No.4 Unit Overheating and Turbocharger Surging on a Two-Stroke Main Engine
Your ship is a 6-cylinder low-speed two-stroke direct-drive bulk carrier, 14 days out, running at 68% MCR on HFO in a moderate following sea. Over the last 8 hours the exhaust gas temperature on No.4 unit has risen from 335 °C to 385 °C while the other units sit between 330 and 340 °C. In the last two hours the turbocharger has begun surging at intervals, the scavenge air pressure fluctuates by about ±0.15 bar, and the scavenge air temperature after the cooler has risen from 38 °C to 47 °C. The scavenge space drain on No.4 discharges a thin oily residue. There is no funnel smoke and no abnormal noise apart from the surge. The next port is 30 hours away and the charterer has already warned about delays. The second engineer wants to stop the engine immediately and inspect No.4 unit; the master wants to hold speed.
1. Diagnose the fault and explain which evidence distinguishes a scavenge space fire from a defective fuel injector or a leaking exhaust valve.
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Diagnosis: a scavenge space fire on No.4 unit, most probably fed by blow-by past worn piston rings or a leaking exhaust valve spindle.
The reasoning follows the fire triangle inside the scavenge air belt: combustible material (cylinder oil and carbon deposits lying in the belt), an ignition source (hot blow-by gas past worn rings or a burnt exhaust valve), and air at scavenge pressure.
The decisive evidence is where the heat appears, not how much there is:
- An injector or exhaust valve fault raises the exhaust gas temperature of that unit only. The scavenge air temperature and the turbocharger stay normal, because the fault is upstream of the scavenge space.
- Here the exhaust gas temperature rose and the scavenge air temperature after the cooler rose by 9 °C. That places the heat source downstream of the cooler, i.e. inside the scavenge belt itself.
- The turbocharger surges because the fire produces a hot, unsteady gas pulse that disturbs the matching between compressor and scavenge receiver, so the compressor momentarily stalls and recovers. Surging is not a normal symptom of a bad injector.
- The oily discharge from the No.4 scavenge drain fits burning deposits rather than a fuel-side fault.
Confirm without opening anything: compare piston underside and stuffing box drain temperatures, scan the scavenge belt with an infrared thermometer, listen for a popping or rumbling at the relief valves, and watch the crankcase oil mist reading. A rising oil mist reading would mean the fire is not yet contained to the scavenge space.
2. State your immediate actions and say whether you would stop the engine or keep it running. Justify each step.
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Keep the engine running at reduced load and cut fuel to No.4; stop only if the fire does not respond.
- Inform the bridge and the chief engineer at once. A speed reduction affects the passage plan and the charter party, and the master must be told that the engine may have to be stopped, so he can decide where that can safely be done.
- Reduce engine speed to the slow range (about 25–30% MCR). This cuts the heat and air input into the scavenge belt and slows the rate at which deposits are consumed.
- Cut fuel to No.4 by lifting the pump index or opening the puncture valve. With no combustion in that unit there is no further blow-by feeding the fire.
- Do not stop the engine immediately. A running engine still scavenges, so the air flow carries burning material out of the space and cools the belt. Stopping traps the fire in a hot, still, unventilated space where it can distort the scavenge belt and the piston. Stopping becomes the correct choice only if the fire persists at reduced speed, the scavenge air temperature continues to climb, a relief valve lifts, or the crankcase oil mist alarm operates.
- If you must stop: stop, engage the turning gear, keep the main lube oil pump running, and smother the scavenge space with CO2 or steam through the fixed connection. Do not open scavenge doors, crankcase doors or air cooler covers until the space has cooled for 30–60 minutes - a rush of air into smouldering deposits causes a flare-up.
- Station a man with an extinguisher and an IR thermometer at the unit, keep the fire party standing by, and inform the bridge before every change of condition.
3. What inspection and planned maintenance actions follow the incident, and what operational changes would you make to prevent recurrence?
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Inspection after controlled cooling:
- No.4 piston withdrawn: ring sticking and ring groove wear, broken or badly worn rings, liner wear and ovality, and the state of the liner lubrication.
- Exhaust valve and seat, valve spindle and guide for burning or leakage.
- Scavenge space: burnt deposits, condition of the drains and their non-return valves, condition of the air cooler, and any distortion of the belt or doors.
- Stuffing box and piston rod gland, since these are the usual route for oil into the scavenge space.
- Turbocharger: compressor fouling, blade condition and bearing play, because surging stresses thrust bearings.
Root causes to close out: worn rings or liner allowing blow-by; over-lubrication or the wrong cylinder oil feed rate; blocked or neglected scavenge drains; a leaking exhaust valve; and prolonged low-load running that lets deposits accumulate.
PMS and operational changes:
- Add scavenge drain checks and blow-through to the weekly routine, and record drain quantities.
- Bring liner wear measurements forward and compare against the wear rate curve.
- Verify the load-dependent cylinder lubricator settings against the fuel sulphur content.
- Avoid sustained running below about 25–30% MCR; if long low-load periods are unavoidable, run the turbocharger water washing and scavenge space inspections at shorter intervals.
- Brief the watchkeepers on the early signs (exhaust gas temperature spread plus scavenge air temperature rise) so the next fire is caught earlier, and raise an SMS observation so the fleet benefits from the finding.