Pilot Injection in Dual-Fuel Marine Engines

A compression-ignition engine has no spark plug. Every ignition event depends on hot compressed air raising injected fuel past its autoignition temperature. Diesel does that easily at typical marine compression ratios of 14:1 to 18:1. Natural gas mixed into the scavenge air does not: the lean premixed charge sits far below its autoignition temperature throughout the compression stroke. Without an ignition source, the gas-air mixture passes through the cylinder unburned.

The solution is the pilot injection: a small quantity of diesel fuel injected near top dead centre that autoignites within milliseconds and provides the ignition kernels the gas charge needs. The same fundamental requirement applies to methanol, dimethyl ether, and ammonia dual-fuel variants, though the pilot quantities and timing differ. This article examines how pilot injection works across the two dominant large slow-speed dual-fuel architectures, the WinGD X-DF low-pressure Otto cycle and the MAN Energy Solutions ME-GI high-pressure diesel cycle, and traces the engineering trade-offs in pilot quantity, timing, injector design, NOx, and methane slip that have driven the progressive reduction from 5 percent pilot fractions toward sub-1-percent micro-pilot operation.

Why gas charges don’t autoignite in CI engines

Diesel autoignition relies on a combination of high compression temperature and the physical properties of diesel fuel: its cetane number (typically 40 to 55 for MGO) quantifies how readily it ignites under compression. Natural gas has an effective methane number (a scale analogous to octane number for knock resistance) that sits at 70 to 100 depending on composition, which means it resists autoignition rather than encouraging it.

At a compression ratio of 16:1, the end-of-compression temperature in a WinGD X-DF cylinder in gas mode is roughly 500 to 550 degrees Celsius at full load, elevated somewhat by turbocharger preheating. Diesel fuel autoignites at around 250 degrees Celsius under those pressure conditions. A lean natural-gas-air mixture at lambda 2.0 (twice the stoichiometric air quantity, consistent with Tier III NOx control on the X-DF) has an autoignition temperature that exceeds 600 degrees Celsius under typical marine compression conditions. The compressed gas charge does not ignite. It needs the pilot.

This isn’t a marginal shortfall. It’s a fundamental thermodynamic boundary: lean premixed gas combustion and diesel-cycle self-ignition operate in different corners of the temperature-equivalence-ratio map. The pilot bridges them.

The two combustion architectures: Otto cycle vs diesel cycle

Low-pressure premixed Otto cycle (WinGD X-DF, Wartsila DF)

In the WinGD X-DF and Wartsila DF families, gas enters the cylinder during the scavenging phase. Gas admission valves (GAVs) mounted in the cylinder liner or ports inject low-pressure gas (typically 0.6 to 1.0 bar gauge above scavenge pressure, or roughly 4 to 6 bar absolute for large bore engines) into the incoming scavenge air. By the time the scavenge ports close and compression begins, the cylinder contains a uniform lean mixture of gas and air at a global lambda of approximately 1.9 to 2.1 at full load.

This is Otto-cycle combustion in a two-stroke framework. The pilot injection then fires at roughly 5 to 15 degrees before top dead centre, autoigniting in the hot compressed atmosphere and initiating multiple flame kernels that propagate through the premixed lean charge. Combustion is complete within 20 to 40 degrees of crank angle after TDC, depending on engine bore, charge temperature, and methane number.

The advantage of this approach is lean-burn NOx control. At lambda 2.0, peak combustion temperatures stay well below the thermal NOx formation threshold of approximately 1,800 Kelvin. WinGD’s X-DF design achieves NOx levels in gas mode that comply with IMO MARPOL Annex VI Regulation 13 Tier III without selective catalytic reduction (SCR) on many vessel types, a significant practical advantage in areas where urea supply is constrained.

The disadvantage is methane slip. Unburned methane from the premixed charge escapes through scavenge-exhaust overlap, crevice volumes near the piston crown, and incomplete combustion at the cylinder walls. At partial loads below approximately 25 percent MCR, methane slip rises sharply because mixture preparation deteriorates and the pilot has less thermal energy per unit of gas charge to guarantee complete flame propagation. Methane slip carries a 20-year global warming potential roughly 84 times that of CO2 per unit mass (IPCC AR5 values), so managing it matters for the overall climate case for LNG propulsion.

High-pressure diesel-cycle gas injection (MAN ME-GI, ME-GA)

The MAN Energy Solutions ME-GI injects natural gas at high pressure, around 300 bar, directly into the cylinder near top dead centre after the scavenge ports have closed and compression is nearly complete. There is no premixed gas-air charge. Instead, the high-pressure gas jet enters hot compressed air and combustion proceeds as a diffusion flame: gas mixes with air at the jet boundary and burns progressively, much as diesel combustion proceeds.

The pilot in an ME-GI context is smaller in its functional role. It fires slightly before the gas injection, creating an ignition zone of burning diesel that the high-pressure gas jet then enters. Combustion of the gas proceeds in diesel-cycle fashion, with the flame front moving outward from the injection jet rather than propagating inward from multiple premixed kernels.

This is fundamentally a diesel-cycle combustion event with gas as the main fuel, not an Otto-cycle event with a diesel ignition aid. The ME-GI thus avoids the scavenge-overlap methane slip that afflicts low-pressure Otto engines. MAN cites in-cylinder methane slip below 0.1 percent for ME-GI in gas mode under normal load conditions, compared with figures ranging from 1 to 3.5 percent for low-pressure premixed engines across their load range according to IMO MEPC discussion documents.

The trade-off is NOx. Diffusion combustion at the stoichiometric jet boundary produces high local temperatures, pushing NOx well above Tier III limits. ME-GI vessels operating in Tier III Emission Control Areas (ECAs) under MARPOL Annex VI Regulation 13 require SCR systems fitted and operational in those zones. For routes that avoid ECAs, the lack of methane slip and the high engine efficiency across the load range make ME-GI a strong choice.

The ME-GA (gas/ammonia) is MAN’s ammonia-capable variant, following the same high-pressure injection architecture with modifications to handle ammonia’s corrosivity and lower flame speed.

Pilot injector architecture

Dedicated pilot injectors vs shared injectors

The two combustion architectures produce different injector arrangements. On WinGD X-DF and Wartsila DF low-pressure engines, the cylinder head carries dedicated pilot injectors separate from the main fuel injectors used in liquid diesel mode. A typical large bore X-DF cylinder has one or two pilot injectors and two to three main injectors arranged around the head perimeter. In gas mode, only the pilot injectors fire. In liquid mode, the pilot injectors may be deactivated or remain active in a small contribution role while the main injectors carry full load.

The separation of pilot and main injectors is driven by the extreme range of fuel quantities involved. In gas mode, the pilot must deliver perhaps 0.5 to 1 percent of full-load fuel energy per cycle: for a large bore X-DF at 70 percent MCR, that may be 0.1 to 0.3 kilograms of MGO per cylinder per hour, or fractions of a milligram per injection event. Designing a single injector to deliver accurately across a 100:1 ratio from micro-pilot to full diesel load is not practical with conventional needle-and-spring injector technology. Hence the separation.

On MAN ME-GI engines, the gas injection is handled by dedicated gas valves with separate liquid backup injectors, again keeping the two fuel paths distinct. The pilot for ME-GI is a small diesel admission that fires ahead of the gas injection.

Micro-pilot injector design

A micro-pilot injector is a high-pressure fuel injector with very small orifice diameters, typically 0.10 to 0.20 millimetres per orifice, compared with 0.30 to 0.50 millimetres for a main fuel injector on the same engine. The small orifice produces fine atomization at the very low fuel quantities involved, with Sauter Mean Diameter (SMD, the volume-to-surface-area representative droplet size) in the range of 10 to 25 micrometres.

Fine droplets are required because the pilot must vaporize and ignite within a narrow crank-angle window. At rated speed on a large slow-speed two-stroke (roughly 80 to 120 RPM), each degree of crank rotation takes 70 to 100 microseconds. The pilot must ignite within 3 to 8 degrees of injection timing, leaving less than 1 millisecond for vaporization and ignition delay. Larger droplets with their lower surface-area-to-volume ratio cannot achieve this on the compressed gas charge alone.

Common rail fuel injection on two-stroke engines is standard on modern dual-fuel designs. The pilot fuel rail operates at pressures between 800 and 1,500 bar, depending on manufacturer, with electronically controlled pilot injectors receiving injection timing and quantity signals from the engine management system on each cycle. This electronic control is the mechanism that allows pilot quantity and timing to be varied independently across the load range, an essential capability for NOx and methane slip management.

Fuel valve and injector design for two-stroke marine engines covers the needle geometry, seat materials, spring loads, and differential pressure calculations that determine injector opening and closing behaviour. Many of those principles apply directly to pilot injectors, scaled to smaller orifice diameters and lower per-cycle fuel masses.

Pilot fuel supply system

The pilot fuel supply is kept separate from the main fuel system on most dual-fuel installations, with a dedicated pilot service tank, pilot fuel pump, pilot rail, and filter train. MGO is the standard pilot fuel because its low sulfur content (0.1 percent mass by mass for ECA operation, or 0.5 percent globally under MARPOL Annex VI Regulation 14) matches the emission profile of the gas mode operation. Using high-viscosity HFO as a pilot would require heating and would introduce a higher sulfur contribution inconsistent with the low-emission character of the gas mode cycle.

The pilot fuel filter is typically rated at 5 micrometres absolute filtration, finer than the 10 to 25 micrometre filters used on main fuel systems. The reason is the small orifice diameter: a 0.15-millimetre orifice can be blocked by particles that pass freely through a main injector with a 0.4-millimetre orifice. Filter maintenance frequency on pilot circuits is therefore higher than on main fuel systems, typically checked and replaced at the manufacturer’s stated interval rather than only on differential-pressure indication.

Pilot quantity: energy fraction and physical quantities

Full-load micro-pilot on WinGD X-DF2.0

WinGD’s second-generation X-DF platform, introduced commercially in the early 2020s, targets pilot fuel fractions below 1 percent of total fuel energy in gas mode at full load. For a large bore X-DF engine such as the 12X92DF at its maximum continuous rating of approximately 65,880 kW (quoted from WinGD’s engine programme data), operating at a specific fuel oil consumption (SFOC) for pilot fuel of around 1 to 2 grams per kWh in gas mode, the total pilot fuel consumption at full load is in the range of 66 to 132 kilograms per hour for the 12-cylinder variant.

Expressed differently: if the total fuel energy is represented by a gas SFOC of approximately 165 grams of LNG equivalent per kWh (a representative X-DF2.0 gas mode figure at ISO reference conditions), and the pilot contributes 1 to 2 grams per kWh in diesel equivalent, the pilot energy fraction sits at approximately 0.7 to 1.3 percent. WinGD’s own product descriptions have consistently stated sub-1-percent micro-pilot as the target for X-DF2.0 in gas mode at full load.

Pilot quantity at partial load

The pilot does not scale linearly with engine load. At 25 percent MCR, the gas charge per cycle is reduced to roughly one-quarter of full-load charge, but the minimum pilot quantity required for reliable ignition of that charge is bounded by injector minimum delivery and ignition physics. The pilot fraction at 25 percent MCR is typically 3 to 8 percent, and at very low loads (10 to 15 percent MCR) can exceed 10 percent. This partial-load pilot enrichment is one of the reasons methane slip rises at low loads: the additional pilot combustion raises in-cylinder temperature and pressure slightly, but the much-reduced gas charge still leaves more unburned methane per unit of useful work.

Below approximately 15 to 20 percent MCR (the exact threshold varies by engine type and installation), X-DF engines commonly run in pilot-only mode: essentially diesel mode with the gas admission valves closed. This threshold is the “gas mode lower load limit” and it is set by the manufacturer based on combustion stability testing, not regulatory requirement.

ME-GI pilot quantities

MAN ME-GI pilot fractions are lower in absolute terms because the gas injection itself creates favourable combustion conditions through the diffusion-flame mechanism. Typical ME-GI pilot fractions at full load are cited at 1 to 5 percent of total fuel energy, varying by engine generation and operating conditions. The pilot in ME-GI does not need to propagate a flame through a large premixed volume; it only needs to create a stable ignition zone for the incoming high-pressure gas jet. This distinction allows ME-GI to function reliably at low pilot fractions without the same lean-burn flame propagation constraint that sets the X-DF micro-pilot’s lower limit.

Wartsila DF medium-speed pilot fractions

Wartsila’s four-stroke DF engines (including the Wartsila 50DF covered at Wartsila 50DF dual-fuel engine) use a similar lean-premixed Otto-cycle approach to WinGD X-DF but in a trunk-piston medium-speed configuration. Pilot fractions on Wartsila DF four-stroke engines are in the range of 1 to 5 percent, with modern variants targeting the lower end of that range. Four-stroke engines differ from two-stroke in scavenging: the lack of scavenge port overlap (compared with uniflow two-stroke designs) reduces methane slip during the gas exchange process, though in-cylinder slip remains.

Comparison table: low-pressure Otto vs high-pressure diesel cycle

Ignition physics: from pilot spray to full-charge combustion

Autoignition delay and the crank-angle budget

The pilot fuel, injected at high pressure as fine droplets into compressed gas-air mixture, must vaporize, mix with surrounding gas at a locally near-stoichiometric equivalence ratio, and reach autoignition temperature before the piston moves significantly past TDC. This sequence takes a physical minimum time, the ignition delay, which at typical marine large-bore operating conditions (compression temperature around 500 to 550 degrees Celsius, pressure 80 to 100 bar at injection timing) is approximately 0.3 to 1.0 milliseconds.

At 90 RPM, each degree of crank rotation takes 0.185 milliseconds. A 1.0-millisecond ignition delay consumes 5.4 degrees of crank angle. If the pilot is injected at 10 degrees BTDC, autoignition occurs near TDC to slightly after TDC, which is near-ideal for pressure rise. If ignition delay is extended by lower compression temperature (partial load, low intake temperature) or a larger pilot droplet SMD, the effective injection timing must be advanced to keep the heat release centered near TDC.

Ignition kernel distribution

Each orifice of the pilot injector produces a separate fuel spray jet that forms an ignition kernel when it autoignites. A pilot injector with 4 to 6 orifices arranged symmetrically around the injector axis produces 4 to 6 ignition kernels distributed around the cylinder volume. From each kernel, a turbulent premixed flame propagates radially outward through the lean gas-air mixture at a turbulent flame speed of typically 0.5 to 2.5 m/s (laminar flame speed of methane-air at lambda 2.0 is roughly 0.15 m/s; turbulence in the compressed cylinder charge amplifies this by a factor of 5 to 15).

With multiple kernels distributed across the cylinder cross-section, the flame fronts meet somewhere near the midplane of the charge within approximately 15 to 25 degrees of crank rotation after ignition. Total combustion duration (10 to 90 percent heat release) is typically 20 to 40 degrees on a large bore X-DF engine at full load. The number of ignition kernels, their spatial distribution, and the local equivalence ratio at each kernel location determine whether all kernels successfully propagate flames or whether some kernels fail.

Knock: the upper combustion stability boundary

Knock in a dual-fuel Otto-cycle engine occurs when the unburned gas-air mixture ahead of the propagating flame fronts reaches autoignition conditions from the combined pressure and temperature rise of the early combustion event. Unlike gasoline engine knock, which can produce sharp metallic noise and occurs rapidly, large-bore marine engine knock is often detected by cylinder pressure sensors as an irregular pressure signature rather than audible detonation. WinGD X-DF engines and Wartsila DF engines use cylinder pressure sensors as the primary knock detection mechanism.

Knock risk increases when:

• Gas equivalence ratio is higher (richer charge, closer to stoichiometric)
• Pilot timing is advanced relative to TDC (early heat release raises end-gas temperature and pressure before the flame consumes it)
• Methane number of the gas supply is low (heavier hydrocarbon fractions in LNG reduce the methane number, lowering the knock resistance)
• Inlet charge temperature is high

When the engine management system detects knock, it retards pilot timing to delay the start of combustion, giving the early heat release less time to raise end-gas pressure before the flame front arrives. It may also reduce gas admission quantity to lean the mixture further. Persistent knock in multiple cylinders is a trip condition: the engine switches to liquid diesel mode until the cause is identified.

The methane number (MN) of LNG cargo varies with field origin. North American LNG (Sabine Pass, Freeport, Cameron) tends toward MN 90 to 98 because it is mostly pure methane. Australian LNG (Ichthys, Gorgon, Prelude) and Qatari LNG contain higher ethane and propane fractions, bringing MN down to 70 to 85. The engine control system on X-DF and Wartsila DF engines accepts methane number as an input and adjusts knock margins accordingly. MARPOL Annex VI Regulation 13 and the IGF Code both require that the engine management system can respond to varying gas composition within the design envelope.

Misfire: the lower combustion stability boundary

Misfire is the failure of the pilot to initiate and propagate combustion through the gas-air charge. It occurs when:

• Pilot quantity is below the minimum reliable ignition dose for the given charge conditions
• Pilot injection pressure is insufficient for adequate atomization (injector fault, low rail pressure)
• Gas-air mixture is too lean to support flame propagation from the available ignition kernels
• Compression temperature is too low (cold start, abnormal charge temperature)

Misfire produces a cycle with no combustion, releasing unburned methane directly to exhaust. The cylinder pressure trace shows no pressure rise after TDC. Modern dual-fuel engine control systems monitor individual cylinder pressure on every cycle and can detect single-cycle misfires.

At partial load, the combination of reduced charge density and the physical lower bound on pilot quantity produces the most challenging ignition conditions. The lean limit for premixed methane-air combustion is approximately lambda 2.3 to 2.5 at the temperature and pressure conditions relevant to early combustion (before the flame has raised pressure further). Engine designers set the minimum gas load limit (typically 20 to 25 percent MCR for X-DF) partly by where the ignition margin becomes unacceptably small given the pilot quantity floor set by injector capability.

Pilot timing: setting the crank-angle window

The injection-timing sweep and its effects

Pilot injection timing, expressed in degrees before top dead centre (BTDC), governs where the heat release event falls in the crank-angle cycle. Earlier timing means the pilot is injected into a less fully compressed charge (lower temperature and pressure), extending ignition delay. The subsequent heat release occurs earlier in the expansion stroke. Later timing means injection into a fully compressed charge with shorter ignition delay, but also less time for the flame to propagate before the piston descends.

At full load on a WinGD X-DF large-bore engine, typical pilot timing is in the range of 4 to 12 degrees BTDC. At partial load, timing may advance somewhat to compensate for the lower charge temperature, which would otherwise extend ignition delay and push the heat release too late in the expansion stroke.

Specifically, advancing pilot timing at partial load:

• Compensates for longer ignition delay at lower charge temperatures
• Maintains combustion efficiency despite the smaller, less reactive gas charge
• Risks advancing the heat release too early (toward more negative work on the piston during compression) if over-corrected

The engine management system interpolates from calibrated maps of optimal timing across the load-speed-methane-number space, with closed-loop correction from cylinder pressure feedback.

Pilot timing and methane slip

The relationship between pilot timing and methane slip is not simple. Later pilot timing (closer to TDC or slightly after) tends to reduce scavenge-related methane losses because the combustion pressure is higher during the early expansion stroke, leaving less opportunity for unburned methane to escape. But timing that is too late produces incomplete combustion, increasing slip from within-cylinder incomplete burn.

IMO MEPC discussions on methane slip (including work under MEPC 80 toward the 2023 IMO GHG Strategy) have highlighted that low-load operation is the dominant source of methane slip from the X-DF engine family. At loads below 25 percent MCR, methane slip can rise to 4 to 8 grams per kWh on early X-DF installations, according to in-service measurement programs cited in IMO MEPC 80 submissions. At full load, slip figures of 1 to 2 grams per kWh are more typical for X-DF in gas mode, with X-DF2.0 and later variants with iCER (intelligent Control by Exhaust Recycling) achieving lower figures through EGR-enabled charge temperature management.

MAN ME-GI in-cylinder methane slip at full load is below 0.1 grams per kWh in normal gas mode operation, a figure that has been confirmed in MEPC working documents. This is the primary methane-slip advantage of the high-pressure diesel cycle over the low-pressure Otto cycle.

Timing and NOx

Earlier pilot timing in gas mode raises peak combustion temperatures by shifting the heat release toward TDC, which raises NOx. This creates a knock-NOx trade-off: the timing window that minimizes knock risk overlaps with the timing window that minimizes NOx. Engine developers calibrate the timing maps to stay within IMO MARPOL Annex VI Regulation 13 Tier III NOx limits (3.4 g/kWh at 130 RPM and below for Tier III, according to the NOx Technical Code 2008 correction formula) while maintaining acceptable combustion stability margins.

On X-DF engines operating under Tier III, the lean-burn approach gives considerable headroom: typical gas-mode NOx at lambda 2.0 is around 1 to 2 g/kWh, well below the 3.4 g/kWh Tier III limit, without requiring SCR. See NOx Tier I, II, and III requirements and MARPOL Annex VI Regulation 13 for the full regulatory framework.

Backup diesel mode and pilot-only operation

Mode switching mechanics

All dual-fuel marine engines can operate in pure diesel mode, with gas admission fully closed and all fuel coming from the main diesel injectors. Mode switching from gas to diesel is automatic and takes a few seconds. The engine management system closes the gas admission valves (or the gas control valve upstream), confirms gas pressure has been relieved from the gas admission system, and ramps up the main diesel injection to take over fuel delivery.

During the transition, the pilot injector continues operating and the main injectors open progressively. The crossover takes 3 to 10 seconds in normal automated switching, depending on engine type and load level. In an emergency (gas supply trip, gas leak detection, crew-initiated emergency stop on gas fuel), the crossover is immediate: gas closes and diesel opens on the same cycle, with a brief period of reduced power that the propulsion control system accommodates via pitch or speed correction.

The IGF Code (MSC.391(95), Part A, section 5 and Part G for gas fuel systems) requires that dual-fuel installations maintain the capability to switch to a non-gas fuel at any time and under any operating condition without loss of propulsion or power below the minimum required level. This shapes the pilot system design: the pilot injectors and their fuel supply must be available at all times during gas-mode operation, and the switch to diesel must be possible even if the pilot system has a fault.

Pilot-only mode (partial gas-off range)

At very low loads, some X-DF installations operate in what is effectively a pilot-dominated mode: gas admission is reduced to very low flow rates, and the pilot contributes a substantial fraction of the total cycle energy. Below the minimum gas mode load limit (typically 15 to 25 percent MCR), the engine switches fully to liquid mode with all fuel from the main injectors and the pilot either reduced or shut off depending on configuration.

This low-load gas-off behaviour matters for port approach and departure operations. A vessel slowing for pilotage or berthing at 10 to 15 percent MCR may be in diesel mode for that portion of the voyage. The IGF Code’s requirement for continuous gas-diesel switching capability is partly designed to accommodate exactly this operational reality.

Application across fuel types: LNG, methanol, and ammonia

LNG dual-fuel (X-DF, ME-GI, Wartsila DF)

LNG is the dominant gas fuel for dual-fuel marine engines, and the pilot injection systems on all major LNG-fuelled engine families were primarily developed for methane-dominant natural gas. The LNG fuel system delivers boil-off gas or forced-vaporized LNG to the gas admission valves (on X-DF) or gas injection pumps (on ME-GI). The pilot diesel initiates combustion regardless of LNG composition, with the engine management system adjusting timing and knock margins based on the methane number of the specific cargo.

WinGD publishes X-DF product descriptions noting methane number range from 70 to 100 as within the engine’s designed operating envelope, with the control system managing timing accordingly. Below MN 70, the engine may need to reduce gas load fraction or operate with higher pilot quantities to maintain combustion stability.

Methanol dual-fuel (MAN ME-LGI, WinGD X-DF-A methanol variant)

Methanol has a lower heating value of approximately 19.9 MJ/kg, compared with approximately 50 MJ/kg for natural gas (on a lower heating value basis). Methanol also has a high octane rating and requires temperatures above its autoignition temperature of around 385 degrees Celsius to ignite under compression. MAN’s ME-LGI (Liquid Gas Injection) injects methanol directly into the cylinder, with a diesel pilot providing the ignition event. The pilot fractions for methanol dual-fuel are similar to LNG dual-fuel, typically 1 to 5 percent.

The methanol as marine fuel article covers the fuel properties, storage requirements, and regulatory status in detail. From a pilot injection perspective, the key difference is that methanol is injected as a liquid (not a gas) and the cylinder contains air only (not a premixed methanol-air mixture), so the combustion proceeds more like diesel diffusion combustion than Otto-cycle premixed combustion. This reduces methane slip to near zero (there is no scavenge-borne methanol loss) but introduces its own combustion challenges including the potential for aldehydes in the exhaust.

See also methanol marine engines overview for the full engine comparison.

Ammonia dual-fuel (MAN ME-GA, test programmes)

Ammonia has a significantly lower flame speed than methane (laminar burning velocity approximately 0.07 m/s versus 0.37 m/s for methane-air at stoichiometric conditions) and a very narrow flammability range (16 to 25 percent by volume in air versus 5 to 15 percent for methane). Both properties make ammonia harder to ignite and sustain in combustion than methane.

MAN’s ME-GA, the ammonia-capable variant of the ME-GI, is designed for high-pressure ammonia injection following the same diffusion-flame principle as the ME-GI for gas. The pilot fuel contribution for ammonia is larger: MAN has indicated pilot fractions of approximately 5 to 10 percent for ammonia operation, with the pilot providing not just ignition but a sustained high-temperature zone to support ammonia combustion. The ammonia as marine fuel article and ammonia marine engines overview cover the complete landscape of ammonia combustion challenges, including NOx from nitrogen in the fuel and the potential for N2O emissions with worse climate impact than CO2.

Regulatory framework: MARPOL Annex VI and the IGF Code

MARPOL Annex VI Regulation 13 and pilot NOx

MARPOL Annex VI Regulation 13, enforced by flag states and port states, sets NOx emission limits across three tiers based on engine rotational speed. Tier III limits apply to engines installed after 1 January 2016 that operate in designated NOx ECAs (the North American ECA and the US Caribbean ECA under MARPOL Annex VI Appendix VII; the North Sea ECA and Baltic Sea ECA are also designated). The Tier III limit for engines below 130 RPM is 3.4 g/kWh.

For X-DF engines in gas mode, the lean-burn Otto cycle produces NOx well below 3.4 g/kWh without additional abatement. For ME-GI engines in gas mode, SCR is required for Tier III compliance. In liquid diesel mode, both engine types produce NOx consistent with Tier II (the global limit), not Tier III, unless additional abatement is fitted. This means dual-fuel vessels entering Tier III ECAs operate in gas mode (for Tier III compliance on X-DF) or switch between gas mode and SCR-diesel mode (for ME-GI).

The pilot injection itself contributes to NOx because the stoichiometric combustion zone around the pilot droplets operates at temperatures above the thermal NOx threshold. With a micro-pilot below 1 percent, this contribution is small but measurable. Pilot NOx control was one of the engineering drivers behind the progressive reduction in pilot fraction from the 3 to 5 percent first-generation values toward sub-1-percent micro-pilot.

IGF Code: gas fuel system safety and pilot fuel backup

The IGF Code (MSC.391(95)), which SOLAS Chapter II-1 Part G requires for ships using low-flashpoint fuels (flashpoint below 60 degrees Celsius), includes specific requirements for gas fuel systems that bear directly on pilot injection system design.

Part A (general requirements) requires redundancy in the fuel supply system: if any single component of the gas fuel system fails, the ship must be able to continue on its remaining fuel capability. Part G (natural gas) and the related parts covering methanol require that the gas fuel system can be isolated and the vessel can operate on pilot fuel (or backup liquid fuel) alone within defined safety limits.

The IGF Code’s gas detection and shutdown requirements shape pilot system design: a gas detection event in the engine room triggers automatic closure of the master gas valve, isolating the gas fuel supply, and the engine reverts to diesel mode via the pilot injectors and main injectors within the code’s required switch time. The pilot fuel system must therefore be reliable independently of the gas fuel supply.

SOLAS Chapter II-2 Regulation 4 (fire safety for gas fuel ships) cross-references the IGF Code requirements and applies them to the engine room gas management framework.

Operational considerations and maintenance

Pilot injector wear intervals

Pilot injectors wear faster than main fuel injectors on the same engine because they operate with very small orifices that are sensitive to erosive wear from high-velocity fuel flow, and their fine filtration requirement means any filter bypass event causes disproportionate injector damage. Pilot injector overhaul intervals on WinGD X-DF installations are typically in the 4,000 to 8,000 running-hour range, compared with 8,000 to 16,000 hours for main injectors on the same engines. The exact interval is set by the engine builder’s maintenance schedule and may be shortened in service based on injector performance monitoring.

Injector performance monitoring on modern X-DF and Wartsila DF engines uses cylinder pressure data: the engine management system tracks combustion quality indicators (heat release rate shape, peak pressure, 50 percent burn point) on a per-cylinder, per-cycle basis. Degrading pilot injector performance shows up as erratic ignition timing, longer ignition delay, or increasing misfire frequency before a full injector failure occurs. Predictive maintenance based on pressure trace analysis is now standard practice on most LNG-fuelled vessel operators.

Filter maintenance

Pilot fuel filters at 5-micrometre absolute rating plug faster than main fuel filters. The pilot circuit is also more sensitive to water contamination, which can produce injector corrosion and orifice enlargement. Operators’ procedures for MGO quality verification should specifically check the pilot fuel service tank for free water, as water separation in a small pilot service tank may not proceed as reliably as in a large main bunker tank with heating coils.

Cold start: diesel mode only

Dual-fuel engines start in diesel mode and transition to gas mode after reaching operating temperature, typically after several hundred hours of operation on a new installation or after a planned hot standby period in service. Cold start in gas mode is not approved under the engine builders’ operating procedures because:

• Cold cylinder liner temperatures quench flame propagation near the liner wall, increasing methane slip and reducing combustion efficiency
• Compression temperatures at cold start are lower, extending pilot ignition delay beyond the design window
• The gas fuel system itself requires warming before the gas admission valves reach their designed operating temperature range

IGF Code Annex 1 (guidelines on the application of gas fuel systems) recommends that gas fuel system commissioning include verification of mode-switching behaviour from diesel to gas and back under controlled conditions before first commercial operation.

Gas mode lower load limit and port operations

Vessels operating LNG-fuelled propulsion in busy port approaches often face the practical constraint that their gas mode lower load limit (15 to 25 percent MCR) coincides with the speed range used for harbour manoeuvring. Vessel operators in ports with Tier III ECAs need to plan mode-switching sequences carefully: switching to diesel mode before reaching the gas mode load limit avoids the risk of misfire or high methane slip during the transition, but it means diesel-mode NOx in an ECA that may require Tier III compliance.

Some engine builders and ship operators have addressed this by extending the gas mode lower load limit through control system modifications and pilot quantity optimization on specific engine builds, but the IGF Code’s safety requirements (specifically the gas fuel system shutdown capability) set a minimum gas mode limit below which reliable automatic shutdown and changeover to diesel cannot be guaranteed by the system design.

Limitations

The following constraints bound the practical applicability of the information in this article.

Pilot injection performance data comes from manufacturer product documentation, MEPC working group submissions, and published academic work on large-bore dual-fuel combustion. In-service performance varies by gas composition, ambient conditions, age and condition of the injector hardware, and the specific engine variant installed. The figures cited here (pilot energy fraction below 1 percent on X-DF2.0, methane slip below 0.1 percent on ME-GI full load) reflect manufacturer-stated design targets and values cited in IMO submissions, not guaranteed performance for any specific vessel.

The methane number ranges cited for specific LNG cargoes are illustrative values based on field compositions published in trade literature and LNG industry sources. Actual cargo MN must be determined from the cargo quality certificate on each loading.

Ammonia dual-fuel is a developing technology. The pilot fraction figures cited for MAN ME-GA (5 to 10 percent) reflect publicly available MAN documentation as of the article date. These figures are from pre-commercial development and test programmes and may change as the technology matures and in-service experience accumulates.

The regulatory framework described (MARPOL Annex VI Regulation 13 Tier III, IGF Code MSC.391(95)) is current as of mid-2026. IMO regulations are amended by resolution; readers should verify current text through IMO’s official document portal.

This article does not cover diesel-only engines or engines designed for liquefied petroleum gas (LPG) or hydrogen, which involve different combustion and pilot injection architectures.

See also

• WinGD X-DF Dual-Fuel Engine Architecture
• Common Rail Fuel Injection on Two-Stroke Engines
• Fuel Valve (Injector) Design for Two-Stroke Marine Engines
• Methane Slip from LNG Dual-Fuel Engines
• LNG as Marine Fuel
• LNG Fuel System
• Methanol as Marine Fuel
• Methanol Marine Engines Overview
• Ammonia as Marine Fuel
• Ammonia Marine Engines Overview
• NOx Tier I, II, and III Requirements
• MARPOL Annex VI Regulation 13: NOx Tier
• IGF Code
• MAN B&W ME-C Electronic Control Overview
• Wartsila 50DF Dual-Fuel Engine
• Piston Crown Cooling in Slow-Speed Engines
• Tier III Compliant Two-Stroke Engines

Relevant calculators:

• Engine - Pcomp vs Pmax Ratio
• Fuel Pump - Delivery Stroke
• Injector - Fuel Injection Timing
• Formaldehyde / HC Calculator - Dual-fuel engines