Marine and power-plant checklist for 4-stroke dual-fuel engine gas-to-diesel transfer, gas admission valves (SOGAV type), pilot fuel stability and gas trips.
← All technical guides · Category: Dual-Fuel Engines · Updated: · 9 min read
| Operating / Transfer Mode | When It Happens | What the Engine Does | Diagnostic Watchpoints |
|---|---|---|---|
| Diesel-to-Gas Transfer (Normal Entry) | Within the load range and warm-up conditions set by the engine maker, after the gas valve unit (GVU) passes its pre-start checks | Ramps gas admission up while ramping diesel injection down over the transfer time set by the maker | Engine temperatures inside the maker’s gas-mode window, stable pilot fuel pressure, and GVU gas pressure tracking charge-air receiver pressure plus the set differential |
| Gas Mode (Lean-Burn Otto Cycle) | Normal running on gas within the maker’s load range | Low-pressure natural gas admitted at each cylinder inlet (for example by SOGAV-type solenoid valves) and ignited by a small diesel pilot injection | Air-fuel ratio control (exhaust waste gate or charge-air bypass), knock sensor levels, and cylinder EGT deviation against the maker’s alarm limits |
| Normal Gas-to-Diesel Transfer | Operator command before manoeuvring, port entry or a planned fuel changeover | Replaces gas admission with main diesel injection; the double block-and-bleed valves then close and the gas line downstream is vented | Main diesel supply pressure and injection readiness, and confirmation that gas line venting (and inert-gas purge, where fitted) has finished |
| Gas Trip / Emergency Transfer | Automatic, on safety-system triggers such as heavy knock, misfire, gas pressure deviation, gas detection or loss of ventilation in the gas pipe duct | Gas admission stops, the two shut-off valves close and the bleed valve opens, and the engine keeps running on oil fuel (IGF Code 9.4.4–9.4.5 and 10.3.2) | Fuel actuator response, governor speed dip, turbocharger surge margin, and the first-out alarm in the engine safety system log |
On LNG carriers, dual-fuel container ships, DFDE vessels and onshore power plants, a 4-stroke dual-fuel (DF) engine runs on the lean-burn Otto cycle in gas mode. Low-pressure natural gas is admitted at each cylinder’s inlet port during the intake stroke by an electronically controlled gas admission valve, mixes with a large excess of charge air, and is ignited near top dead centre (TDC) by a small diesel pilot injection.
Because the lean air-gas mixture works inside a narrow corridor between the knock limit (mixture too rich or charge air too hot) and the misfire limit (mixture too lean or weak pilot ignition), any sudden disturbance in gas supply pressure, charge-air receiver pressure, methane number (MN) or pilot injection timing can push one or more cylinders outside safe limits and trigger a gas trip.
Each cylinder relies on close coordination between the cylinder control electronics, the gas admission valve and the pilot fuel injector (in some designs the pilot and main diesel nozzles share one injector body; in others they are separate). Over running hours, carbon build-up on the pilot nozzle tip affects spray atomisation, delaying ignition and causing intermittent misfire or harsh late combustion.
Similarly, if a gas admission valve suffers filter fouling, coil resistance drift or seat wear, that cylinder’s air-fuel ratio drifts away from the rest of the bank. The control system compensates by trimming that valve’s opening duration within a limited range set by the maker; once a cylinder reaches its trim limit, an EGT deviation or knock/misfire alarm follows.
Under the IGF Code (IMO Resolution MSC.391(95)), gas-fuelled machinery is protected by gas detection that alarms at 20% of the lower explosion limit (LEL) and activates the safety system at 40% LEL on two detectors, with 30% and 60% LEL used for ventilated ducts around gas pipes in machinery spaces (15.8 and Table 1). Double-wall gas pipes or ducts in machinery spaces need mechanical underpressure ventilation of at least 30 air changes per hour (9.6 and 13.8). Class surveyors, port State control officers and vetting inspectors may ask to see the related records, so keep fuel changeover, gas detection and ventilation test records easy to find.
Our interactive 4-stroke DF, SG & GVU simulator lets your engineering team rehearse Lab 01 (GVU tightness test, nitrogen purge and diesel-to-gas switchover, the free-trial lab) and Lab 02 (cylinder knock and pilot nozzle coking diagnostics) before stepping into the Engine Control Room.
The IGF Code (10.3.2) does not set a time in milliseconds. It requires the engine to keep running on oil fuel without interruption when the gas supply is shut off, and the automatic changeover to cause minimum fluctuation in engine power. In practice the safety system stops gas admission and closes the double block-and-bleed valves while diesel injection takes over at once; the engine maker’s documentation gives the exact response for your engine.
EGT spread just after gas entry is most often caused by uneven pilot ignition (carbon on a pilot nozzle tip), a sticking or poorly calibrated gas admission valve on one cylinder, or inert gas left in the gas line if the venting or purge sequence did not finish.
You can run interactive fault scenarios on our 4-Stroke DF, SG & GVU Simulator at marvelsolutionz.com/lng-academy/ (Lab 01 has a 5-minute free trial; full 12-module Self-Paced enrolment is from $395 USD + 5% sales tax).
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