WinGD X-DF Engine: Low-Pressure Dual-Fuel Architecture

Background and corporate lineage

WinGD (Winterthur Gas & Diesel) traces its heritage to Sulzer Brothers in Winterthur, Switzerland, where the first slow-speed marine two-stroke diesel entered service in 1905. The Sulzer RTA series, introduced in 1983, established the uniflow-scavenging crosshead configuration that the current X-series continues. Electronic common-rail injection arrived with the RT-flex in 2001, putting variable injection timing, variable injection pressure, and hydraulic exhaust-valve actuation under software control. The common-rail fuel injection article describes that architecture in detail.

In January 2015, Wartsila’s two-stroke division and CSSC (China State Shipbuilding Corporation) formed a joint venture, with CSSC taking a 70% stake. Wartsila divested its remaining 30% in June 2016. CSSC has held WinGD at 100% ever since, with headquarters remaining in Winterthur. The corporate lineage article at /wiki/wingd-corporate-history covers the full ownership timeline.

The X-DF product line was announced from this base in the early 2010s, when two competing slow-speed manufacturers (then MAN B&W and Wartsila two-stroke) were developing parallel but architecturally opposite responses to LNG as a marine fuel. MAN B&W chose high-pressure gas injection in the diesel cycle. WinGD chose low-pressure gas admission in the Otto cycle. Those two architectural choices are the axis around which every technical comparison in this article turns.

WinGD crossed the 1,000-engine order milestone for the X-DF family, confirming it as the most-ordered low-pressure dual-fuel slow-speed engine in the world. The current production range spans six bore sizes from 52 mm to 92 mm (the X52DF through X92DF series) and has spawned three fuel-variant branches: X-DF-A for ammonia, X-DF-M for methanol, and the HP (high-pressure) variant for operators who prefer a diesel-cycle gas mode. The X-DF-HP is described briefly in the comparison table at the end of this article; the main body addresses the canonical low-pressure Otto-cycle architecture.

The Otto-cycle vs diesel-cycle choice

The central architectural question for a dual-fuel slow-speed two-stroke engine is when and how to introduce the gas fuel into the cylinder.

The MAN ME-GI uses a high-pressure diesel cycle: natural gas is compressed to roughly 300 bar by a high-pressure compressor train on board, stored in a high-pressure buffer vessel, and injected directly into the cylinder near top dead centre (TDC) at the end of the compression stroke, alongside a small diesel pilot. The gas ignites by compression heat, just as conventional diesel fuel does. Combustion is diffusion-controlled, with the gas burning as it mixes with the hot compressed air at the injector tip. This is the same combustion mode as a diesel oil cycle. Peak thermal efficiency is high. Methane slip is very low, typically 0.15 to 0.35% of fuel mass at 75 to 100% load, because gas exposure to crevice volumes and cool wall layers is brief.

The WinGD X-DF takes the opposite path. Gas is supplied to the engine at approximately 16 bar from the ship’s LNG fuel system, far below the 300 bar of the ME-GI. The gas is not injected at TDC; instead, a gas admission valve (GAV) opens each cylinder’s gas feed during the scavenging phase, admitting gas into the cylinder while the intake air charge is still entering. Gas and air mix across the full cylinder volume. Compression then begins with a premixed lean charge. Near TDC, a small diesel pilot injection ignites the mixture. Combustion propagates as a premixed flame through the lean charge. This is the Otto cycle in the thermodynamic sense: the heat release occurs approximately at constant volume after premixed ignition, in contrast to the ME-GI’s constant-pressure diffusion combustion.

Each approach has direct engineering consequences.

Gas supply simplicity. The X-DF needs only the low-pressure gas handling system that any LNG-fuelled ship carries for boil-off management: a fuel-gas supply unit (FGSU), pressure regulators, and the double-walled gas piping mandated by the IGF Code (MSC.391(95)). The ME-GI adds a high-pressure compressor set, a buffer vessel at 300 bar, and the safety systems for high-pressure gas on board. The capital cost of the ME-GI gas compression train adds roughly USD 2 to 4 million to a newbuild, depending on engine size.

Tier III NOx without aftertreatment. Lean premixed Otto-cycle combustion runs at a global air excess ratio (lambda, λ) of roughly 1.8 to 2.2 at full load. This means the cylinder charge contains 80 to 120% more air than stoichiometrically required for complete combustion. The diluted, cool charge lowers peak flame temperatures and, by extension, the thermal NOx that scales with the Zeldovich mechanism. Engine-out NOx in gas mode on a slow-speed X-DF (rated below 130 rpm) runs approximately 2.0 to 3.0 g/kWh. The MARPOL Annex VI Regulation 13 Tier III limit for the same rated-speed band is 3.4 g/kWh. The X-DF in gas mode is inherently Tier III compliant without a selective catalytic reduction (SCR) catalyst or an EGR loop.

Methane slip. Lean premixed combustion distributes gas across the full cylinder volume during scavenging. Some of that gas enters the small crevices around piston rings and between the piston crown and cylinder liner wall. These volumes are cold enough that combustion doesn’t reach them. The trapped gas exits through the exhaust as unburned methane. In the ME-GI, gas is injected late and burns in a confined diffusion flame near the injector, greatly limiting crevice exposure. Measured bench-test slip for the standard low-pressure X-DF (LBSI architecture) runs 0.5 to 2.5% of fuel mass at 75 to 100% load, rising steeply below 50% load. The ME-GI bench-test slip is 0.15 to 0.35% at the same loads. Methane slip from LNG dual-fuel engines addresses the climate-impact arithmetic in detail.

This trade-off is the fundamental reason why ME-GI dominates LNG carrier and VLCC propulsion (where full-load methane slip is commercially and regulatorily critical) while X-DF dominates dual-fuel container ships and bulk carriers (where simpler gas supply infrastructure and guaranteed Tier III compliance without SCR outweigh the slip disadvantage).

System architecture: from LNG tank to combustion chamber

Fuel gas supply unit (FGSU) and gas valve unit (GVU)

Compressed and vaporized natural gas is supplied to the engine from the ship’s Type C LNG fuel tanks via a fuel gas supply unit, typically at 8 to 20 bar depending on tank pressure and engine demand. Immediately upstream of the engine the gas passes through a gas valve unit (GVU), which contains the master shutoff valves, pressure-reducing regulators, gas filters, leak-detection sensors, and vent valves routed to safe external locations. The GVU is a classification-society-approved safety barrier: any detected gas leak or pressure anomaly triggers automatic isolation and engine trip to liquid mode. IGF Code Section 7 (Part A) mandates the double-isolation principle, so the GVU contains two independent shutoff valves in series for each gas feed circuit.

Double-walled gas manifold

A double-walled pipe manifold runs along the engine length, supplying each cylinder unit. The annular space between the inner gas pipe and the outer casing is continuously monitored for gas concentration. A detection alarm at or above 20% of the lower flammable limit (LFL) triggers a warning; at 40% LFL the gas supply isolates automatically and the engine trips to liquid mode. This double-wall requirement under IGF Code Section 7.4 applies to all gas piping in machinery spaces.

Gas admission valve (GAV)

Each cylinder unit has one GAV mounted in the cylinder cover, directly analogous in location to the fuel injector on a diesel-only engine. The GAV is a hydraulically actuated poppet valve driven by the engine’s servo-oil system (the same common-rail hydraulic circuit that actuates the exhaust valve and, on X-DF engines, the fuel injectors). Opening duration is a few milliseconds per cycle, timed to start shortly after the exhaust valve closes during late scavenging, when the cylinder is already full of fresh charge air at moderate pressure.

Gas admission timing is a software parameter set by the engine management system. Advancing GAV opening toward the exhaust valve close event increases the fraction of gas admitted during scavenging overlap, raising the risk of gas short-circuiting directly into the exhaust (a primary source of slip). Retarding GAV opening too far reduces mixing time before TDC, increasing the risk of incomplete combustion. The control system optimises GAV timing against cylinder pressure feedback in real time.

Micro-pilot injection

Near TDC, a small diesel pilot injection provides the ignition energy to initiate combustion in the premixed lean charge. The pilot injector is physically co-located with the engine’s main fuel valve, sized for the small pilot quantity. In the original X-DF1.0, the pilot represented roughly 3 to 5% of total fuel energy at full load, falling at lower loads. In the X-DF2.0 generation, WinGD achieved micro-pilot operation, with the pilot fraction below approximately 1% of total fuel energy at full load. This reduction matters because the pilot injection itself burns in a stoichiometric or near-stoichiometric diffusion flame, which is the highest-temperature, highest-NOx combustion event in an otherwise lean Otto-cycle cylinder. Minimising pilot quantity reduces this local NOx source.

The pilot injection in dual-fuel engines article describes the ignition physics and the comparison with the ME-GI pilot in greater depth.

Cylinder pressure sensing and dynamic combustion control

Every cylinder of an X-DF engine carries a piezoelectric cylinder pressure transducer. The pressure traces feed the engine’s central combustion control software continuously. The control system uses these signals to:

• Verify that each cylinder ignited on time (presence of a pressure rise after pilot timing; absence means misfire)
• Detect knock: the characteristic high-frequency pressure oscillations caused by autoignition of the premixed charge ahead of the propagating flame front
• Balance cylinder-to-cylinder peak pressure and indicated mean effective pressure (IMEP) by adjusting individual GAV timing and pilot quantity per cylinder
• Modulate the gas fraction smoothly as load changes

This per-cylinder closed-loop feedback is what makes micro-pilot operation viable. Without real-time pressure feedback, the ignition margin at lean lambda is too narrow to operate reliably with a fraction of a percent pilot.

Combustion process: Otto cycle at lean lambda

Scavenging and gas admission sequence

The uniflow scavenging system on X-DF engines (as on all modern large slow-speed two-strokes) works in the following sequence per cycle:

1. The piston descends through the power stroke. Near bottom dead centre (BDC), the scavenge ports in the lower cylinder liner open, admitting turbocharger-pressurized fresh air.
2. The exhaust valve in the cylinder cover opens. The cylinder is scavenged: exhaust gases are expelled upward and out through the exhaust valve while fresh air enters through the scavenge ports.
3. As the piston rises again and begins to close the scavenge ports, the GAV opens. Natural gas at 16 bar enters the cylinder and begins mixing with the trapped air charge. The gas mass admitted is metered by controlling GAV open duration.
4. The exhaust valve closes. The GAV closes shortly after. The cylinder is sealed with a premixed gas-air charge.
5. Compression begins. The charge heats adiabatically. Near TDC, the micro-pilot is injected, self-ignites, and creates multiple ignition kernels in the lean premixed mixture.
6. The flame propagates through the lean charge, releasing heat approximately at constant volume. Peak cylinder pressure and temperature are lower than in a stoichiometric diesel cycle because the diluted charge absorbs more heat per unit of temperature rise (higher specific heat capacity of the diluted mixture).
7. Expansion, and the cycle repeats.

Lambda target and the knock-lean-limit window

The charge air-to-gas ratio, expressed as lambda (λ), is the operative control variable for combustion stability. At full load, X-DF engines run at approximately λ = 1.8 to 2.2. This represents 80 to 120% excess air over stoichiometry.

Running lean creates a working margin between two failure modes:

Lean limit (misfire): Below a critical lambda, the mixture is too lean to sustain stable flame propagation from the pilot kernels. Cylinder pressure fails to rise; the misfire is detected by the combustion control system. The engine management reduces the gas fraction on that cylinder and can trigger an automatic trip to liquid mode if multiple cylinders misfire simultaneously.

Knock limit (autoignition): Above a critical lambda (richer mixture) or at high inlet charge temperature, the end-gas ahead of the propagating flame autoignites before the flame reaches it. Knock produces high-frequency pressure oscillations that stress the cylinder cover, piston crown, and liner. The control system detects knock from the pressure sensor, immediately retards pilot timing or reduces the gas fraction to bring the cylinder back within the window.

The width of the knock-lean-limit window depends on the methane number (MN) of the gas. Methane number is an index of knock resistance, analogous to octane number for gasoline: pure methane is defined as MN = 100; gases with higher concentrations of heavier hydrocarbons (ethane, propane, butane) have lower methane numbers. A gas with MN = 70 has a significantly narrower knock margin than MN = 90. Most LNG cargoes from well-established export terminals fall in the MN 70 to 90 range. The engine control system monitors methane number indirectly through cylinder pressure analysis and adjusts pilot timing and gas fraction accordingly, but engines are typically rated for a minimum methane number (around MN = 70 for the standard X-DF range) and may apply a small power derating at lower MN gas.

Why lean lambda gives Tier III without aftertreatment

The thermal NOx formation rate follows the Zeldovich mechanism, which scales extremely steeply with peak local flame temperature. Stoichiometric combustion (λ = 1.0) reaches adiabatic flame temperatures around 2,200 to 2,500 K in a diesel or gas engine. At λ = 2.0, the same fuel-air mixture has a lower heating value per unit volume, and the adiabatic flame temperature drops to roughly 1,700 to 1,900 K. NOx formation rates fall by approximately two orders of magnitude for every 200 K drop in flame temperature in this range.

The result is that engine-out NOx in X-DF gas mode at full load is approximately 2.0 to 3.0 g/kWh for a slow-speed engine rated below 130 rpm. The MARPOL Annex VI Regulation 13 Tier III limit for that speed band is:

The engine-out value is within the Tier III limit without any additional aftertreatment. In the four designated Nitrogen Emission Control Areas (the North American NECA effective 1 January 2016 under MEPC.190(60), the US Caribbean NECA from 1 January 2016 under MEPC.202(62), and the Baltic and North Sea NECAs from 1 January 2021 under MEPC.286(71)), a Tier III-required X-DF vessel can comply in gas mode on the X-DF technology alone. No urea tank, no SCR catalyst, no wet scrubber loop. This is a direct competitive advantage over the ME-GI, which requires EGR engagement in NECA for Tier III compliance, and over diesel-only engines fitted with SCR systems.

In liquid diesel mode, the lean-burn advantage disappears: the engine operates as a conventional diesel and meets Tier II only. Tier III in liquid mode requires SCR or, on X-DF2.0 with iCER, the EGR system described in the next section.

X-DF2.0: iCER and the methane slip problem

The methane slip problem for low-pressure engines

When X-DF1.0 entered service in 2017, its methane slip in LBSI mode was the principal environmental liability of the low-pressure architecture. At full load, bench-test slip was approximately 3 to 5 g/kWh (expressed in mass of methane per kilowatt-hour of delivered work). At part load (50% MCR and below), slip rose steeply, driven by increased crevice-volume fractions and reduced combustion temperatures.

Methane has an IPCC AR5 100-year global warming potential (GWP-100) of 28 relative to CO2 on a mass basis. A 4 g/kWh slip rate on an X-DF engine burning LNG at roughly 150 g/kWh (SFOC equivalent) corresponds to approximately 2.7% of the fuel methane mass escaping unburned. At GWP-100 = 28, this adds 112 g-CO2eq/kWh to the climate footprint of the engine, partially or fully offsetting the 25% tank-to-wake CO2 benefit of LNG over HFO. The per-fuel well-to-wake LNG Otto vs diesel comparison quantifies this trade-off numerically.

The IMO MEPC methane-slip work stream (reflected in the LCA Guidelines for marine fuels adopted at MEPC 80 in 2023) and the EU FuelEU Maritime Regulation both use WtW intensity as the compliance metric and apply GWP-100 = 28 to methane. High slip rates on low-pressure engines produce worse WtW scores, which directly affect CII rating and EEXI compliance calculations under MEPC.336(76) and MEPC.328(76) respectively.

iCER: Intelligent Control by Exhaust Recycling

WinGD introduced iCER as the defining technology of the X-DF2.0 generation. The system is essentially a low-pressure exhaust gas recirculation (EGR) circuit designed specifically to address methane slip without requiring the high-pressure wet-scrubber EGR found on the ME-GI for Tier III.

In iCER operation:

1. A fraction of exhaust gas is extracted from the exhaust gas receiver, downstream of the turbocharger turbine, at relatively low pressure (approximately 0.5 to 1.5 bar above ambient).
2. The extracted gas passes through an EGR cooler, which reduces its temperature to approximately 60 to 80°C.
3. The cooled EGR passes through an EGR scrubber, which removes sulphur dioxide (from any residual liquid fuel combustion or sulphur traces in the gas) and particulate matter.
4. The scrubbed, cooled EGR is reintroduced into the scavenge air receiver, where it mixes with fresh charge air before the cylinder intake.

The recirculated exhaust gas has two effects. First, it displaces oxygen in the scavenge charge, which means the engine must admit proportionally more air to maintain the same lambda; the overall charge mass per cycle is higher, but the oxygen content per unit mass is lower. Second, the inert CO2 and H2O in the recirculated gas reduce the adiabatic flame temperature, further lowering NOx even below the already-low X-DF1.0 gas-mode levels.

The primary methane slip benefit comes from a different mechanism: the EGR effectively reduces the valve-overlap slip contribution. In a uniflow-scavenged two-stroke engine, there is a brief period when both the scavenge ports and the exhaust valve are open simultaneously. During this overlap, some freshly admitted gas-air mixture can escape directly into the exhaust before combustion. The higher charge density with EGR, combined with modified scavenging dynamics, reduces this short-circuit loss.

WinGD states that iCER reduces methane slip by approximately 50% compared to X-DF1.0 at equivalent loads. In bench-test conditions at full load, X-DF2.0 with iCER achieves slip of approximately 1.5 to 2.5 g/kWh (roughly 1.0 to 1.7% of fuel mass), bringing it significantly closer to the high-pressure ME-GI level and substantially improving the WtW CO2 balance.

The iCER system also enables Tier III compliance in liquid diesel mode without a separate SCR system. The EGR dilution reduces in-cylinder oxygen concentration and peak flame temperature in diesel mode, bringing NOx below the Tier III limit with EGR rates of roughly 25 to 35%. This is directly analogous to the MAN ME-GI EGR-for-Tier-III liquid-mode approach, but the X-DF2.0 uses the same iCER circuit for both the gas-mode slip reduction and the liquid-mode Tier III function, rather than running separate SCR and EGR systems.

The EGR retrofit on two-stroke engines article describes the MAN ME-GI’s EGR architecture and the retrofit option for existing Tier II engines.

Engine family: bore sizes and configurations

The current WinGD X-DF production range (as of 2024) spans six standard bore sizes:

The X82DF and X92DF are the most common choices for large container ships; the X52DF and X62DF appear on smaller dual-fuel vessels. The stroke-to-bore ratios in the X-series are in the range 3.4 to 3.7:1, consistent with the ultra-long-stroke trend of modern slow-speed engine design that improves thermal efficiency and specific fuel consumption.

All of these engines share the uniflow-scavenging, crosshead architecture described in two-stroke marine diesel engine fundamentals and crosshead diesel engine architecture overview. The X-DF gas-handling hardware (GAV, GVU, double-walled manifold, iCER on X-DF2.0) is layered onto this base architecture.

Engine ratings follow the standard slow-speed convention of maximum continuous rating (MCR) in kW and rated speed in rpm. The 920 mm bore X92DF at 10 cylinders produces roughly 84,000 kW at approximately 80 rpm; the 520 mm bore X52DF at 5 cylinders produces roughly 10,000 kW at approximately 100 rpm. Specific values vary with stroke selection and the contracted power reduction for EEXI compliance (MEPC.328(76)) via engine power limitation (EPL) or shaft power limitation (ShaPoLi).

Comparison: X-DF low-pressure Otto vs MAN ME-GI high-pressure diesel

The ME-GI’s methane-slip advantage is significant from a WtW climate perspective: its measured bench-test slip of 0.15 to 0.35% at 75 to 100% load compares to 0.5 to 2.5% for the standard X-DF (LBSI) and roughly 0.5 to 1.5% for X-DF2.0 with iCER. At GWP-100 = 28, the difference between 0.3% and 1.5% slip translates to approximately 17 g-CO2eq/kWh additional warming impact from the X-DF2.0, a difference large enough to affect CII rating calculations over a full voyage profile. For vessels spending significant time at low load (slow-steaming, port approach, anchorage), the X-DF slip penalty grows further because slip rates rise steeply below 50% MCR on all low-pressure designs.

The X-DF’s NOx advantage is equally concrete. An ME-GI vessel trading to the Baltic or North Sea NECAs must engage the EGR loop in NECA, incurring scrubber wash-water management and closed-loop EGR maintenance obligations. An X-DF vessel in the same trade runs in gas mode and is inherently compliant without any system engagement. For a vessel spending 40% of voyage time in European NECAs, this operational simplicity is commercially meaningful.

The high-pressure compressor requirement on ME-GI vessels adds roughly USD 2 to 4 million in capital cost and a continuous parasitic power draw of approximately 200 to 500 kW depending on engine size, which debits from the effective propulsive power and slightly worsens the SFOC. The X-DF avoids this but incurs the recurring capital and maintenance cost of GAV overhauls (see Maintenance section).

Multi-fuel variants: X-DF-A and X-DF-M

X-DF-A: ammonia

Ammonia (NH3) is a candidate zero-carbon marine fuel under the IMO Net-Zero Framework, carrying no carbon in its molecular structure. The X-DF-A extends the X-DF platform to ammonia as the primary fuel. As of May 2026, WinGD completed Factory Acceptance Testing (FAT) for the X-DF-A, confirming commercial readiness.

The X-DF-A architecture retains the low-pressure Otto-cycle premixed combustion concept but requires extensive adaptation:

Safety and toxicity. Ammonia is acutely toxic at concentrations above 25 ppm (NIOSH IDLH: 300 ppm) and an explosion risk at 15 to 28% by volume in air. The IGF Code ammonia annexe and the forthcoming IMO interim guidelines require tighter double-walled containment, stronger ventilation in machinery spaces, and ammonia-specific detection and ESD systems. Crew exposure limits are far more demanding than for LNG.

Combustion properties. Ammonia has a lower flammability range (15 to 28% vol, versus methane at 5 to 15%) and a higher autoignition temperature (651°C versus ~537°C for methane). It burns more slowly and requires a higher pilot fraction to achieve stable ignition. WinGD addresses this with an enhanced pilot system, using a larger diesel pilot (or a dedicated pilot fuel in some design variants) to provide sufficient ignition energy for the lean ammonia-air charge. The lean burn at high lambda still delivers Tier III NOx in gas mode, but combustion stability requires tighter control.

N2O emissions. Ammonia combustion produces nitrous oxide (N2O, GWP-100 = 273 per IPCC AR6) as a by-product of incomplete combustion at lean conditions. N2O regulation for marine engines is not yet finalized at IMO, but it’s already in scope of the IMO LCA Guidelines. Early engine tests suggest N2O slip can be a more significant climate liability than the methane slip on LNG engines if not carefully managed through combustion optimization.

Ammonia slip. Unburned ammonia exiting the exhaust is both a toxic air pollutant and a climate concern. Mitigation requires optimized combustion and potentially a downstream oxidation catalyst. The IGF Code and forthcoming IMO interim guidelines impose minimum engine-out ammonia limits.

The X-DF-A does not enter the market as a pure-ammonia engine. Ships will carry both ammonia fuel and a secondary liquid fuel (typically MGO) for pilot and backup, with a fuel-switching capability analogous to the X-DF gas-to-diesel switch.

X-DF-M: methanol

Methanol (CH3OH) is a low-carbon marine fuel with a well-established supply chain and an increasing newbuild orderbook. The X-DF-M adapts the X-DF platform to methanol. The delivery of the Tema Maersk in June 2026 on a WinGD X-DF-M engine marked a recent commercial milestone.

Methanol combustion in a slow-speed two-stroke engine is architecturally different from gas admission:

Liquid injection. Methanol is liquid at ambient conditions and is injected through the fuel injectors, not through a gas admission valve. The X-DF-M uses a Liquid Gas Injection Methanol (LGIM) style injection system, where methanol is injected at high pressure through modified fuel valves sized for methanol’s lower energy density (approximately 50% of HFO by volume). A diesel pilot ignites the methanol spray.

No methane slip. Because methanol contains no C-C bonds and is not a gaseous fuel, there is no methane slip. The primary combustion concerns are incomplete combustion of methanol itself (formaldehyde and methanol slip as unburned fuel, both regulated) and NOx.

Tier III NOx. Methanol’s oxygen content (32% by mass) partially supports lean combustion at lower equivalence ratios than HFO, giving lower flame temperatures and NOx. Depending on the injection system and engine load, methanol combustion can achieve Tier III in some operating modes without SCR or EGR. The exact Tier III compliance map depends on the engine sub-type and certification by the classification society.

The methanol marine engines overview and methanol as marine fuel articles cover the full comparison of methanol and LNG from a ship-design and WtW perspective.

Fuel mode switching and tripping

Commanded gas-to-liquid switch

A commanded switch from gas to liquid mode, performed under way at any load above roughly 20% MCR, proceeds as follows:

1. The engine management system receives the gas-to-liquid switch command.
2. The liquid fuel injection quantity ramps up on each cylinder, moving the setpoint progressively toward full diesel operation.
3. Simultaneously, the GAV open duration on each cylinder ramps down, reducing the gas fraction.
4. As the gas fraction approaches zero, the pilot injection quantity increases to full diesel injection.
5. The GVU master shutoff valves close, isolating the gas supply to the engine.
6. The engine is now in full liquid diesel mode.

The full switch takes approximately 3 to 5 minutes. Engine speed and propulsive power are maintained throughout.

Commanded liquid-to-gas switch

The reverse sequence begins with verifying gas supply quality and pressure. The gas valve train opens progressively. The GAV begins operating at a low gas fraction while liquid fuel remains the dominant combustion fuel. The gas fraction increases step by step, with the combustion control system monitoring every cylinder’s pressure trace for knock and misfire events. Full gas mode is reached when the pilot-only injection sustains stable combustion.

Liquid-to-gas switching is subject to checks not required for gas-to-liquid: gas quality verification (methane number adequacy), gas supply pressure confirmation, and confirmation that the GVU leak-detection system shows no alarms. These checks typically add 2 to 5 minutes to the switch time compared to the gas-to-liquid direction.

Automatic fault trip

If any of the following conditions are detected during gas operation, the engine management system executes an automatic trip to liquid mode in approximately 2 to 5 seconds:

• Knock exceeding the threshold on one or more cylinders for more than a set number of consecutive cycles
• Misfire detected on two or more cylinders simultaneously (a single cylinder misfire triggers an alarm but not necessarily a trip)
• Gas leak alarm at the GVU or in the double-walled manifold
• Loss of gas supply pressure below the minimum required for stable GAV operation
• Combustion control system fault that prevents per-cylinder monitoring

The automatic trip does not interrupt propulsive power: the liquid fuel injection system takes over within the trip sequence. The gas isolation valves close and the GVU vents to safe locations.

NOx certification and EIAPP under NTC 2008

Engine family and type approval

X-DF engines are certified under the NOx Technical Code 2008 (NTC 2008), made mandatory by IMO Resolution MEPC.177(58). Each bore size (X52DF, X62DF, X72DF, X82DF, X92DF) and each generation (X-DF1.0, X-DF2.0) constitutes a separate engine family with a parent engine (the worst-case cylinder count and rating for NOx) and member engines (all other configurations within the family). The parent undergoes the full E3 test cycle (four modes at 100%, 75%, 50%, and 25% load on the propeller curve) for both gas-mode and liquid-mode NOx characterization.

The EIAPP Certificate issued after type approval records the Tier II NOx in liquid mode and the Tier III NOx in gas mode, confirming the dual-mode compliance status. Ships trading to NECAs show the EIAPP Certificate to Port State Control inspectors as evidence that no SCR system is needed in gas mode.

Dual-mode Technical File entries

The NTC 2008 Technical File for an X-DF engine carries two certified operating modes:

• Liquid mode (Tier II): injection timing, fuel injector specifications, charge-air settings, turbocharger model.
• Gas mode (Tier III): GAV timing limits, gas supply pressure range, pilot quantity range, methane number operating range, iCER EGR rate (for X-DF2.0), cylinder pressure monitoring parameters.

The parameter check method at surveys verifies that the as-fitted engine matches both sets of Technical File parameters. A PSC inspector boarding a Tier III-required vessel in the Baltic NECA will verify that the engine is in gas mode and that the gas-mode parameters match the Technical File.

EEXI and CII: dual-fuel engine treatment

EEXI under MEPC.328(76)

The Energy Efficiency Existing Ship Index (EEXI), applicable to existing ships above 400 GT on international voyages with EEXI surveys mandatory from 1 November 2022, measures the specific CO2 emission rate of the ship at a reference speed. For a dual-fuel vessel, the EEXI calculation uses the fuel conversion factor (Cf) and specific fuel oil consumption (SFOC) applicable to the reference operating fuel.

X-DF vessels can claim EEXI credit for reduced SFOC in gas mode, because the Cf for LNG (2.750 g-CO2/g-fuel) is lower than for HFO (3.114) or VLSFO (3.151), and because the X-DF SFOC in gas mode is slightly better than diesel-only operation (the Otto cycle’s lower compression ratio is partially offset by lean-burn efficiency). The EEXI EPL and ShaPoLi article covers the calculation method and the power-limitation options in detail.

CII under MEPC.336(76)

The Carbon Intensity Indicator (CII), mandatory for ships above 5,000 GT from 1 January 2023, measures actual annual CO2 intensity (gCO2/capacity-nautical-mile) and rates vessels A through E annually. For X-DF vessels, CII is calculated from the actual fuel consumption logs covering both gas-mode and diesel-mode operation. Gas-mode operation credits the lower Cf of LNG. However, methane slip from gas-mode operation is not captured in the current CII calculation methodology (which counts only CO2-equivalent from combustion CO2, not from unburned methane emissions). This means an X-DF vessel with high methane slip can achieve a better CII rating than its true WtW climate impact would warrant.

The IMO is developing amendments to capture methane slip within the GHG intensity calculation framework under its 2023 IMO Strategy on Reduction of GHG Emissions from Ships. If and when methane slip is folded into CII-equivalent GHG intensity, X-DF vessels with high slip rates will see their ratings worsen, creating additional commercial pressure toward X-DF2.0 iCER or X-DF-HP variants.

IGF Code compliance architecture

Every X-DF-equipped ship must comply with the IGF Code (MSC.391(95)), which entered into force on 1 January 2017 and applies to all ships using gases or low-flashpoint fuels. The principal IGF requirements relevant to X-DF installation are:

Fuel containment. LNG fuel is stored in Type C IMO pressure vessels, typically cylindrical or bilobed tanks on deck or in a hold space. The IGF Code Section 6 specifies minimum distances from accommodation and lifesaving appliances, ventilation requirements for the tank hold space, and the pressure-relief system sizing.

Fuel preparation room. The gas conditioning equipment (vaporizers, low-duty compressors, pressure regulators) occupies a designated fuel preparation room with gas-tight boundaries, mechanical ventilation sufficient for at least 30 air changes per hour, and continuous gas detection. The fuel preparation room is treated as a hazardous area under the IGF Code and must meet ATEX/IECEx electrical equipment standards.

Double-walled fuel piping. All gas piping in machinery spaces is double-walled under IGF Code Section 7.4, with the annular space monitored for gas concentration. The GVU, double-walled manifold, and associated detection loops described earlier satisfy this requirement.

ESD architecture. The IGF Code mandates an emergency shutdown (ESD) system with two independent shutdown layers: ESD-1 (local GVU isolation) and ESD-2 (full gas system isolation from a remote location). Both are required to operate on loss of electrical power (fail-safe closure).

Bunkering. LNG bunkering procedures must comply with IGF Code Section 18, including simultaneous operations (SIMOPS) restrictions, bunkering-plan documentation, crew training under STCW Section V/3, and compatibility checks between the bunker vessel or terminal and the ship’s fuel containment system.

Classification societies certify IGF Code compliance through the notation system: DNV’s Gas Fuelled notation, LR’s LNG Fuelled notation, and equivalent notations at ABS, BV, NK, KR, and CCS. The notation confirms that the fuel system design, installation, and commissioning testing meet the IGF Code and the society’s own rules for gas-fuelled ships.

Maintenance: X-DF-specific items

X-DF maintenance shares the majority of its scope with any electronically controlled common-rail two-stroke engine. The alpha-lubricator system, exhaust valve overhauls, piston inspections, turbocharger cleaning, and common-rail hydraulic servicing apply to all RT-flex/X-series engines. The X-DF adds three maintenance categories specific to the gas-handling hardware:

Gas admission valve (GAV) overhauls

The GAV is exposed to repeated thermal cycling (the valve face transitions between cylinder combustion temperatures and cooler gas supply conditions each cycle), high-velocity gas flow, and the possibility of liquid carryover from the gas supply. WinGD specifies GAV inspection intervals comparable to fuel injector overhaul intervals, typically 6,000 to 12,000 hours in service, depending on gas quality and operating profile. GAV overhaul involves valve-face inspection and re-lapping, seat inspection and replacement if worn, hydraulic actuator seal check, and performance verification after reassembly. A full set of GAVs on a 10-cylinder engine represents a maintenance outlay comparable to one complete set of fuel injector overhauls.

iCER system maintenance (X-DF2.0)

The iCER EGR circuit introduces three new maintenance items:

• EGR cooler: deposits from exhaust gas accumulate on the cooler tubes, reducing heat transfer and increasing pressure drop. Cleaning intervals depend on fuel sulphur content and exhaust gas temperature; WinGD recommends inspection every 2,000 to 3,000 hours and cleaning when deposit thickness degrades cooler performance.
• EGR scrubber: the wet scrubber removes sulphur oxides and particulate matter from the recirculated exhaust. The wash water circuit requires periodic chemistry checks, sludge removal, and nozzle inspection. The closed-loop wash water from the EGR scrubber is regulated under MEPC.184(59) and must be retained on board for shore disposal.
• EGR blower: the low-pressure blower that drives the recirculated gas into the scavenge receiver requires bearing and impeller inspection at standard centrifugal blower intervals (typically 8,000 to 12,000 hours).

Double-walled gas piping

The double-walled manifold and supply piping require visual inspection for outer-pipe integrity at each dry-docking (5-year intervals), pressure testing of the inner pipe, and functional testing of all gas-detection sensors in the annular space. Any inner-pipe repair requires full gas-freeing of the supply system and class society oversight under IGF Code Section 7.4.

Cylinder pressure sensor replacement

The combustion control system depends on accurate cylinder pressure signals. Pressure transducers are consumable items, subject to thermal fatigue and signal drift. Replacement intervals vary with engine type but are typically 12,000 to 24,000 hours in full gas-mode operation. Degraded sensors produce false knock or misfire alarms and, if left uncorrected, can cause the control system to restrict gas fraction unnecessarily, reducing gas substitution and worsening SFOC in gas mode.

Limitations

Methane slip on low-pressure engines

The low-pressure Otto-cycle architecture has a structural methane slip disadvantage relative to high-pressure diesel-cycle engines. Even with iCER on X-DF2.0, bench-test slip of 1.0 to 1.7% at full load compares unfavorably to 0.15 to 0.35% for the ME-GI. In-service measurements consistently show higher slip than bench-test values, particularly at part load. For vessels that slow-steam at 50% MCR or below for significant fractions of a voyage, the actual WtW climate benefit of X-DF over HFO can be small or negative depending on the upstream LNG methane leakage rate. Ship operators taking on LNG as a transition fuel on X-DF engines need to verify that their operating profile keeps average load high enough to stay within the WtW break-even boundary.

Tier III available only in gas mode

X-DF Tier III compliance applies only in gas mode. In liquid diesel mode, the engine operates at Tier II. X-DF vessels must burn gas continuously in NECAs for Tier III compliance. If gas supply is exhausted, contaminated, or the gas system is shut down for maintenance, the vessel is non-compliant with Tier III in NECA until gas mode is restored. X-DF2.0 with iCER mitigates this by enabling Tier III in liquid mode via EGR, but iCER must be operational and engaged. Vessels without iCER have no liquid-mode Tier III pathway other than an additional SCR system.

Methane number dependency

X-DF performance is sensitive to the methane number (MN) of the bunkered LNG. Gas with MN below about 70 (high ethane, propane content from some US or North African terminals) narrows the knock margin, requiring engine derating or conservative pilot timing. This creates uncertainty for vessels bunkering from diverse global terminals. The ME-GI, operating on the diesel cycle, is indifferent to methane number because it has no premixed charge subject to knock.

Part-load slip spike

Methane slip on all low-pressure engines rises steeply at loads below 50% MCR. Vessels spending extended time at low load (port approach, slow-steaming, dynamic-positioning operations) face significantly degraded WtW performance compared to full-load design-point figures. This characteristic is well-documented in the methane slip deep dive article and is a major driver of the 2020s trend toward X-DF-HP variants in applications where load flexibility is required.

Ammonia and N2O regulation still developing

The X-DF-A (ammonia) variant is entering commercial service against a regulatory background that is still being drafted. IMO interim guidelines for ammonia as fuel were not finalized as of mid-2026. N2O emissions from ammonia combustion are not yet captured in CII or EEXI calculations, meaning a ship’s official carbon-intensity scores could look better than actual WtW impact. Classification societies are certifying early X-DF-A vessels under flag-state equivalency approvals rather than a completed IGF Code ammonia annexe. Operators ordering X-DF-A tonnage face regulatory uncertainty around N2O emission limits, ammonia slip limits, and the bunkering safety regime.

See also

• Pilot Injection in Dual-Fuel Marine Engines
• Common Rail Fuel Injection: Two-Stroke Engines
• MAN B&W ME-C Electronic Control Overview
• Methane Slip from LNG Dual-Fuel Engines
• LNG as Marine Fuel
• Methanol Marine Engines Overview
• Ammonia Marine Engines Overview
• IGF Code: Low-Flashpoint Fuel Ships
• MARPOL Annex VI Reg 13: NOx Tier Limits
• WinGD Corporate History
• EGR Retrofit on Two-Stroke Engines
• Per-Fuel Well-to-Wake: LNG Otto vs Diesel
• What is EEXI?
• What is CII?
• Tier III Compliant Two-Stroke Engines
• Emission Control Areas

Related calculators:

• CH4 Methane Slip Calculator: LNG engines
• LNG Well-to-Wake Calculator: Otto / Diesel modes
• EEXI Attained Calculator
• NOx Tier II / Tier III Compliance Check
• SFOC to CII Conversion