I.Symptoms of the fault occurrence
The three generating diesel engines of the Kote nenhai wheel are all Japanese Yanmar diesel engines, model EY26, with a power of 1620KW and a rated speed of 720r/min, which are four-stroke six-cylinder medium-speed diesel engines.
One month had elapsed since the comprehensive overhaul of the No. 2 generator diesel engine. During a routine inspection of the engine room while on duty and refueling during a voyage from Malaysia to Africa, it was observed that the exhaust temperatures of cylinders No. 1 and No. 4 of the No. 2 generator diesel engine were significantly higher than those of the other cylinders. A comparison with the manufacturer's specifications indicated that these temperatures had exceeded the standard values provided in the manual. Despite this anomaly, all other operational parameters of the generator diesel engine remained within normal limits, and no abnormal sounds, vibrations, or odors were detected.
II. Analysis of Possible Causes for the High Exhaust Temperature Fault
The high exhaust temperature of a generator diesel engine can be attributed to several factors, primarily related to the fuel system, intake and exhaust systems, component wear, and other miscellaneous issues. However, the main causes are typically associated with the quality of fuel combustion and air exchange.
1. Fuel System Issues
(1) Excessive Fuel Viscosity or Poor Fuel Quality
1) Excessive Fuel Viscosity
① The Yanmar generator diesel engines on board the Kote Nenhai vessel use heavy fuel oil (HFO) when the load exceeds 25%. If the load does not meet the required threshold during HFO operation, insufficient fuel flow and excessive heat loss can lead to low oil temperature or high viscosity. This results in poor atomization, causing delayed combustion within the cylinder and prolonged afterburning, ultimately leading to elevated exhaust temperatures.
② The fuel system on Kote Nenhai is shared between the main engine and the generator diesel engines. After berthing, when the main engine is shut down, the small fuel consumption of the generator diesel engines makes it difficult for the automatic fuel heating system to maintain optimal temperature. Manual adjustments to reduce steam volume are often necessary. If the steam volume is reduced excessively, it can cause the oil temperature to drop too low, resulting in poor atomization and extended afterburning, thereby increasing exhaust temperature.
2) Poor Fuel Quality
Fuel properties such as cetane number and viscosity directly impact ignition and combustion efficiency. Low cetane numbers prolong the ignition delay period, leading to rough combustion and late ignition, which increases peak combustion pressure and exhaust temperature [3]. Conversely, excessively high cetane numbers can result in incomplete combustion and deteriorated combustion quality, also raising exhaust temperature. High fuel viscosity impairs atomization, leading to incomplete combustion and further increasing exhaust temperature. Therefore, poor fuel quality affects atomization and combustion, contributing to higher exhaust temperatures across the entire generator diesel engine.
(2) Fuel Supply and Injection System Failures
1) Injector Malfunction
High exhaust temperature may be caused by low valve opening pressure due to broken or loose pressure springs, loose lock nuts, improper installation, or stuck injectors. These conditions can lead to early injection onset and delayed injection termination, extending the injection duration and degrading atomization quality. This prevents proper mixing of fuel with fresh air, worsening combustion quality and causing severe afterburning, overheating of components, and increased exhaust temperature. Additionally, an enlarged injector nozzle diameter can impair atomization and combustion, while poor sealing of the injector nozzle's cone surface or erosion-induced holes and cracks can cause fuel leakage, extending the afterburning period and further deteriorating combustion quality. In such cases, the injector should be inspected for carbon buildup, and tests for valve opening pressure, sealing, and atomization quality should be conducted to diagnose the issue.
2) Fuel Injection Pump Failure
Exhaust temperature can also rise due to excessive wear between the plunger and sleeve, reducing sealing performance and injection pressure, leading to poor atomization and a smaller injection advance angle. Severe wear between the valve and valve seat can similarly reduce sealing performance, causing a drop in residual pressure in the high-pressure fuel line and a reduction in injection volume, both of which affect atomization quality and combustion process, ultimately deteriorating combustion quality and increasing exhaust temperature.
3) Damaged Fuel Filter
A damaged fuel filter can degrade the quality of fuel entering the combustion chamber, allowing larger fuel particles that cannot fully decompose and burn, leading to poor atomization and combustion quality, and consequently increasing exhaust temperature. This scenario can be ruled out if only a single cylinder exhibits abnormally high exhaust temperature.
2. Intake and Exhaust System Issues
1) Intake Valve Malfunctions
One possible cause of intake valve malfunction is severe wear on the top of the intake valve stem, leading to excessive valve clearance. This affects the timing of the intake valve, causing it to open late and close early. Consequently, the amount of fresh air entering the combustion chamber is significantly reduced, resulting in lower compression pressure at the end of the compression stroke, incomplete fuel combustion, deteriorated combustion quality, and increased exhaust temperature.
Another potential issue is severe wear on the sealing cone surface of the intake valve or erosion of the valve disc, or the intake valve getting stuck in the open position. These conditions prevent the intake valve from closing tightly, allowing fresh air to leak out of the combustion chamber through the intake valve, reducing air volume, deteriorating combustion quality, and increasing exhaust temperature.
This situation can be diagnosed by measuring the valve clearance of the intake valve and conducting a sealing test on the valve disc.
2) Exhaust Valve Malfunctions
Exhaust valve malfunctions can be caused by several factors:
- Erosion of the exhaust valve disc, leading to small holes.
- The exhaust valve being stuck in the open position.
- Pockmarks and pits on the sealing line of the exhaust valve disc, resulting in poor sealing.
These issues allow fresh air to leak from the exhaust valve, reducing the air volume in the cylinder, lowering the compression pressure, deteriorating combustion quality, and increasing exhaust temperature.
This situation can be identified by checking the sealing condition of the exhaust valve disc and verifying the flexibility of the exhaust valve.
3) Valve Drive Mechanism Malfunctions
Malfunctions in the valve drive mechanism can arise from:
- Loose adjusting screws for the rocker arm that adjusts valve clearance.
- Severe wear on the rocker arm contacting the intake valve stem.
- Bent or deformed pushrods.
- Severe wear on the roller and cam.
These conditions lead to an increase in intake valve clearance, altering the timing of the intake valve, causing it to open late and close early, reducing the amount of fresh air entering the combustion chamber, deteriorating combustion quality, and increasing exhaust temperature.
Additionally, overly tight adjusting screws can result in insufficient valve clearance for both intake and exhaust valves. When the valves reach their maximum operating temperature, they may not close tightly, leading to leakage, intensified erosion of the valve disc, reduced air volume in the cylinder, deteriorated combustion quality, and increased exhaust temperature.
This situation can be checked by measuring the clearance of the intake and exhaust valves.
III. Fault Resolution Measures and Lessons Learned
1. Measures Taken for Handling
1) Upon discovering the malfunction during a routine refueling shift, I immediately proceeded to the generator diesel engine to conduct a thorough inspection of the fault symptoms. Subsequently, I initiated the standby No. 1 generator diesel engine, synchronized the power supply, and then disconnected the No. 2 generator diesel engine. After completing the oil change operation, I decided to shut down the engine for detailed inspection. The initial inspection revealed that the overall position of the throttle lever was within normal limits. Next, I removed the fuel injectors from the cylinder heads and conducted valve opening pressure tests, atomization tests, and sealing tests on all removed injectors.
The tests indicated that the valve opening pressures were slightly lower than specified. After adjusting the valve opening pressures to the required values, the injectors were reinstalled on the generator. Based on the firing sequence (1-5-3-6-2-4), I made minor adjustments to the fuel supply to each cylinder. However, after restarting and running the generator diesel engine for a period, the same high exhaust temperature issue persisted.
2) Following this, I inspected the high-pressure fuel pump and adjusted the fuel supply timing. Despite these adjustments, the exhaust temperature of the affected cylinders remained elevated. Suspecting fouling in the exhaust pipes, I dismantled the exhaust manifolds for inspection but found minimal soot with no abnormal conditions. I then turned the engine over and measured the valve clearances of the intake and exhaust valves for each cylinder. Measurements revealed that the intake valve clearances for cylinders 1 and 4 were significantly larger than the normal working clearance values. Upon closer inspection, it was evident that the intake valve stems and rocker arms exhibited severe wear, and the screws for adjusting valve clearance on the rocker arms were loose. Finally, I adjusted the intake valve clearances to the specified value (0.3 mm) and tightened the adjusting screws. Additionally, I replaced the filter screen at the turbine's intake end. After running the generator diesel engine for some time, the exhaust temperatures returned to the normal operating range.
2. Lessons Learned
Upon reviewing the auxiliary engine log, measuring seawater temperatures, and consulting with the engine room personnel, it was discovered that the exhaust temperatures of cylinders 1 and 4 had been consistently higher than those of the other cylinders for several days while navigating through the relatively cooler waters of the China Sea. Initially, the temperature difference was not significant, leading the chief engineer to overlook the issue and delay investigating the cause of the elevated exhaust temperatures in cylinders 1 and 4. No immediate action was taken to address the excessive intake valve clearance and loose adjusting screws, which accelerated wear between the rocker arms and intake valve stems, further increasing the valve clearance. Over time, the fault worsened, with the exhaust temperature difference approaching critical levels, causing overheating and affecting the service life and safety of the components.
Although no major accidents occurred, the incident serves as a profound lesson. It underscores the importance for engineers to promptly analyze and address any faults, regardless of their apparent severity, to prevent escalation into more serious issues. Engineers must enhance their awareness of equipment safety management, improve daily maintenance practices, and maintain a high level of responsibility to ensure the safe operation of the vessel.