Background
Dual-fuel slow-speed two-stroke marine engines burn natural gas, methanol, ammonia, or other low-sulphur fuels alongside diesel as a backup. In gas-mode operation, the gas-air mixture is too lean to autoignite under compression alone (unlike diesel, which self-ignites at typical compression ratios). The engine therefore needs a separate ignition source. Marine dual-fuel engines use pilot injection: a small quantity of diesel fuel injected at the end of compression that self-ignites and creates ignition kernels propagating into the gas-air charge.
Pilot injection bridges the gap between diesel-cycle and Otto-cycle operation. From the diesel side, the pilot is a familiar high-pressure injection of fuel into hot compressed air. From the Otto side, it functions as a distributed spark ignition system, with multiple ignition kernels initiating flame propagation through the lean mixture.
The pilot is small (typically 1 to 5 percent of total fuel energy at full load), but it must be precisely timed and uniformly distributed for reliable ignition. Small variations in pilot timing or quantity can shift combustion characteristics, methane slip, and emissions significantly.
This article covers pilot injection architecture, pilot quantity selection, ignition physics, and operational considerations across LNG, methanol, and ammonia dual-fuel applications.
Pilot injector architecture
Pilot fuel system
The pilot fuel system is independent of the main liquid fuel system on most dual-fuel engines:
- Pilot fuel tank: small dedicated tank for diesel pilot fuel (typically MGO for cleanliness)
- Pilot fuel pumps: high-pressure pumps for the pilot rail
- Pilot common rail: high-pressure rail (typically 1,000 to 1,500 bar) feeding pilot injectors
- Pilot injectors: one per cylinder, dedicated to pilot delivery
- Pilot fuel filter: very fine filtration (typically 5 micrometres) protecting the small pilot orifices
The independent system ensures pilot fuel quality and supply reliability even if the main fuel system has problems.
Combined system on some engines
Some dual-fuel engines use the same fuel injectors for both pilot (in gas mode) and full diesel (in liquid mode). This simplifies the engine but requires the injector to deliver across a wide range of fuel quantities. The injector must be capable of accurately delivering 1 percent of full quantity (the pilot in gas mode) and 100 percent of full quantity (full diesel in liquid mode).
Dedicated micro-pilot injector
Some recent engines use a dedicated micro-pilot injector with very small orifices (0.15 to 0.3 mm) optimised for tiny pilot doses. These engines have separate main fuel injectors for full liquid mode operation. The two-injector approach gives best precision in both gas and liquid modes.
Pilot quantity
Gas mode pilot
In gas mode, pilot quantity is small but precisely controlled:
- Standard pilot: 3 to 5 percent of total fuel energy at full load
- Reduced pilot: 1 to 2 percent on advanced engines
- Micro-pilot: below 1 percent, achievable on the most advanced designs
Pilot quantity is typically expressed as percentage of total fuel energy (gas + pilot). Lower pilot fraction means more of the fuel energy comes from gas, with corresponding methane slip and emission characteristics.
Pilot at low load
At reduced load, the gas quantity per cycle is smaller, but the pilot quantity may not scale proportionally. Below approximately 25 percent of full load, the pilot fraction may rise to 5 to 10 percent because the minimum reliable pilot dose is bounded by injector capability.
Pilot at zero gas (liquid mode)
In liquid mode (gas off), the engine runs on diesel alone. The pilot injector may be deactivated, with all fuel coming through the main injector(s); or it may be active, contributing a small fraction of fuel.
Tier III pilot considerations
Tier III emissions compliance requires careful pilot management. The pilot itself produces NOx because it burns at high temperature and stoichiometric ratio. Reducing pilot quantity reduces NOx but eventually compromises ignition reliability.
Ignition physics
Pilot ignition kernel
The pilot fuel, injected near top dead centre, autoignites within 0.5 to 2 milliseconds (the ignition delay) due to the high temperature of the compressed gas-air mixture. Ignition occurs at multiple points within the spray pattern: each pilot droplet vaporises, mixes with surrounding gas-air mixture at locally favourable air-fuel ratio, and ignites.
The result is multiple ignition kernels distributed through the cylinder wherever the pilot spray reached. Each kernel is a small ignited region that grows by flame propagation into the surrounding gas-air mixture.
Flame propagation
From each kernel, flame propagates radially outward through the lean gas-air mixture at typical flame speeds of 0.2 to 0.5 m/s. With multiple kernels (one per pilot orifice times the number of pilot injectors), the flame fronts cover the cylinder volume in a few milliseconds, completing the heat release.
Lean limit
If the gas-air mixture is too lean (below approximately 0.6 lambda inverse, i.e. above lambda 1.7), flame propagation fails and combustion is incomplete. The lean limit sets the minimum gas-air mixture richness for reliable operation.
Quenching
Near cool walls (cylinder liner, piston crown), flame propagation can be quenched by heat loss. Quenching leaves regions of unburned mixture that contribute to methane slip. Pilot spray patterns that ignite the bulk of the mixture far from cool walls minimise quenching.
Pilot timing
Pilot timing relative to TDC
Pilot injection is typically timed at 5 to 25 degrees BTDC (before top dead centre), depending on engine design and operating conditions. Earlier timing gives more time for the pilot to ignite and the flame to spread; later timing provides higher in-cylinder pressure and temperature at injection.
Variable pilot timing
Modern engines vary pilot timing as a function of:
- Engine load: pilot timing typically retards at higher load (to control peak pressure)
- Methane number: pilot timing advances at higher methane number (more knock-resistant) to capture more expansion work
- Ambient conditions: timing adjusts for inlet temperature and pressure
- Fuel mode: different timing for gas vs liquid mode
Knock prevention
If pilot timing is too advanced (or gas charge too rich), the gas-air mixture may autoignite before the pilot, causing knock. The engine control system monitors cylinder pressure for knock signatures and retards pilot timing if knock is detected.
Misfire detection
If the pilot fails to ignite, the gas-air mixture passes through the cylinder unburned, producing a misfire. Misfires are detected by absence of pressure rise after pilot timing, with the engine control system isolating the affected cylinder.
Pilot atomisation
Pilot atomisation is more demanding than main injection atomisation because:
- Pilot quantities are very small, requiring precise metering
- Pilot droplets must vaporise and ignite within milliseconds
- Pilot spray pattern must produce well-distributed ignition kernels
- Cylinder gas density is high (peak compression), restricting spray penetration
Modern pilot injectors achieve fine atomisation (Sauter Mean Diameter typically 15 to 40 micrometres) through:
- High injection pressure (1,000 to 1,500 bar)
- Small orifice diameters (0.15 to 0.4 mm)
- Multiple orifices per pilot injector (4 to 8 orifices)
- Optimised spray cone angles for chamber coverage
Pilot fuel management
Pilot fuel type
The diesel pilot is typically MGO for fuel quality reasons:
- Low sulphur (matches Tier III emissions targets)
- Clean (low catalytic fines, low water, low particulates)
- Low viscosity (good atomisation without heating)
- Reliable supply chain
Some engines support pilot operation on LSFO or HFO if MGO is unavailable, but with reduced injector lifetime and emission performance.
Pilot fuel consumption
For an engine at 70 percent load with 3 percent pilot fraction:
- Total fuel energy at 70% MCR: ~25 MWh per hour
- Pilot fuel energy: 0.03 × 25 = 0.75 MWh per hour
- Pilot mass flow at MGO LCV 42,500 kJ/kg: 0.75 × 3600 / 42.5 = 63.5 kg per hour
Annual pilot fuel consumption is typically 200 to 400 tonnes per ship. Pilot fuel is therefore a meaningful operating cost item.
Pilot fuel storage
Ships carry separate pilot fuel storage:
- Service tank: 5 to 20 cubic metres
- Storage tanks: 50 to 200 cubic metres
- Bunkering arrangements: separate from main fuel bunkering
Pilot fuel quality is verified through routine sampling.
Application across fuel types
LNG dual-fuel
LNG dual-fuel engines use diesel pilot ignition. WinGD X-DF engines use pilot fractions of 2 to 5 percent at full load. MAN B&W ME-GI engines, with high-pressure gas injection, use even smaller pilots (1 to 3 percent) because the gas is injected directly into hot air rather than premixed lean.
Methanol dual-fuel
Methanol dual-fuel engines (MAN B&W ME-LGI) use diesel pilots similar to LNG engines. Methanol’s lower energy content per unit volume requires larger fuel injection volumes than LNG, but pilot fractions are similar.
Ammonia dual-fuel
Ammonia dual-fuel engines, expected to enter commercial service in the mid-2020s, will require larger pilots than LNG or methanol because ammonia is more difficult to ignite. Pilot fractions of 5 to 15 percent are anticipated.
Operational issues
Pilot injector wear
Pilot injectors operate at very high pressures with very small orifices. They wear faster than main injectors:
- Typical pilot injector overhaul interval: 4,000 to 8,000 hours
- Main injector overhaul interval: 8,000 to 16,000 hours
Operators carry sufficient pilot injector spares for the planned voyage profile.
Fuel filter integrity
The very fine filtration on pilot fuel (5 micrometres) requires diligent filter maintenance. Filter blockages quickly degrade pilot performance.
Cold start in gas mode
Engines typically start in liquid mode (full diesel) and switch to gas mode after warming up. Cold start in gas mode is generally not permitted because:
- Cold cylinders quench flame propagation
- Pilot ignition is unreliable below operating temperature
- Methane slip is much higher
Emergency operation
If the gas supply fails, the engine immediately switches to liquid mode. The pilot injector either stops or continues with a small contribution; the main injector takes over the full fuel delivery. The transition is automatic and takes a few seconds.
Pilot trip
Some engine faults trigger automatic gas mode shutdown:
- Knock detection in multiple cylinders
- Misfire in multiple cylinders
- Loss of pilot fuel supply
- Loss of common rail pressure
- Operator-initiated emergency shutdown
In all cases, the engine reverts to liquid mode for safe continued operation.
Related Calculators
- Pilot Fuel Quantity Calculator
- Pilot Fuel Consumption Calculator
- Pilot Timing Optimisation Calculator
- Lambda Air-Fuel Ratio Calculator
- Methane Slip Estimation Calculator
See also
- WinGD X-DF Dual-Fuel Engine Architecture
- MAN B&W ME-C Electronic Control Overview
- Common Rail Fuel Injection on Two-Stroke Engines
- Fuel Valve (Injector) Design for Two-Stroke Marine Engines
Additional calculators:
- Engine - Pcomp vs Pmax Ratio
- Fuel Pump - Delivery Stroke
- Injector - Fuel Injection Timing
- Formaldehyde / HC Calculator - Dual-fuel engines
Additional formula references:
References
- WinGD. (2023). X-DF Pilot Fuel System Engineering Specifications. Winterthur Gas & Diesel.
- MAN Energy Solutions. (2023). ME-GI and ME-LGI Pilot Injection Manual. MAN Energy Solutions.
- DNV. (2022). LNG as Marine Fuel: Pilot Ignition Best Practices. DNV.
- Heywood, J. B. (2018). Internal Combustion Engine Fundamentals (2nd ed.). McGraw-Hill.
- Lloyd’s Register. (2022). Guidance Notes for Dual-Fuel Marine Engines.