Basic knowledge of marine diesel engines

1. Why is a high-pressure fuel pump required to be zeroed? How is a return port type high-pressure fuel pump zeroed?

A: The purpose of zeroing is to ensure that when the oil adjustment handle (wheel) is in the stop position, all high-pressure fuel pumps immediately stop supplying fuel, ensuring the diesel engine can be shut down quickly.

2. If, during normal operation, a diesel engine needs to temporarily stop fuel supply to a cylinder for some reason, how do you properly deactivate the high-pressure fuel pump for that cylinder?

A: During navigation, a vessel may need to temporarily stop fuel supply to a cylinder for various reasons, such as replacing a high-pressure fuel pipe or experiencing a cylinder malfunction. 

In such cases, shutting off the fuel pump's inlet valve should not be used to deactivate the high-pressure pump. 

This is because if the pump continues to operate without fuel supply, the plunger may become stuck, or at least the sleeve may become excessively worn. 

The correct method for deactivating the pump is to lift the plunger's roller to disengage it from the fuel pump cam. 

The method for lifting the roller depends on the diesel engine model.

3. What is the structure and working principle of the injector?

A: The fuel injector, also called a fuel head, is used to spray fuel delivered by the high-pressure fuel pump through the high-pressure fuel pipe into the combustion chamber in a well-atomized state. 

Modern diesel engines use hydraulically actuated valve injectors (also known as closed-type injectors). While their structural forms and sizes vary between older engines, their operating principles and main components remain the same.

4. What precautions should be taken when disassembling and installing the fuel injector on the engine? What should be inspected and cleaned after disassembly?

A: The fuel injector is a very precise coupling component, and its interior must be kept extremely clean. 

When removing it from the engine, wrap any loose fuel pipes with a clean cloth, and plug the injector holes in the cylinder head with a wooden plug or other object. 

After removing the injector, inspect the nozzle oil holes for carbon deposits. After cleaning the exterior, perform a test pump inspection. 

When disassembling the injector, first loosen the spring adjustment screw to prevent damage to the needle valve core. 

When reinstalling the injector, inspect the seating surface between the injector and the combustion chamber, ensuring it is clean and smooth. 

Then, carefully reassemble and connect the pipes. During the test run, check the sealing surface between the injector and the combustion chamber and all pipe joints for leaks. 

After disassembling the injector, clean and store the disassembled parts properly, avoiding damage to precision components such as the needle valve.

Check the surface finish of the needle valve core, ensuring it slides freely within the guide, but no gaps are detectable. Use a wire slightly thinner than the nozzle to clear each nozzle. Be careful not to let the wire break into the hole. 

Check the spring tension of the injector spring for deformation and cracks. Check that the ejector rod in the injector is not bent and that the wear in the recess where the rod end meets the spring seat is normal. 

During reassembly, thoroughly clean and purge the disassembled parts with clean diesel and compressed air. Do not wipe the precision joints with old cotton cloth or touch them with your hands. Dip them in clean diesel and brush them before reassembly.

 During reassembly, first install the ejector rod, spring seat, and spring into the injector body (do not tighten the spring yet). Then, secure the nozzle to the injector body with the locknut. At this point, place the injector on a test bench for a pump pressure test.

5. How should I perform a pump test on the injector?

Answer: The main areas of inspection and adjustment for the injector are its valve opening pressure, atomization quality, and the sealing performance of the needle valve assembly. This inspection is performed using a manual oil pump test device. 

To determine the injector's condition, perform some necessary cleaning on the exterior of the injector before performing a pump test. Do not disassemble it.

 Place the injector on the test bench, connect the oil line, loosen the bleed screw on the injector, and pressurize the oil pump to release air until oil begins to escape. 

Then, tighten the screw. Afterwards, perform the following pressure test:

1. Valve opening pressure test: Slowly depress the oil pump lever to gradually increase the pressure. 

Pay attention to the pressure indicated on the pressure gauge until the needle valve opens and emits a slight spraying sound. 

At this time, the pressure gauge pointer suddenly jumps, which is the valve opening pressure value. Practice has shown that after a long period of use, the valve opening pressure of the injector will generally decrease. 

It should be adjusted according to the value specified in the manual. 

2. Needle valve assembly sealing test: The sealing test can be divided into valve face sealing and needle valve guide face sealing test.

(1) Valve face sealing test: Wipe the nozzle dry and pay attention to increase the oil pressure to and maintain it lower than the specified valve opening pressure. Check that there is no oil dripping from the nozzle. Slight moisture is allowed. 

(2) Needle valve guide face sealing test: Similarly, increase the oil pressure to below the valve opening pressure, release the oil pump pressure rod, and let the oil pressure drop by itself. Note the time required for the pressure drop to 4.91Mpa. 

For a good nozzle, it should be no less than 6 seconds. If the needle valve moves smoothly along the guide surface and the pressure drop time is short, check the seal between the nozzle and the injector body before repeating the test. 

If the nozzle and injector body contact surfaces are intact and the seal is good, but the pressure drop time is too short, this indicates a poor seal between the needle valve guide surfaces due to wear, resulting in leakage. Consider replacing the needle valve assembly.

3. Atomization Quality and Oil Drip Check: If all the above checks are normal, close the buffer valve in front of the pressure gauge and pump rapidly to check the atomization angle, fineness, and uniformity.

A crisp "chirping" sound should be heard during the spraying of the fuel. Finally, pump rapidly 10 times continuously. If there is no oil dripping or slight "leakage" at the nozzle, the fuel is considered normal.

6. Based on the test pump results, how can you diagnose and address various injector faults?

Answer: 

1. The spray hole is clogged, preventing atomization and forming oil droplets.

Troubleshooting: Clear and clean the spray hole before repeating the pump test.

2. The needle valve is not functioning properly, the spray direction is unilateral, the oil droplets are large, and they are not fully atomized.

Troubleshooting: Check the flexibility of the needle valve.

3. Atomization is good, but the valve is not closing properly, resulting in oil dripping.

Troubleshooting: Lay the valve and valve seat.

4. The spray is functioning properly, the spray pattern on the paper is evenly distributed, and the nozzle tip is clean.