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WinGD X-DF Dual-Fuel Engine Architecture

WinGD X-DF is the low-pressure dual-fuel slow-speed two-stroke engine family from WinGD (Winterthur Gas & Diesel), the largest single category of new-build LNG-fuelled marine engines through the late 2010s and 2020s. X-DF burns natural gas in Otto-cycle mode, drawing the gas-air mixture into the cylinder during scavenging and igniting it with a small diesel pilot injection at the end of compression. This architecture differs fundamentally from the MAN B&W ME-GI high-pressure gas injection approach. This article describes the X-DF system architecture, gas admission, ignition, methane slip mitigation, fuel mode switching, and operational characteristics. Visit the home page or browse the calculator catalogue for related propulsion engineering tools.

Contents

Background

WinGD (Winterthur Gas & Diesel) corporate history: the WinGD lineage runs Sulzer Brothers (Winterthur, founded 1834; first marine diesel 1898; first marine two-stroke 1905) → New Sulzer Diesel Ltd. (1990 spin-off) → Wartsila NSD Corporation (April 1997 merger of New Sulzer Diesel with Wartsila Diesel Oy) → Wartsila Switzerland Ltd. (2006). On 19 January 2015 the Wartsila two-stroke business and CSSC (China State Shipbuilding Corporation) closed a joint venture, with CSSC taking 70% and Wartsila retaining 30% at founding. In June 2016 Wartsila divested its remaining 30%; CSSC has owned WinGD 100% since June 2016. Headquarters remain in Winterthur, Switzerland. The Sulzer RTA series (uniflow scavenging, hydraulically actuated central exhaust valve) was introduced in 1983, and RT-flex (common-rail electronic) entered commercial service in 2001; both are direct ancestors of the current X-series and X-DF.

When natural gas began emerging as a marine fuel in the early 2010s, the two leading slow-speed engine manufacturers (then MAN B&W and Wartsila two-stroke, now WinGD) developed parallel but architecturally distinct approaches. MAN B&W chose high-pressure gas injection (HPGI), in which natural gas is compressed to roughly 300 bar and injected directly into the cylinder near top dead centre, alongside a diesel pilot. The HPGI approach uses a diesel cycle (compression ignition) for both fuels, providing diesel-equivalent combustion efficiency and limited methane slip.

WinGD chose low-pressure gas operation with the Otto cycle: gas is admitted at relatively low pressure (typically 16 bar) into the cylinder during scavenging, mixed with the air charge, and ignited at the end of compression by a small diesel pilot injection. The Otto-cycle approach uses simpler gas supply systems (no high-pressure compressors) and avoids the capital and operational cost of HPGI. The penalty is methane slip and a smaller knock margin.

Both approaches have found commercial success. MAN B&W ME-GI dominates LNG carrier and large containership applications where fuel efficiency and methane slip are commercially critical. WinGD X-DF dominates the dual-fuel container ship and bulk carrier market where capex sensitivity and gas supply simplicity are decisive. By the early 2020s, X-DF had become the most-installed dual-fuel slow-speed engine in absolute numbers.

This article describes the X-DF architecture, its evolution through the X-DF1.0 and X-DF2.0 generations, and the operational characteristics that distinguish it from competing platforms.

System architecture

Gas valve unit (GVU)

Natural gas is supplied to the engine from the ship’s LNG fuel system at typically 16 to 20 bar absolute. Before reaching the engine, the gas passes through a gas valve unit (GVU) containing:

  • Master shutoff valves
  • Pressure regulation
  • Gas filtration
  • Leak detection
  • Vent valves to safe locations

The GVU is a critical safety element: it must reliably isolate the engine from the gas supply on detection of any fault.

Gas manifold

A double-walled gas manifold runs along the engine length, supplying each cylinder. The double-walled construction provides containment if the inner pipe leaks; the annular space is monitored for gas detection.

Gas admission valve (GAV)

Each cylinder has a gas admission valve (GAV) mounted in the cylinder cover. The GAV is a hydraulically actuated valve that opens during scavenging to admit gas into the cylinder. Gas flow is rapid (a few milliseconds) and timed precisely against the engine cycle.

GAV opening typically begins shortly after the exhaust valve closes during the late scavenging phase, when the cylinder is filled mostly with fresh air at moderate pressure. The GAV closes well before TDC to prevent over-mixing and to allow the gas-air mixture to reach a homogeneous state before ignition.

Diesel pilot

Each cylinder has a small fuel injector providing the diesel pilot that ignites the gas-air mixture. Pilot injection quantity is typically 1 to 5 percent of the total fuel energy at full load, growing to a larger fraction at reduced load. The pilot injector is similar in construction to the main fuel injector on a liquid-only engine but sized for the smaller pilot quantity.

In some recent designs, the pilot is replaced by a micro-pilot of even smaller quantity (below 1 percent), permitted by improved injector design and ignition stability.

Cylinder pressure sensors

X-DF engines include cylinder pressure transducers in every cylinder. The pressure traces are used by the engine control system to:

  • Verify timely ignition of the gas-air mixture
  • Detect knock or misfire
  • Balance cylinder-to-cylinder performance
  • Enable closed-loop combustion control

Combustion process

Otto cycle in marine context

In Otto-cycle gas operation:

  1. During scavenging, the cylinder is filled with fresh air through the scavenge ports.
  2. Late in scavenging, the GAV opens and admits gas. Gas mixes with air to form a relatively lean mixture (typically lambda 2.0 to 2.5, i.e. 100 to 150 percent excess air).
  3. The exhaust valve closes; the cylinder is sealed; compression begins.
  4. Near top dead centre, the diesel pilot is injected. The pilot self-ignites and creates multiple ignition kernels in the gas-air mixture.
  5. The flame propagates through the gas-air mixture, releasing heat and producing the power stroke.

The mixture leanness (high lambda) is a defining feature of X-DF combustion. Lean operation:

  • Limits flame temperature and therefore NOx formation
  • Provides knock margin (rich mixtures are more prone to autoignition)
  • Allows the engine to meet IMO Tier III without additional aftertreatment in gas mode
  • Reduces fuel consumption per unit of work compared to richer operation

Lean limit and knock

The lean limit is set by ignition reliability: too lean, and flame propagation fails. The knock limit is set by mixture autoignition before pilot injection: too rich (or too high a methane number), and the mixture autoignites before the pilot, producing knock.

The X-DF operating window is between these limits, and the engine control system continuously monitors cylinder pressure traces to keep operation within the window.

Methane number sensitivity

Different LNG sources have different methane numbers (a measure of resistance to autoignition). High-methane LNG (from gas with low ethane and propane content) has higher methane number and is more knock-resistant. Low-methane LNG (with significant heavier hydrocarbons) has lower methane number and reduces knock margin.

X-DF engines are typically rated for a minimum methane number (commonly 80) and may derate slightly when operating on lower-methane gas. The control system handles methane number variations through pilot quantity and timing adjustments.

Generations: X-DF1.0 and X-DF2.0

X-DF1.0

The first-generation X-DF entered service in 2017. It was rated to meet Tier III in gas mode without additional aftertreatment. Methane slip was approximately 4 g/kWh at full load, comparable to most low-pressure gas engines of the era.

X-DF2.0

The second-generation X-DF entered service in approximately 2020 with the addition of iCER (Intelligent Control by Exhaust Recycling). iCER is essentially a low-pressure exhaust gas recirculation (EGR) system that recirculates a fraction of exhaust gas through the scavenge receiver. The recirculated gas dilutes the cylinder charge with already-burned products, reducing methane slip by 50 to 70 percent and lowering NOx and CO emissions in both gas and liquid modes.

X-DF2.0 with iCER achieves methane slip of approximately 1.5 to 2 g/kWh at full load, comparable to the best high-pressure gas injection figures.

X-DF-A

WinGD has announced X-DF-A for ammonia operation, expected to enter service in the mid-2020s. The architecture extends the X-DF concept to ammonia as fuel, retaining the diesel pilot for ignition and adapting the GAV for ammonia handling.

Methane slip

Methane slip is the unburned methane that escapes the cylinder and exits via the exhaust. Low slip is important because methane is a potent greenhouse gas (warming potential ~28x CO2 over 100 years).

Sources of slip

Three sources contribute to methane slip:

  1. Crevice volume: gas trapped in the small crevices between piston, ring, and liner that does not reach combustion temperatures
  2. Quenching: flame extinction near cool walls, leaving local pockets of unburned mixture
  3. Scavenging losses: gas-air mixture short-circuiting through the cylinder to the exhaust during scavenging

Mitigation

Modern X-DF engines mitigate slip through:

  • iCER: recirculation of exhaust dilutes the charge and reduces unburned methane in residual gas
  • Optimised injection timing: GAV timing matched to scavenging characteristics to minimise short-circuit
  • Improved combustion chamber geometry: piston bowl optimised to reduce quenching volumes
  • Pilot quantity tuning: pilot ignition kernel intensity matched to mixture conditions
  • Cylinder pressure feedback: closed-loop control keeps each cylinder near optimal slip-vs-NOx tradeoff

Fuel mode switching

X-DF engines can switch between liquid and gas modes during operation. The switching procedure:

Gas to liquid

  1. Bypass setpoint to 100 percent liquid mode
  2. Liquid fuel injection ramps up while gas injection ramps down
  3. Diesel pilot quantity increases to full diesel injection
  4. GAV closes; gas supply isolated
  5. Engine runs entirely on diesel/HFO/LSFO

Liquid to gas

  1. Verify gas supply pressure and quality
  2. Start gas pilot pump
  3. Open gas isolation valves
  4. Begin GAV operation at low gas fraction
  5. Ramp up gas fraction while ramping down liquid fuel
  6. Stabilise at full gas operation with diesel pilot only

Mode switching takes several minutes and can be performed at any load above approximately 20 percent MCR.

Tripping behaviour

If a fault is detected during gas operation (knock, misfire, gas leak), the engine automatically trips to liquid mode. The transition is rapid (a few seconds) and does not interrupt engine operation.

Operational characteristics

Knock detection

X-DF engines monitor cylinder pressure for knock signatures: high-frequency oscillations characteristic of gas autoignition. On knock detection, the control system retards pilot timing or reduces gas fraction until knock subsides.

Misfire detection

Failure of pilot ignition results in a misfire: gas-air mixture passes through the cylinder unburned. Misfires are detected via cylinder pressure trace (no pressure rise after pilot timing). The control system isolates misfiring cylinders and may switch to liquid mode if multiple cylinders misfire.

Cylinder balancing

As with ME-C, each cylinder’s pilot timing, pilot quantity, and GAV timing are individually adjustable through software. The control system uses cylinder pressure feedback to balance cylinder-to-cylinder.

Tier III without aftertreatment

The lean Otto-cycle operation provides Tier III NOx compliance in gas mode without additional aftertreatment. This is a key commercial advantage of X-DF for ECA-trading vessels.

Tier II/III in liquid mode

In liquid mode, X-DF engines meet Tier II as standard and require EGR or SCR for Tier III. iCER provides Tier III in liquid mode without separate aftertreatment.

Maintenance

GAV overhaul

Gas admission valves are the principal new maintenance items unique to X-DF. They are inspected at intervals comparable to fuel injector overhauls (typically every 8,000 to 12,000 hours) and overhauled or replaced as needed.

Pilot injector overhaul

Diesel pilot injectors are smaller versions of standard fuel injectors and have similar overhaul requirements.

Gas piping inspection

Double-walled gas piping must be inspected periodically for leaks in either pipe wall and for proper functioning of leak detection systems.

iCER maintenance

iCER systems include exhaust gas filters, scrubbers, and coolers that require periodic cleaning. The iCER cooler in particular tends to accumulate deposits and must be cleaned every 2,000 to 4,000 hours.

See also

References

  • WinGD. (2023). X-DF Engine Family Technical Reference. Winterthur Gas & Diesel.
  • WinGD. (2023). iCER System Operation and Maintenance Manual. Winterthur Gas & Diesel.
  • DNV. (2022). LNG as Marine Fuel: Methane Slip and Mitigation Strategies. DNV.
  • Heywood, J. B. (2018). Internal Combustion Engine Fundamentals (2nd ed.). McGraw-Hill.
  • Lloyd’s Register. (2022). Guidance Notes for Dual-Fuel Marine Engines.