Basic knowledge of marine diesel engines

1. What are the requirements for the mating parts of a high-pressure oil pump? What faults are likely to occur during use?

Answer: The main components of a high-pressure oil pump are the plunger and sleeve, and the valve and valve seat, two tightly coupled parts.

The plunger must be able to reciprocate freely within the sleeve and maintain a good seal even under high oil pressure. There should be no significant oil leakage. Minor oil leakage is acceptable to lubricate the mating surfaces. Fuel is corrosive to metals, so the plunger and sleeve should be made of steel with high wear and corrosion resistance and sufficient strength, such as special steels such as nickel steel and nickel-chromium steel, and should be surface hardened. The sleeve is typically made of ball bearing steel, although high-quality pig iron is also available. During use, the plunger and sleeve may suffer from excessive wear, galling, binding, and seizure. Excessive wear, often referred to as a "loose core," results in poor sealing, severe oil leakage, reduced injection pressure, poor fuel atomization, and incomplete combustion. Uneven wear on the plungers and sleeves of each cylinder's high-pressure fuel pump can lead to uneven fuel flow to each cylinder, causing unstable diesel engine operation at low loads. Excessive wear of the plunger and sleeve assembly is not only related to material and manufacturing quality, but also to fuel quality, such as viscosity, sulfur content, and impurities. If the plunger is stuck or galled, the high-pressure fuel pump will no longer pump fuel to the cylinders. Seizing and galling can be caused by poor fuel purification and the ingress of impurities into the assembly. Improper installation or insufficient clearances can also contribute. Failure to adhere to proper operating procedures during fuel conversions, resulting in sudden changes in fuel supply temperature, can also cause the plunger to stick and gall the sleeve. The valve and valve seat assembly must not only ensure a leak-proof seal, but also slide freely within the valve seat, opening and closing smoothly and effortlessly. During use, valves and valve seats can also develop faults such as wear, valve stem seizure, galling, and valves becoming permanently open due to debris. These valve and valve seat faults are also primarily caused by poor fuel purification and quality. Similar failures can also occur if the valve is deformed or damaged by impact.

2. How to check the sealing of a high-pressure oil pump?

Answer: Mainly check the sealing performance of the high-pressure oil pump's plunger assembly, the oil discharge (inlet) valve, and the valve seat.

To check a return hole-type oil pump, install a pressure gauge at the oil discharge pipe joint. Manually pry the high-pressure oil pump's lever. When the oil pump pressure reaches the pressure specified in the manual, stop pumping oil and allow the pump plunger to remain stationary. If this pressure is maintained for about half a minute, the high-pressure oil pump's discharge valve is considered to be properly sealed. Remove the discharge valve and check the plunger's sealing using the same method. If the seal is poor, identify the cause and repair or replace the assembly. For return valve-type high-pressure oil pumps, the above method should be used only if the return valve is properly sealed. If a pressure gauge is not available, suffocate the oil pipe and pry the lever using the above method. After oil pressure is established, if the plunger rises very slowly or no longer, the pump's plunger is properly sealed.

3. What precautions should be taken when disassembling a high-pressure fuel pump?

Answer: A high-pressure fuel pump is an extremely precise component. Furthermore, its operation must maintain a certain timing relationship with the diesel engine. Failure to do so will cause changes in injection timing and confusion. Therefore, the following precautions must be taken when disassembling a high-pressure fuel pump:

1) During disassembly, pay attention to the original assembly and relative positions of each pair of components, such as the plunger and sleeve, the oil delivery valve and valve seat, the adjusting gear ring and control rack, the oil pump housing, and gaskets. Mark each component during disassembly and place them together. This is especially important when disassembling several fuel pumps simultaneously. Finally, store or clean all the components in pairs. Do not separate them arbitrarily to avoid mistakes.

2) During disassembly and until reassembly, the high-pressure fuel pump's plunger, sleeve, drain valve, and other precision components must not be impacted by hard objects or placed randomly to avoid damage or contamination. When cleaning the oil sump, place some soft cushioning material at the bottom. After the final rinse with clean oil, store them together. The entire disassembly and cleaning process must not be performed in an environment exposed to fly ash.

3) Do not rotate or adjust the roller assembly, the relative position of the ring gear, rack assembly, or the eccentric shaft of the valve-type oil pump, or any other components, including the adjusting screws, unless necessary. They should generally be reassembled as they were to prevent disruption of injection timing.

4) When removing the high-pressure oil pump, all oil delivery and high-pressure oil pipes should be wrapped and sealed with clean cotton cloth to prevent contamination.

4. What precautions should be taken when inspecting and cleaning a high-pressure oil pump?

Answer: When inspecting a high-pressure oil pump, the most important thing is to ensure a clean environment. The workbench, tools, and all contact surfaces must also be clean.

Pump parts can generally be cleaned with clean light diesel or kerosene. Oil or other coatings on some parts may be dissolved during cleaning. Be careful not to allow these residues to adhere to the smooth surfaces of the parts. Also, avoid scratching or damaging the parts. Precision mating parts such as the oil pump plunger and sleeve should only be cleaned with a soft brush or cloth. If it cannot be immediately wiped off, soak it in oil and then wipe it. Never scrape it with knives, sandpaper, emery, or other hard objects. Carefully inspect the mating surfaces of the plunger and sleeve (the plunger's bevel groove end is most susceptible to wear) and the bevel groove for wear, burrs, or damage. Sometimes, dark gray spots may be found on the mating surfaces, which is often caused by water or acid in the fuel. If there is burrs, spots, or other damage, the entire pair must be replaced as appropriate. Minor oil rust spots or burrs on the large end of the oil pump sleeve can be polished away. If they cannot be repaired, the entire pair must be replaced. If the oil pump plunger and sleeve are intact, check the fit after cleaning. If they are too loose, the entire pair should be replaced. Continuing to use a damaged or loose oil pump plunger and sleeve pair will reduce fuel delivery accuracy and affect diesel engine performance. The cleaning method for the oil drain valve assembly is the same as that for the oil pump plunger sleeve. After cleaning, carefully inspect the contact surfaces and the large end surface of the oil drain valve seat for smoothness.

5. What precautions should be taken when installing the high-pressure oil pump?

Answer: After the disassembled high-pressure oil pump is reassembled, it should be rechecked for correct assembly before installation on the diesel engine. Neglecting this inspection could lead to damage to oil pump components or even shattering the pump body during cranking. Inspection method: For small oil pumps, use the handle of a hammer to press on the plunger guide to see if it can move up and down. For large high-pressure oil pumps, inspect the internal transmission components and external moving parts such as the rocker arm and ejector rod through the designated inspection window to verify that they are properly reassembled. Once confirmed, install the pump on the diesel engine.

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