Basic knowledge of marine diesel engines

1. What are the stationary and moving parts of a diesel engine?

Answer: Stationary parts include: engine base, engine frame, cylinder block and cylinder liners, cylinder head, etc. Large diesel engines have guide plates on the engine frame; medium and small diesel engines have the engine frame made as a single unit or integrated with the cylinder block.

Moving parts include: piston, piston pin, piston rod, connecting rod, crosshead and slide plate, crankshaft, flywheel, timing mechanism from the crankshaft timing gear to the valves and high-pressure pump, and various pumps driven by the engine itself.

2. What systems are in a diesel engine?

Answer: 1) Fuel system: This includes the fuel pump, coarse and fine filters, pressure relief valve, pressure gauge, high-pressure fuel pump, injectors, etc., and a governor connected to the high-pressure fuel pump.

2) Lubricating oil system: This includes the lubricating oil pump, coarse and fine filters, lubricating oil cooler, pressure relief valve, temperature gauge, pressure gauge, and some engines also have an automatic temperature regulating valve. In dry crankcase diesel engines, two oil pumps are typically installed, one of which is a standby pump. In reversible engines, a reversing valve is added to maintain a consistent oil flow direction.

3) Cooling system: Open systems include seawater pumps; closed systems include seawater pumps, freshwater pumps, freshwater coolers, automatic temperature control valves, expansion tanks, pressure gauges, and temperature gauges.

4) Starting air system: This includes the main starting valve, starting control valve, air distributor, cylinder starting valves, safety valves, pressure reducing valves, and pressure valves.

5) Intake and exhaust system: This includes the exhaust gas turbocharger, air cooler, scavenging air box, intake and exhaust valves, and their timing control mechanisms. Additionally, reversible diesel engines have their own reversing mechanism.

3. What are the structural differences between dry and wet bushings in diesel engines? What are the more complex aspects of ensuring airtightness and watertightness in the cylinder liner structure of a two-stroke engine compared to a four-stroke engine? A: A dry-type diesel engine cylinder liner is a simple thin-walled cylinder fitted inside the cylinder block. The cylinder block already has an inner wall, and the outside of the liner does not directly contact the cooling water. A wet-type liner, after being fitted into the cylinder block, forms a water jacket with the cylinder block, and the outside of the liner directly contacts the cooling water, requiring watertight seals at both the top and bottom. Two-stroke diesel engines have ports on the cylinder, so more watertight seals are needed. For cylinder liners where exhaust gas exits through the upper port of the cylinder, the seals must not only be watertight but also heat-resistant. Therefore, in addition to rubber rings, copper ring seals are added to the exhaust port seals of this type of cylinder liner.

4. How are diesel engine cylinder liners fixed? What issues should be considered during disassembly and assembly?

A: Diesel engine cylinder liners are fixed by the cylinder head pressing tightly against the shoulder of the cylinder block. Some use a copper gasket for sealing between the shoulder of the cylinder liner and the shoulder of the cylinder block. Others do not use a gasket, allowing the shoulder of the cylinder liner to directly contact the shoulder of the cylinder block. Both the upper and lower parts of the cylinder liner must be sealed with heat-resistant and oil-resistant rubber rings to create a cooling space. The lower part of the cylinder liner is not fixed, allowing it to extend freely downwards when heated.

Removing the cylinder liner from a small diesel engine is generally simple. In large diesel engines, greater force is required when lifting the cylinder liner, generally necessitating the use of appropriate lifting tools and hydraulic jacks. When lifting the cylinder liner, ensure the hoist chain is pulled vertically. Before reinstalling the cylinder liner, strictly prevent dirt or scale from accumulating between the cylinder liner and cylinder block contact surfaces, as this can cause assembly difficulties. Therefore, the contact surfaces must be thoroughly cleaned before reinstallation. Additionally, care must be taken to prevent dirt or scale from falling into the oil filling channels of the cylinder liner. The area of the rubber ring inserted into the annular groove should be 90% of the groove's cross-sectional area. After the rubber ring is inserted into the groove, its cross-sectional diameter should be 80%–85% of its free-state cross-sectional diameter. Before fitting the cylinder liner to the outer diameter, the size of the sharp corner and circumference of the rubber ring should be appropriate to ensure sufficient deformation within the groove and a tight fit. If the rubber ring is too loose, it will easily slip out of the groove during installation; if it is too tight, it will be stretched thin. Generally, the inner diameter of the rubber ring in its free state should be 1%–3% smaller than the groove bottom diameter, with a 1–2mm protrusion at the cross-section, which is sufficient to prevent leakage. Before installing the cylinder liner, apply a layer of soft soap or grease to the outside of the rubber ring to reduce friction and prevent damage during installation. All rubber rings used should be dedicated to this purpose.

If the cylinder liner cannot be seated by its own weight or with slight external force at the beginning of installation, it is likely because the rubber ring is too thick. In this case, do not apply excessive force to prevent cylinder liner deformation; check the rubber ring specifications.

After the cylinder liner is installed, a hydrostatic test should be performed to check the sealing performance of the rubber ring. Simultaneously, the cylinder diameter should be measured again to determine if there is any deformation.

5. What are the requirements for a hydrostatic test of the cooling water space after cylinder liner assembly?

Answer: After cylinder liner assembly, a hydrostatic test must be performed on the cooling water space to determine if the shoulder plane and all rubber rings of the cylinder liner can maintain watertightness.

The hydrostatic test pressure is generally specified as 0.4–0.6 MPa or 1.5 times the operating pressure of the cooling water pump used. The pressure should be increased slowly during the test, and there should be no leakage for at least 5 minutes after reaching the specified test pressure.

To ensure the accuracy of the test, air in the cooling water space must be completely eliminated. All shut-off valves, instrument joints, and the test pump connected to the cooling water space must be tight and leak-free.

A high-flow-rate power pump should not be used for the test; a small-displacement hand pump is generally used (however, in actual operation, hydrostatic tests are not performed after cylinder liner replacement on ships, as leaks are generally not observed if the installation is correct).