where combustion duration is no longer the dominant determinant.

The certified NOx that gets compared against these curves is a weighted average over the applicable test cycle, not a single-point reading. NTC 2008 prescribes $E_{\text{NOx,cert}} = \sum_{i=1}^{N} W_i \cdot \frac{m_{\text{NOx},i}}{P_i}$, where $W_i$ is the weighting factor of mode i from NTC 2008 Appendix III, $m_{\text{NOx},i}$ is the NOx mass flow at mode i in g/h, and $P_i$ is the brake power at mode i in kW. The cycle (E2, E3, D2, C1 or D1) is chosen by engine type as set out earlier. Compliance is the inequality $E_{\text{NOx,cert}} \leq E_{\text{NOx},T}(n)$ evaluated at the engine’s rated speed, not its operating speed.

Tier III NECAs and the dual-mode certification

Tier III is geographically conditional in a way Tier I and Tier II are not. A Tier III engine carries two certified states on one EIAPP Certificate: it meets Tier III in NECA mode and Tier II outside NECA, and the NTC 2008 Technical File documents the parameter set and engagement logic for both. The four NECAs in force as of 2026 took effect on the keel-laying dates already covered above: the North American and US Caribbean NECAs from 1 January 2016 (Resolutions MEPC.190(60) and MEPC.202(62)), and the Baltic and North Sea NECAs from 1 January 2021 (Resolution MEPC.286(71)). An engine installed on a ship keel-laid before its NECA’s effective date is not pulled into Tier III by the designation.

That dual-mode structure is why the achievable-limit reasoning behind the Tier III curve differs from the combustion-rate scaling that shapes Tier I and Tier II. The Tier III plateau at $n \geq 2{,}000$ rpm of 2.0 g/kWh sits below the $n < 130$ rpm plateau of 3.4 g/kWh, inverting the Tier I and Tier II ordering. The curve is set near the practical after-treatment limit regardless of rated speed, so the roughly 80% reduction quoted at every speed is delivered by SCR or LNG operation rather than by exploiting how combustion rate falls with speed.

SCR urea dosing arithmetic

The rule of thumb that an SCR system consumes urea solution at about 5% of fuel mass, $\dot{m}_{\text{urea}} \approx 0.05 \cdot \dot{m}_{\text{fuel}}$, is a first-order estimate, not a fixed ratio. It derives from the SCR stoichiometry (1 mole of $NH_3$ per mole of NO, 17 g $NH_3$ per 30 g NO), the urea-decomposition factor (1 mole of urea yields 2 moles of $NH_3$, 60 g urea per 34 g $NH_3$), the 32.5% urea concentration of AUS-32, and a typical reduction from a 12 g/kWh Tier II baseline down to 2 g/kWh Tier III. The explicit dosing rate is $\dot{m}_{\text{urea,solution}} = \frac{(E_{\text{NOx,in}} - E_{\text{NOx,out}}) \cdot P \cdot \frac{60}{30} \cdot \frac{1}{0.325}}{1000}$ kg/h, where $E_{\text{NOx,in}}$ and $E_{\text{NOx,out}}$ are the SCR-inlet and SCR-outlet NOx in g/kWh, P is the engine brake power in kW, the 60/30 factor is the urea-to-NOx mass ratio under stoichiometric decomposition (60 g urea per 30 g NO via 2 $NH_3$), and 0.325 is the AUS-32 urea-by-mass fraction. The actual ratio runs 3% to 7% depending on the engine NOx baseline, the conversion-efficiency target, the ammonia-slip allowance and the AUS-32 concentration. The SCR urea consumption calculator implements the detailed stoichiometry.

Worked example: 14,000 kW two-stroke on a Rotterdam-Singapore run

Take a 14,000 kW MAN 6S70ME-C two-stroke main engine at a rated speed of 91 rpm, installed on a container ship keel-laid on 1 January 2022 and trading between Rotterdam (Baltic NECA) and Singapore (no NECA). Tier III applies in the Baltic and North Sea NECA; Tier II applies on the rest of the voyage. At 91 rpm both limits sit on the $n < 130$ rpm plateau: the Tier III limit is 3.4 g/kWh and the Tier II limit is 14.4 g/kWh. The engine carries HP-SCR upstream of the turbocharger.

In bypass mode outside NECA the certified NOx is 13.8 g/kWh, inside Tier II. In active mode inside NECA the certified NOx is 3.0 g/kWh, inside Tier III. AUS-32 consumption inside NECA at 80% MCR (11,200 kW shaft power, 180 g/kWh fuel) is about 100 kg/h, or roughly 17 m³ over a 7-day NECA transit at that load. The EIAPP Certificate records both modes; the Technical File documents the parameter set and the SCR-engagement logic; the NOx Record Book logs the NECA entry timestamps, which PSC cross-checks against the AIS track, together with the SCR-engagement timestamps.

The same curve evaluates cleanly at other canonical speeds. A medium-speed engine at 500 rpm faces $45 \cdot 500^{-0.2} = 12.96$ g/kWh under Tier I, $44 \cdot 500^{-0.23} = 10.30$ g/kWh under Tier II (a 20.5% cut), and $9 \cdot 500^{-0.2} = 2.59$ g/kWh under Tier III (an 80.0% cut). A high-speed engine at 1,800 rpm faces $45 \cdot 1800^{-0.2} = 9.92$ g/kWh under Tier I, just above the 9.8 g/kWh plateau, $44 \cdot 1800^{-0.23} = 7.52$ g/kWh under Tier II, and $9 \cdot 1800^{-0.2} = 1.98$ g/kWh under Tier III, just below the 2.0 g/kWh plateau.

Limitations

Reg 13 is a test-bed certification regime, and the gap between certified and in-service NOx is the first limitation to keep in view. The certified value is a weighted-average measured on the manufacturer’s test bed under the prescribed cycle (E2, E3, D2, C1 or D1) and is treated as representative of the in-service envelope only while the engine stays inside its certified parameter range, which is what the parameter check method verifies. A ship that runs predominantly at very low load, at off-design speed, or with a worn fuel-injection system can emit materially more NOx than its certified figure while still passing a documentary survey.

The certified figure also assumes a fuel oil within the ISO 8217 band recorded in the Technical File. High cat-fines, high asphaltene content or water contamination shift the engine outside its certified NOx band, which is why the Reg 18 bunker-delivery-note cross-check matters even though it is a separate regulation. A second limitation is the engine-family presumption: a member engine inherits the parent’s certified NOx without its own full re-test, so a member that has drifted from the family parameters can carry a certified value it would not actually meet on a direct measurement.

Tier III itself is bounded by geography and by hardware. It applies only inside the four designated NECAs and only to engines on ships keel-laid on or after each NECA’s effective date, so a Tier III-capable engine trading outside NECA is under no obligation to engage its after-treatment. The 80% reduction depends entirely on the SCR or EGR system being in its active state, sized correctly, and held in the catalyst window (roughly 300 to 450 °C for the vanadium-titania SCR catalyst); reduction efficiency falls at low load from cooler exhaust and at very high load from ammonia oxidation. The regulatory model treats the system as binary on/off, but in service the dosing and recirculation rate are modulated, so a NECA-mode certified value is an upper-bound design point rather than a guaranteed continuous figure.

The numbers in this article are engineering estimates for orientation, not certification outputs. The retrofit-cost ranges (about USD 1.5 to 2.5 million for a 6,000 to 10,000 TEU container ship), the urea-consumption rule of thumb near 5% of fuel mass, and the worked-example certified figures are typical industry values; a specific installation’s certified NOx, EIAPP entries and AUS-32 demand must come from the engine’s own NTC 2008 Technical File and the recognised-organisation survey, not from a generic limitation-bounded estimate like the ones here. The companion calculators carry the same caveat: they size and screen, they do not certify.

The scope edges are worth stating plainly, because most disputes at survey turn on them. Engines below 130 kW are outside Reg 13 entirely, as are emergency-only engines under Reg 3.1, though an emergency engine pressed into normal-load service falls back in scope in full. Reg 3.2 leaves flag-state discretion for some applications on ships under 24 m. Pure gas engines came into scope under the 2014 amendments by Resolution MEPC.258(67) for ships under the IGF Code, with spark-ignition gas engines tested on cycle G1 or G2 from ISO 8178. A replacement engine on a pre-2000 ship takes the Tier in force at its fitting date and cannot inherit the pre-2000 exemption; a major-conversion engine is re-certified at the Tier in force at the conversion date and cannot retain its original Tier; and a Tier III dual-mode engine outside NECA is held only to Tier II, with its SCR or EGR allowed in bypass.

Four confusions recur often enough to flag. Rated speed, not operating speed, sets the limit in the curve. NOx Tier I, II and III under Annex VI Reg 13 are a marine-engine regime distinct from road-diesel Euro VI, with different numerical limits and units. The EIAPP Certificate has no five-year expiry; it is voided instead by major conversion, by replacement of NOx-relevant components outside the certified family, or by failure of the parameter or simplified measurement check. And the Reg 14 sulphur cap and the Reg 13 NOx tier are independent: a SECA-only ECA imposes no Tier III, a NECA-only ECA imposes no 0.10% sulphur limit, and only a joint SECA plus NECA imposes both.

The regulatory basis for everything above is the primary text and its resolutions: MARPOL Annex VI Regulation 13 for the scope, tier definitions and NECA framework; the NOx Technical Code 2008 for the certification methodology, test cycles, parameter check and simplified measurement; Resolution MEPC.176(58) for the 2008 amendments that established Tier II, Tier III and the NECA framework; Resolution MEPC.177(58) for the mandatory adoption of NTC 2008; Resolution MEPC.272(69) for the 2016 amendments to NTC 2008; and Resolutions MEPC.190(60), MEPC.202(62) and MEPC.286(71) for the North American, US Caribbean, and Baltic plus North Sea NECA designations respectively.

See also

• MARPOL Annex VI: parent annex
• MARPOL Convention: top-level treaty
• NOx Tier I/II/III overview: summary article with tier comparison table
• Baltic Sea SECA + NECA
• North Sea SECA + NECA
• North American ECA
• US Caribbean ECA
• Mediterranean SECA 2025
• IMO 2020 sulphur cap
• Per-fuel WTW: LNG Otto vs diesel
• Ammonia marine engines overview
• Methanol marine engines overview
• Paris MOU port state control
• Calculator catalogue

Related calculators

• MARPOL Annex VI - NOx Tier III Limit
• MARPOL Annex VI - NOx Tier II Limit
• NOx Tier Compliance Check
• NOx Tier I / II / III Limits
• MARPOL Annex VI/13 - NOx emissions
• MARPOL Annex VI/10 - Port state control NOx
• SCR urea consumption
• MARPOL Annex VI/18 - Bunker delivery note