What causes a scavenge fire in a two-stroke marine engine?

A scavenge fire happens when unburnt fuel, cylinder lubricating oil and carbon deposits that have accumulated in the scavenge air spaces of a two-stroke crosshead engine reach their ignition temperature, most commonly set off by hot combustion gas blowing past a worn, stuck or broken piston ring and igniting the oily buildup around the piston skirt and scavenge ports. The underlying conditions are almost always present for some time before the fire itself: worn or broken piston rings and a worn or scored liner allow gas to blow past into the scavenge space; poor combustion from faulty or worn fuel injectors, incorrect injection timing, low compression or sustained overloading leaves unburnt fuel to collect rather than being consumed; cylinder oil feed rate that is too high, too low, or mismatched to the fuel's sulphur content and the resulting base number requirement leaves either excess unburnt oil or inadequate ring lubrication; and infrequent draining and cleaning of the scavenge spaces lets oily sludge build up into a genuinely combustible layer.

The warning signs, when caught early, are fairly distinct: a rising exhaust temperature on the affected unit relative to the others, a loss of power and rough or irregular running, an abnormal scavenge space or scavenge drain temperature picked up by fitted sensors, and in a developing fire, smoke or sparks visible at the scavenge drains, unusual noise, and paint blistering or discoloring on the scavenge casing where it is hot to the touch.

On confirming a scavenge fire the immediate response is to reduce engine speed, cut or minimize fuel to the affected unit where the engine allows individual unit fuel cut off, and increase cylinder lubrication to help protect the liner and rings, while never stopping the engine dead if it can be safely kept turning, since a stopped piston can seize in a hot liner. The fire is generally left to burn itself out under controlled conditions while the casing is cooled from the outside with hoses or a fixed smothering system using steam or CO2 if fitted; the scavenge space doors must never be opened while the fire is active or suspected still hot, because the inrush of fresh air can flash the accumulated deposits into a much larger secondary event.

Once cooled, a full inspection of the liner, piston rings, ring grooves and scavenge ports is needed before the unit is put back into service, since restarting on a damaged ring or liner invites a repeat fire. Prevention is entirely about the conditions that allow buildup in the first place: sound combustion, rings and liners in good condition confirmed by regular inspection, correctly dosed and load matched cylinder oil, and a disciplined routine of draining and cleaning the scavenge spaces rather than leaving it until the next major overhaul.