Under what circumstances is a cylinder lift inspection required for an engine? What aspects should be considered and checked during a cylinder lift?

Cylinder lifting involves removing the cylinder head of a diesel engine and lifting out the piston for inspection, measurement, and repair of the piston, cylinder head, and cylinder liner. Aside from factory repairs, cylinder lifting inspections on operating vessels depend on two situations:

1. Abnormal accidental cylinder lifting.

2. Periodic planned cylinder lifting. Periodic planned cylinder lifting is determined according to the engine's instruction manual and on a case-by-case basis. After a certain period of operation, cylinder lifting inspection is necessary even if no abnormalities occur. 

Clerly, periodic cylinder lifting inspection is a powerful measure to prevent engine accidents, as it can detect abnormalities early, allowing us to take timely measures to prevent the accident from escalating. The following aspects should be noted when performing this work:

(1) Safety measures before cylinder lifting:

Before cylinder lifting maintenance, safety training should be provided to the personnel involved in the maintenance work, explaining the purpose and method of cylinder lifting, and assigning tasks so that each participant understands what to do, how to do it, and what to pay attention to, ensuring that the entire cylinder lifting work is carried out in an orderly, safe, and rapid manner. 

The following safety measures must also be taken before the maintenance work begins:

① Inspect the lifting tools.

② Close the main start valve and the shut-off valve of the air cylinder.

③ During the entire cylinder lifting and overhaul process, open all indicator valve cocks.

④ Close the turning gear and lock the handle.

⑤ Hang a sign on the control panel indicating that the engine is under maintenance and not to be moved.

(2) Cylinder head disassembly and inspection:

Before starting work, drain the coolant from the cylinders to be lifted, but not all of it.

Before lifting the cylinder head, ensure that all connected pipes and valves have been removed.

After lifting the cylinder head, perform the following checks:

① Check the coolant level.

② Check if the lower mating surface of the cylinder head is flat.

③ Check for cracks in the cylinder head.

(3) Piston and piston ring inspection:

Before lifting the piston, carefully observe whether there is a step on the upper end of the cylinder liner (a protrusion formed due to the piston not moving). If there is a step, grind it off to avoid pulling the cylinder liner when lifting the piston.

During engine operation, the piston is simultaneously subjected to heat, force, and wear, leading to frequent problems such as piston crown burning, cracking, unilateral wear, and piston ring seizure and breakage. 

Therefore, special attention should be paid to the piston head and skirt for these types of damage.

The piston skirt's anti-friction rings sometimes experience uneven wear, and in severe cases, one side may be worn flat. As long as there is no obvious cylinder scoring on the piston skirt (if cylinder scoring occurs, the center should be aligned), replacing the anti-friction rings is not necessary.

When replacing piston rings, pay attention to the following: ① Most piston ring grooves have different types; do not install the wrong type.

② To prevent the rings from being snagged by the air ports and scratching the cylinder liner, the ring ends should be chamfered. However, modern engines do not allow chamfering.

③ Measure the piston ring thickness to ensure it meets the standard. Also, prepare for checking and measuring the ring wear during the next cylinder reassembly to ensure it is within the normal wear range.

④ Place the ring into the smallest diameter section of the cylinder liner, measure the overlap clearance, remove it, and place it in the ring groove in sequence to measure the top and bottom clearances and the back clearance (the ring thickness should be slightly less than the depth of the ring groove).

After confirming everything is correct, rotate the ring 180° left and right in the groove to check for any jamming. Measure the top and bottom clearances of the ring in the groove, and then arrange the overlap directions of each ring by 180°.

(4) Cylinder Liner Inspection: In most diesel engines, the cylinder liner wear is most severe at or near the top dead center of the first piston ring, gradually decreasing as the wear progresses downwards. Severe cylinder liner wear can cause significant cylindricity and ellipticity, which should be checked during cylinder inspection.

During cylinder inspection, the following defects may sometimes be found:

① Longitudinal scratches and abrasions on the inner surface of the cylinder liner, and even cracks.

② Signs of cylinder scoring—the piston is partially against the cylinder (mostly towards the exhaust side).

③ Cracks or breaks in the ribs between the scavenging holes. ④ Oil line wear (shallowing or flattening), oil filling hole blockage.

⑤ Carbon buildup at the exhaust port.

During cylinder inspection, pay close attention to the cylinder liner for the above defects and correct them as appropriate. If the cylinder liner is severely damaged and cannot be repaired, replace it with a spare part immediately.

(5) Precautions during reassembly:

① Before hoisting the piston into the cylinder liner, shake the cylinder lubricator to check if oil flows out from each lubrication point, and apply cylinder oil along the piston rings and the working surface of the cylinder liner.

② All removed copper gaskets should be tempered before reassembly to restore their plasticity. Gaskets damaged during disassembly should not be used and should be replaced; otherwise, it will be difficult to maintain a seal.

③ Install a tapered guide sleeve at the lower end of the piston rod as a guide to prevent the lower flange of the piston rod from jamming and breaking the sealing ring in the stuffing box.

④ Before installing the piston, move the cylinder head to top dead center before hoisting the piston in. ⑤ Before reinstalling the cylinder head, check the piston top for any foreign objects or tools left behind. Then, replace the cylinder head and tighten the nuts evenly.

After complete reinstallation, restore all oil and water valves that were switched on or off during the cylinder reinstallation process to their original positions. Then, purge the cylinder head with water and pump oil and air from the high-pressure oil pump and injectors. Check for any water or oil leaks.

If circumstances permit and it is deemed necessary, contact the driver's cab to conduct a test run after the cylinder reinstallation to troubleshoot any problems early.