How We Rank the Most Eco-Friendly Marine Engines — the Methodology Behind Our New Emission Ranking

Which main engine actually powers the greenest ships in service? With the Emission Friendly ranking in our engines directory, TrustedDocks answers that question with measured, independently verified data instead of brochure promises: every ranked engine design is scored by the real-world CO₂ performance of the vessels that carry it. Here is exactly how the ranking works — including a subtlety about who "makes" a marine engine that trips up even industry insiders.

Verified emissions, not brochure figures

The foundation is the EU Monitoring, Reporting and Verification (MRV) scheme. Since 2018, every ship above 5,000 GT calling at European ports must monitor its fuel consumption and CO₂ emissions for the full reporting year, have the figures checked by an accredited independent verifier, and publish them through EMSA's THETIS-MRV system. It is the largest audited emissions dataset in shipping.

TrustedDocks links this dataset to its equipment database: for the 2025 reporting year, roughly 14,000 MRV-reporting vessels are matched to the specific main engine installed on board. That connection — verified emissions on one side, the engine room on the other — is what makes an engine-level ranking possible at all.

The metric: CO₂ per capacity-mile (AER)

Each vessel is measured by its Annual Efficiency Ratio (AER): grams of CO₂ emitted per deadweight-tonne nautical mile. This is the same basis the IMO uses for its Carbon Intensity Indicator (CII) rating, and it is built exclusively from quantities that are actually well measured — total annual CO₂, distance sailed, and the ship's deadweight.

We deliberately do not use the "transport work" cargo figures that also appear in MRV reports. Cargo mass is self-reported and noisy enough to produce physically impossible intensities in the raw data, so any ranking built on it would reward reporting style rather than engineering.

Comparing engines fairly across ship types

Raw AER cannot compare a 24,000-TEU container ship with a coastal feeder: economy of scale means large ships are inherently more efficient per tonne-mile, whatever engine they carry. A naive average would simply crown whichever engine happens to sit in the biggest hulls.

So the ranking normalises first: every vessel is scored by its efficiency percentile within its own ship type — tankers against tankers, bulkers against bulkers. A vessel at the 5th percentile is greener than 95% of its direct peers. An engine design's fleet score is then the average percentile of all the vessels carrying it, where 0% is the greenest possible fleet. The familiar A–E grade is derived from that score, exactly as on our vessel-level emissions ranking.

Who "makes" an engine? Designer vs. builder

Here is the subtlety: most large two-stroke engines are not built by the company whose name is on the design. MAN Energy Solutions and WinGD design the engines and license the production; the units themselves are manufactured by licensees such as Hyundai's engine division, Mitsui E&S, Hanwha (Doosan), Dalian Marine Diesel, Hitachi Zosen or CSSC. Classification registers — our data source — record the builder, so the same MAN B&W design can appear once as "Hyundai", once as "Mitsui" and once as "MAN".

Left uncorrected, that fragments one engine design into several small fleets and mislabels the maker. The engine nomenclature, however, identifies the designer unambiguously — a 6G70ME-C10 is a MAN B&W design and a 12X92DF is a WinGD design, whoever cast the block. Our ranking therefore classifies each model by its design brand, merges licensee-built units of the same design into a single entry, and lists the actual builders alongside. You get the answer both ways: who designed the engine, and who built it.

Quality guards

• Operating ships only: vessels emitting less than 500 tonnes of CO₂ in the year (laid-up or barely active) are excluded.
• Main engines only: auxiliary engines and generator sets are not credited with propulsion efficiency.
• One design, one entry: licensee-built units are merged under their design brand, so no design competes against itself.
• Statistical floor: a design needs at least 5 MRV-reporting vessels to be ranked — no single-ship heroes.
• Medians, not means: the displayed AER per design is the fleet median, so individual reporting outliers cannot drag an entry up or down.

What the 2025 data shows

The result passes the engineering smell test. The top of the table is dominated by the latest generations: dual-fuel LNG designs (MAN B&W's ME-GI and WinGD's X-DF families), exhaust-gas-recirculation and high-pressure SCR variants, and modern ultra-long-stroke G-type engines running at low revolutions on large optimised hulls.

Fleet score = average efficiency percentile of the MRV-reporting vessels carrying the design within each vessel's ship type (lower is greener), 2025 reporting year.

Read the numbers with care

An engine does not sail alone. Hull form, vessel size within its segment, operating speed, trade pattern and weather all shape a ship's carbon intensity — a top fleet score means the vessels carrying a design consistently outperform their peers, not that the engine alone explains the gap. Modern engines are typically installed in modern hulls, and the ranking measures the combination honestly rather than pretending to isolate one component. MRV also covers voyages touching the European Economic Area, so fleets trading exclusively outside Europe are not represented.

With those caveats on the table, the pattern is unambiguous: newer, dual-fuel and emission-abated engine designs power measurably greener fleets — and now you can see exactly which ones, design by design, together with the licensees who built them.

Explore the data: the full Emission Friendly engine ranking (updated with each MRV reporting year) and the vessel-level CO₂ efficiency ranking by segment and size class.