Main Engine Exhaust Temperature Spread After Fuel Changeover
A 6-cylinder, 2-stroke slow-speed main engine of 9,000 kW has just completed a changeover from HFO (380 cSt) to MGO while manoeuvring in port. Fifteen minutes after the changeover, the exhaust gas outlet temperatures read: No.1 385 °C, No.2 382 °C, No.3 388 °C, No.4 410 °C, No.5 405 °C, No.6 384 °C. The average is 392 °C and the alarm limit is 420 °C. Turbocharger speed is steady, scavenge air pressure is normal, fuel rack indices are within 2 units of each other, and the fuel viscosity controller is holding 3.5 cSt at the engine inlet. The engine is required to answer manoeuvring commands within the next 10 minutes to berth the vessel.
1. Assess whether the engine may be kept running for berthing, and set out the reasoning that supports your decision.
Read the worked answer
The spread is the key datum, not the absolute value. Cylinders 4 and 5 sit about 18–20 °C above the mean, which is roughly 5 % high, while the others are tightly grouped. No cylinder is near the 420 °C alarm, rack indices are matched, and scavenge pressure is normal, so there is no evidence of a gross load imbalance or air starvation. That points away from a blocked scavenge port or a seized turbocharger and towards a fuel-side cause localised to two units - most likely incomplete combustion from a viscosity/temperature transient during the HFO-to-MGO change, or partially fouled injector nozzles on those cylinders.
Judgement: the engine may be kept on load for the short manoeuvring period because the deviation is moderate, stable and below alarm, and stopping the engine to berth is not a realistic option. The decision is conditional, not a clearance. The duty engineer should log the readings, inform the bridge and the chief engineer, reduce the rate of load change, and monitor the two cylinders every few minutes. If the spread widens beyond about 30 °C, if any unit reaches the alarm, or if the deviation persists after the engine is at steady load, the engine should be stopped for injector inspection. The reasoning that must be shown is: quantify the spread against the mean, check it against alarm and against the other cylinders, exclude air and turbocharger causes using scavenge pressure and rack index, then decide whether the residual risk can be managed for the duration of the manoeuvre.
2. The spread persists at steady load. Describe how you would distinguish between a fuel valve fault and a fuel pump timing/quantity fault using only running observations and a simple onboard check.
Read the worked answer
Running observations first. A worn or coked fuel valve usually produces poor atomisation, so the affected cylinder shows a higher exhaust temperature, darker exhaust, and often a slightly higher fuel rack index or higher firing pressure depending on the failure mode. A fuel pump timing or quantity fault changes the injection characteristic: late injection raises exhaust temperature and lowers firing pressure; over-delivery raises both firing pressure and exhaust temperature; under-delivery lowers both. So the discriminator is the pairing of exhaust temperature with firing pressure and with rack index.
Onboard check: read the indicator/peak pressure for the suspect cylinders and compare with the healthy group. If cylinder 4 shows high exhaust temperature but firing pressure in line with the others, suspect the valve (late or poor combustion rather than excess fuel). If firing pressure is also high and the rack index is high, suspect over-delivery from the pump. If firing pressure is low and exhaust temperature high, suspect retarded timing. A simple confirmatory step is to swap the suspect fuel valve with a known-good unit from a spare or from a cylinder with a normal reading and re-check after a short run; if the fault follows the valve, the valve is the cause. Fuel pump timing is checked against the manufacturer's marks only when the engine is stopped, so it is a follow-up action, not a running check. The answer must show the logic chain: symptom pairing, the swap test, and the recognition that timing verification requires a stopped engine.