1. Scuffing Incident of a Diesel Engine

In May 2023, a TBD620V12 diesel engine underwent an H-level repair according to the maintenance manual and relevant repair procedures. The bench test was conducted on May 31, 2023. The engine started at 9 AM, and a failure occurred on June 1 at 3 PM.

1.1 Fault Symptoms

1) At 11:00 AM on June 1, the engine was running at a speed of 1500 rpm, with the load gradually increasing from 1125 kW to 1250 kW (100% load). At 1190 kW, the diesel engine suddenly stopped.

   - Inspection of the engine’s moving parts and control system showed no apparent abnormalities, and no fault alarms were triggered.

2) After inspection and electronic control adjustments, the bench test continued.

   - At 3 PM on the same day, the engine was restarted and accelerated to 1500 rpm, with the load increased gradually to 625 kW. After approximately 3 minutes of operation at this load, abnormal engine noise was detected, crankcase pressure alarms went off, and white steam was emitted from the breather pipe. Immediate unloading and shutdown were executed. Field assessments indicated a severe scuffing issue in cylinder A2.

1.2 Damage Overview

1)Piston and Piston Rings: The piston rings were stuck in their grooves. The piston exhibited severe aluminum melting and scuffing marks across its entire circumference.

2)Connecting Rod Assembly: The connecting rod bushing showed significant wear. The big-end bearing was intact, with an inner diameter of 128.13 mm, and no visible damage to the connecting rod.

3)Cylinder Liner: A clear crack was observed in the O-ring area of the liner over a 300° circumference. Aluminum residue was evident on the inner wall, with severe scuffing marks.

4)Other Cylinders: Fine aluminum particles were found in all other cylinder liners, with a distinct scuff mark in cylinder A5.

2. Cause Analysis

A thorough investigation was conducted into the repair process and the bench test. Possible causes of scuffing were analyzed as follows:

2.1 Poor Fitment of Piston, Connecting Rod, and Cylinder Liner:

   - The repair was performed by certified personnel following maintenance protocols. Measurements were within standard limits, ruling out poor fitment as a cause.

2.2 Improper Installation:

   - The repair followed strict guidelines. Tightening torques of all bolts met specifications. The engine had run steadily for 7 hours, making improper installation unlikely.

2.3 Poor Quality Lubricating Oil:

   - CD-40 lubricating oil was used as per engine requirements. The oil was new and the system was thoroughly cleaned before use, ruling out oil quality issues.

2.4 Engine Overload or Overspeed:

   - The engine was operated at the rated speed of 1500 rpm, with a maximum load of 1190 kW (close to but not exceeding 100% load). The failure occurred at 625 kW (50% load), eliminating overload as a cause.

2.5 Inadequate Oil Scraping by Piston Rings:

   - New piston rings were installed. Inspection revealed no oil residue on cylinder walls, indicating good oil scraping performance.

2.6 Insufficient Lubricating Oil Pressure:

   - Recorded oil pressure ranged from 0.46 to 0.48 MPa, meeting requirements. Low oil pressure was ruled out.

2.7 Excessive Contaminants in Lubricating System:

   - No significant contaminants were found in the oil filters, and the scuffing occurred uniformly, ruling out localized contamination.

2.8 Carbon Buildup Causing Debris Entry:

   - This was the engine's first test run after repair, with minimal operation time. There was no significant carbon buildup.

2.9 Inadequate Cooling:

   - Severe melting of aluminum on the piston and liner indicated overheating, suggesting poor cooling performance as a potential cause.

2.10 Water Contamination in Lubricating Oil:

    - A crack was found at the O-ring seal, and signs of poor lubrication were evident, indicating possible water leakage into the oil.

Based on the detailed inspection, the following two primary causes were identified:

1) Poor Cooling: The high-temperature expansion of the piston and cylinder liner due to inadequate cooling led to scuffing.

2) Water Contamination in Oil: Water entering the lubricating oil system compromised lubrication, resulting in severe scuffing.

3. Repair Plan

3.1 Technical Repairs

1) Dismantle the piston and connecting rod assembly of cylinder A2.

2) Replace the cylinder liner and O-ring seals, ensuring proper measurements.

3) Check the connecting rod for damage and perform non-destructive testing.

4) Replace the small-end bushing and big-end bearing, ensuring correct fitment.

5) Install new pistons, rings, and pins, following balance requirements.

6) Inspect the cylinder head for abnormalities and proper valve operation.

3.2 Control System Checks

1) Check the actuator’s gear ring for any anomalies.

2) Verify the functionality of the feedback spring and control rod.

3.3 Hazard Mitigation

1) Clean the oil sump and replace the oil.

2) Inspect all cylinders and clean any remaining aluminum debris.

3) Perform water pressure tests to check for leaks.

4. Quality Assurance

4.1 Conduct a new break-in test according to the test protocol.

4.2 After the test, inspect the connecting rod bearings and examine the oil filters for metal particles.

This concludes the detailed analysis and repair plan for the scuffing incident in the diesel engine, focusing on the prevention of future occurrences.