Main Engine Fuel Changeover with Rising Exhaust and Scavenge Temperatures
At 0340, a 48,000 DWT product tanker is proceeding at 13 knots on a MAN B&W 6S50MC-C main engine at 75% load, burning 1.0% sulphur HFO. The vessel will enter a 0.10% sulphur ECA in two hours. The chief engineer orders changeover to MGO. You are the duty engineer. Ten minutes after starting the changeover, No.3 unit exhaust temperature rises from 352°C to 398°C, turbocharger speed fluctuates between 10,600 and 11,400 rpm, scavenge air temperature rises from 38°C to 47°C, and fuel oil inlet viscosity falls to 9 cSt at 40°C. Fuel oil supply pressure is 7.6 bar and fluctuating by ±1.2 bar. The bridge has not been informed. The engine room is manned, and the scavenge drain inspection glass shows a small amount of dark oil. There is no fire alarm and no visible smoke. The chief engineer is off watch and asleep. What do you do?
1. Using the instrument trends, distinguish between a scavenge fire and a fuel-injection or changeover fault, and state your immediate actions as duty engineer.
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The combination matters more than any single reading. A single-unit exhaust-temperature rise with turbocharger-speed instability could be an injector or exhaust-valve fault, but the simultaneous rise in scavenge-air temperature and dark oil in the scavenge drains points to unburnt fuel collecting in the scavenge space, with a real risk of scavenge fire. The falling viscosity and fluctuating fuel pressure suggest the changeover is too fast or the viscosity/temperature control is not stabilised, so atomisation is poor and afterburning is occurring.
Immediate actions: inform the bridge and wake the chief engineer; reduce load to about 50% or lower; pause the changeover and hold on the current fuel; adjust the fuel-oil heater/viscosity controller to bring viscosity back within the engine builder's range; compare the No.3 fuel-pump index with the other units; check the exhaust valve and injector; monitor exhaust temperatures, scavenge-air temperature, turbocharger speed and scavenge drains closely. If scavenge temperature continues to rise, smoke appears from the drains, or paint blisters, treat it as a scavenge fire: cut fuel to the affected unit, stop the engine as required by the emergency procedure, use the fixed smothering system if fitted, and do not open the scavenge space until it has cooled. Do not simply alarm and continue at full load.
2. Assume the condition stabilises and no fire is found. Explain how you would complete the changeover safely before the ECA boundary.
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I would not resume the changeover at 75% load just because the temperature has stopped rising. The engine should stay at reduced load until the cause is understood and the fuel system is stable. First verify the actual fuel viscosity at the engine inlet against the manufacturer's limit, confirm the heater control valve is responding, and check that the changeover valve has not moved too far too quickly. Then resume in small steps, allowing temperatures and fuel-pump indices to settle at each step. I would monitor exhaust temperatures on all units, turbocharger speed, scavenge-air temperature, fuel pressure and any leakage from low-viscosity fuel. If any unit again deviates, stop the changeover and investigate before continuing.
When the fuel is fully changed over, confirm sulphur content from the bunker delivery note and sampling records, raise load gradually, and inform the bridge of the completed changeover and any load limitation. If the ECA boundary cannot be met safely, the chief engineer and master must decide whether to reduce speed, delay entry, or complete the changeover at a lower load; the decision should be recorded and not left to the duty engineer alone.
3. What notifications and records are required, and what would you tell the chief engineer at handover?
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The bridge must be told immediately because reducing load or stopping the engine affects manoeuvring capability and passage timing. The chief engineer must be informed without delay. I would record in the engine log the time the changeover started, the temperatures, viscosity, fuel pressure, load reductions, actions taken and the final fuel in use. If a defect such as a sticking injector or exhaust valve is found, I would raise a maintenance report and, if the event had safety implications, an SMS near-miss or defect report.
At handover I would state: what happened, the current fuel and load limit, which parameters are still being monitored, what checks remain outstanding, and whether the ECA changeover was completed. I would not falsify or omit the event. If the engine cannot safely complete the changeover, the chief engineer and master need that information early enough to make a commercial and regulatory decision.