A Scavenge Fire is a dangerous condition in a marine diesel engine where accumulated lubricating oil, carbon deposits, or combustible material inside the scavenge space (under-piston space) catches fire. It is one of the most common engine room emergencies encountered onboard ships and can lead to severe engine damage if not controlled promptly.
Scavenge fires generally occur in large two-stroke marine diesel engines where unburnt fuel, carbon deposits, and scraped cylinder oil accumulate in the scavenge space.
Like any fire, a scavenge fire requires the three elements of the Fire Triangle:
When these three conditions combine inside the under-piston space, a scavenge fire may develop.
The sequence of events leading to a scavenge fire is:
Poor combustion results in carbon particles and unburnt fuel deposits accumulating inside the scavenge space.
Cylinder lubricating oil scraped down by piston rings collects in the under-piston area.
Due to worn piston rings or liner wear, hot combustion gases escape into the scavenge space.
Burning carbon particles and hot gases increase the temperature of accumulated deposits.
The deposits eventually ignite, resulting in a scavenge fire.
Several factors may contribute to the occurrence of a scavenge fire:
Incomplete fuel combustion leads to carbon deposit formation.
Worn piston rings allow hot combustion gases to enter the scavenge space.
Over-lubrication causes oil accumulation beneath the piston.
Improper valve adjustment may cause blowback of hot gases.
Increased back pressure can force hot gases back into the scavenge ports.
Dirty scavenge spaces and blocked drains increase fire risk.
The following warning signs may indicate a scavenge fire:
Usually activated around 90°C.
The engine may run irregularly and lose efficiency.
Unstable turbocharger operation is a common symptom.
The affected cylinder unit often shows a higher exhaust temperature.
Smoke may appear around the turbocharger inlet.
Burning particles may emerge through drain lines.
The scavenge box becomes unusually hot.
Paint around the affected area may blister or peel due to heat.
Temperature monitoring systems may indicate overheating.
A minor scavenge fire is usually localized to one cylinder unit.
A major scavenge fire requires immediate emergency response.
Proper maintenance is the key to preventing scavenge fires.
Remove carbon deposits and sludge regularly.
Ensure drains are clear and functioning properly.
Check piston ring condition and maintain proper clearances.
Prevent lubricating oil from entering the scavenge space.
Ensure proper atomization and combustion.
Calibrate liners periodically and replace when wear exceeds limits.
Avoid both under-lubrication and over-lubrication.
A severe scavenge fire may result in:
A Scavenge Fire is one of the most serious hazards in a marine diesel engine. It occurs when lubricating oil, carbon deposits, oxygen, and a heat source combine within the scavenge space. Early detection through alarms, regular inspections, proper maintenance of piston rings, cylinder liners, fuel injectors, and scavenge drains are essential to prevent engine damage and ensure safe ship operations.
The main cause is the ignition of accumulated carbon deposits and lubricating oil by hot blow-past combustion gases.
Most scavenge fire alarms are set at approximately 90°C.
It reduces heat generation and prevents the fire from intensifying.
To prevent sparks and burning oil from entering the engine room.
Yes. If left uncontrolled, excessive heat may spread and increase the risk of a crankcase explosion.
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