MARPOL Annex VI — Emission Reduction, EEDI, EEXI & CII

EEDI, EEXI and CII — design efficiency versus annual operation

MARPOL Annex VI Chapter 4 uses three related measures that answer different questions. Keep their scope separate: EEDI and EEXI are technical/design efficiency indices; CII is an annual operational carbon-intensity rating.

Exam distinction: EEDI/EEXI compare technical efficiency with required values; CII reflects achieved annual operation. An EEXI-compliant ship may still receive a poor CII rating because its actual annual fuel use, distance travelled and transport work determine operational intensity.

Continue to the detailed EEDI, EEXI and power-limitation engineering and the separate CII rating bands and SEEMP process. Use the latest Administration guidance and ship-specific certification for attained/required calculations and reduction factors.

1. Primary NOx Reduction: Exhaust Gas Recirculation (EGR) & Slide Valves

While secondary post-treatment systems like Selective Catalytic Reduction (SCR) clean exhaust gases downstream, primary emission controls prevent thermal NOx from forming inside the combustion chamber. Thermal NOx generation spikes exponentially above 1100°C when nitrogen and excess oxygen are subjected to intense flame temperatures.

Marine engine designers employ two principal primary technologies: Exhaust Gas Recirculation (EGR) and slide-valve fuel injectors:

• Exhaust Gas Recirculation (EGR): Diverts up to 40% of exhaust gas back into the engine scavenge air manifold. Carbon dioxide and water vapor in the exhaust gas possess higher specific heat capacities than pure air, dampening peak combustion flame temperatures. Furthermore, lowering scavenge oxygen concentration directly deprives the thermal NOx reaction of reactants.
• Slide-Valve Fuel Injectors: Conventional hole-type injectors feature a small residual fuel cavity (sac volume, ~20–40 mm³) beneath the needle seat. When the needle snaps shut, trapped sac fuel boils and dribbles into the hot cylinder during expansion, generating intense local flame hot spots and soot. Slide-valve injectors feature a sliding cut-off spindle flush with the nozzle tip surface, achieving zero sac volume.

• NOx Cut via Marine EGR: 50–60%
• Slide-Valve Nozzle Volume: 0 mm³ Sac
• Thermal NOx Threshold: >1100°C
• EGR Scrubber Gas Outlet: ≤60°C

The mechanical P&ID of an advanced marine 2-stroke diesel engine Exhaust Gas Recirculation system demonstrates the cleaning, cooling, and blower circuits required:

Alongside EGR, modern electronically controlled engines feature Slide-Valve Injectors that eradicate fuel sac dribble:

2. EEDI, EEXI & Engine Power Limitation (EPL / ShaPoLi)

MARPOL Annex VI Chapter 4 establishes a mandatory technical energy efficiency framework aimed at reducing maritime greenhouse gas emissions:

EEDI (Energy Efficiency Design Index)

Mandatory for new ships constructed on or after 1 January 2013. Sets a regulatory ceiling on grams of CO2 emitted per cargo capacity deadweight-tonne per nautical mile (g CO₂ / (t · nm)). Phased reductions force naval architects to optimize hull lines, bulbous bows, and propulsion machinery.

EEXI (Energy Efficiency Existing Ship Index)

Entered into force on 1 January 2023, applying EEDI-equivalent design efficiency retroactively to all existing ships ≥400 GT. Existing ships that cannot meet their Required EEXI must undergo technical modifications, predominantly through engine power de-rating.

EPL (Engine Power Limitation) & ShaPoLi

The primary compliance mechanism for EEXI. Involves installing a physical wire-sealed mechanical stop on the fuel injection rack, or a software-locked governor limiter (Shaft Power Limitation - ShaPoLi) that legally restricts maximum continuous rating (MCR) to a compliant power level.

The Master EEDI / EEXI Mathematical Equation:

The index represents the fundamental ratio of environmental harm (total carbon mass produced) to commercial utility (transport work accomplished):

The mechanical architecture of an Engine Power Limitation (EPL) and Shaft Power Limitation (ShaPoLi) system demonstrates the tamper-evident locking mechanisms required by surveyors:

3. Carbon Intensity Indicator (CII) & SEEMP Part III Trajectory

While EEDI and EEXI certify the theoretical design efficiency of a vessel's hardware, the Operational Carbon Intensity Indicator (CII) measures actual real-world operational carbon emissions per transport work over an entire calendar year.

Mandatory for all ships ≥5,000 GT (the same threshold as the IMO Fuel Oil Data Collection System - DCS), the attained annual operational CII is calculated as:

Attained CII = Σ (Fuel Mass × C_F) / Σ (DWT × Distance Travelled)   [g CO_2 / (t · nm)]

The Five Operational Performance Rating Bands (A to E):

Every year, each ship's attained CII is compared against the regulatory required CII baseline for its ship type and size, assigning a statutory operational rating:

Warning: The SEEMP Part III Corrective Action Plan

If a ship receives an 'E' rating in a single calendar year or a 'D' rating for three consecutive years, its Ship Energy Efficiency Management Plan (SEEMP Part III) must be revised with an approved Plan of Corrective Actions (PoCA). The plan must detail exact technical steps (hull antifouling, micro-bubble air lubrication, propeller boss cap fins, voyage routing, or operational slow steaming) to regain a 'C' rating within 24 months. Failure to execute invalidates the Statement of Compliance.