1. The Harm of Diesel Engine
Diesel engine failures, often referred to as "burnt tiles," are common quality issues that occur during the testing and operation of diesel engines. If a bearing bush is damaged, the maintenance process becomes complicated. In minor cases, the technician must replace the bearing bush, which increases workload and may not guarantee repair quality. In severe situations, the entire diesel engine needs to be removed from the locomotive for disassembly and repair. When the bearing bush is burned, the crankshaft journal can become deformed due to overheating, leading to discoloration and structural damage. The main bearing hole in the engine block also gets affected, requiring replacement of multiple components, increasing repair time and costs. Moreover, the diesel engine is the core of the locomotive's power system. If it fails while in operation, it can lead to serious accidents, causing significant economic losses and damaging the reputation of the manufacturer, while also disrupting railway transportation.
To prevent such failures, manufacturers have implemented various preventive measures targeting the root causes. However, due to the complex nature of these issues, it is still challenging to completely eliminate them. To reduce economic losses, this article explores the repair methods used after a diesel engine has suffered a failure.
2. Structure of the Main Bearing and Accuracy Analysis of the Body
The engine body serves as the structural framework of the diesel engine. The main bearing holes are formed by assembling the main bearing housing with the cover using bolts. Once the bearing bush and crankshaft are installed, the main bearing is completed. This type of bearing is an open sliding bearing made up of two semi-circular alloy segments. It relies on pressure lubrication to form an oil film that supports the load, ensuring smooth engine operation.
The engine body is typically made from high-strength ductile iron, and the precision of the main bearing holes is critical. These holes serve as reference points for other key features like camshaft and cylinder holes. A new high-power, low-emission, and fuel-efficient diesel engine is produced with strict machining tolerances. The body contains nine main bearing holes, each with a diameter of 265+0.0320 mm, cylindrical tolerance of 0.025 mm, and straightness of 0.03 mm over the full length. Machining is done using a gantry machining center, with precise measurements taken using collimators, bridge gauges, and coordinate measuring machines to ensure compliance with design specifications.
3. Condition of the Main Bearing Hole After Engine Failure
When the crankshaft operates inside the main bearing hole, if the oil film thickness is insufficient, the surface roughness of the parts may cause the oil film to break down. This leads to direct contact between the crankshaft and the bearing, resulting in dry friction. Such conditions cause rapid wear and overheating, potentially melting the bearing alloy and leading to severe damage. Post-failure inspection of several diesel engines shows that the main bearing hole diameters have decreased, the cylinder bore is out of tolerance, adjacent bearings and full-length straightness are off, and in some cases, the bearing caps are stuck. On the crankshaft, the surface of the main bearing hole is deformed due to stress.
4. Repair Methods for the Engine Body After Failure
Since the main bearing hole serves as a reference for other critical features, maintaining its accuracy is crucial. The most important aspect of repairing the main bearing hole is keeping its center position consistent. Any shift in the center can affect the alignment of other components, making the body unsuitable for reuse. The most efficient method is to replace the main bearing cap and re-machine the hole. The main bearing cap is a weak point that is prone to deformation, especially after a failure. Replacing it allows for correction of the hole’s geometry and ensures accurate alignment.
The repair process includes: disassembling the engine, cleaning the body, reassembling the bearing caps according to specifications, performing three-coordinate measurements, replacing damaged caps, calibrating the machine tool, manually checking the results, and finally cleaning and preparing the body for reassembly.
5. Inspection After Body Repair
After repairs, a thorough inspection is essential. This includes checking the repaired areas and retesting non-repaired critical parts such as the camshaft and cylinder bores. Key inspection steps include manual measurement of inner diameters and surface roughness, three-coordinate measurements of diameter, cylindricity, and straightness, and pressure tile testing to ensure proper contact between the bearing and the hole. Pressure tile inspection involves applying red powder to the bearing and checking the contact area, ensuring at least 80% coverage for optimal performance.
6. Conclusion
The key to repairing a failed diesel engine body is maintaining the center position of the main bearing hole. Replacing the main bearing cap is the most effective method. For severe damage, coating or welding repairs may be necessary, but care must be taken to control the amount of material applied and to manage stress after welding. Proper cleaning and preparation before reassembly are also crucial to avoid future issues. After several successful repairs, the bodies met all operational requirements, proving that effective repair methods can restore functionality and extend the life of the engine.
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