The cylinder head of the diesel engine produced by the Wuxi Diesel Engine Factory of FAW Jiefang Company typically uses a nozzle hole copper sleeve sealing structure. This design is crucial for separating cooling water from fuel oil, ensuring proper engine function and preventing contamination. To meet increasingly stringent energy-saving and environmental regulations, the factory introduced a fuel return system, allowing excess fuel to be redirected back to the oil pump for reuse. However, a persistent challenge has been the leakage of cooling water into the fuel return passage due to issues with the copper sleeve press-fit process.
**Processing Process Analysis**
As shown in Figure 1, the copper sleeve for the nozzle hole is machined using imported machining centers and special planes, ensuring that size and positional accuracy meet all design specifications. The artificial deflashing process removes surface flash, while automatic cleaning ensures the cylinder head is free from debris before assembly. Manual chip blowing is used for any residual chips that couldn't be removed during casting. The sealing ring is manually placed into the groove, followed by an automated press that expands the copper sleeve using an expansion rod. Sealing and leak testing are then conducted to detect any air or water leaks.
**Troubleshooting and Improvement**
Leakage in either the large or small end of the copper sleeve can cause serious malfunctions. A manual plugging device was used to test 20 randomly selected leaking cylinder heads, revealing that all nozzle holes had leaks. A fishbone diagram (Figure 2) was employed to analyze the root causes, focusing on human, machine, material, and process factors.
Human error played a role, as some operators failed to monitor rework processes thoroughly, leading to missed defects. Chips from previous machining steps sometimes entered the sealing groove, causing contamination. To address this, training was implemented, and strict protocols were enforced for chip removal and rework documentation.
Machine-related issues included misalignment between the copper sleeve mechanism and the nozzle hole center, which could cause collisions and leaks. Special inspection tools were introduced to align the pressure bar with the nozzle hole axis, but the improvement was limited.
Material and process factors were also examined. The copper sleeves were checked for scratches and surface quality, and the sealing rings were upgraded to more durable materials. Despite these efforts, the main causes of leakage were found to be casting defects and scratches on the copper sleeve.
After extensive testing, two primary causes were identified: casting shrinkage or blistering in the nozzle hole, and scratches on the copper sleeve’s surface. These issues were addressed through tighter quality control with the casting supplier and improved inspection methods.
**Conclusion**
Through comprehensive improvements in casting quality, copper sleeve manufacturing, and internal process controls, the company significantly reduced the leakage rate. Before the improvements, 54 parts were scrapped monthly due to casting defects, but this number dropped to just 3 after the changes. The copper sleeve leakage rate fell from 4.7% to 0.87%, demonstrating the effectiveness of the measures taken.
This case highlights the importance of a systematic approach to problem-solving. By analyzing each factor—human, machine, material, and process—the company was able to identify and resolve the root causes of leakage. The improvements not only enhanced product quality but also strengthened the company’s overall production capabilities, providing a valuable reference for similar sealing challenges in the future.
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