In large slow-speed two-stroke engines, proper cylinder liner lubrication is essential to maintain a protective oil film between the piston rings and cylinder liner. If lubrication is inadequate or improperly controlled, several operational and mechanical problems may occur.

1. Excessive Wear

• When lubrication is insufficient, metal-to-metal contact occurs between the piston rings and the cylinder liner. This results in accelerated wear of both the piston rings and liner surface, ultimately reducing the service life of the engine components.

2. Scuffing and Scoring

• Improper lubrication can cause the breakdown of the lubricating oil film. As a result, deep vertical scratches or scoring marks may develop on the liner surface. If this condition becomes severe, it may lead to piston seizure.

3. Micro-Seizure

• Micro-seizure occurs when localized welding and tearing of metal surfaces takes place between the piston rings and liner. This happens when the lubricating oil film is too thin or insufficient, causing direct metal contact.

4. Corrosive Wear

• Residual fuels contain sulphur, which during combustion forms sulphuric acid. If the cylinder oil does not have a sufficient Base Number (BN) to neutralize these acidic products, the acid can corrode the liner surface, leading to corrosive wear.

5. Piston Ring Sticking

• Poor lubrication and the formation of carbon deposits can restrict the free movement of piston rings within their grooves. This causes piston ring sticking, resulting in poor sealing and increased gas leakage.

6. Blow-by and Loss of Compression

• Worn liners or damaged piston rings allow combustion gases to leak past the piston rings, a condition known as blow-by. This reduces compression pressure, lowers engine efficiency, and increases fuel consumption.

7. Overheating

• Excessive friction due to poor lubrication increases the temperature of the piston and liner surfaces. This overheating may damage the piston crown, piston rings, and cylinder liner.

8. Increased Oil Consumption

• Incorrect cylinder oil feed rates may lead to either excessive oil consumption or insufficient lubrication, both of which negatively affect engine performance and operating costs.

1. Cloverleafing

Description

Cloverleafing refers to an uneven wear pattern on the cylinder liner. The liner develops a lobed or oval shape resembling a clover leaf rather than remaining perfectly circular. This wear pattern usually occurs at specific locations corresponding to the fuel injection points.

Causes

Cloverleafing can occur due to several factors, including:

• Uneven temperature distribution around the circumference of the liner
• Poor fuel atomization, causing localized hot spots
• Incorrect fuel injection timing
• Over-lubrication, which may lead to bore polishing
• High thermal and mechanical stresses acting on the liner

Effects

The consequences of cloverleafing include:

• Poor sealing between the piston rings and liner
• Increased blow-by of combustion gases
• Development of irregular wear patterns on the liner surface

2. Micro-Seizure

Description

Micro-seizure is a condition where localized adhesion occurs between the piston ring and the cylinder liner. Small fragments of metal may tear away from the surfaces, leaving fine scoring marks on the liner.

Causes

Micro-seizure can result from several operating conditions, such as:

• Insufficient lubrication
• Low cylinder oil feed rate
• Low oil viscosity
• Excessive engine load
• Poor distribution of lubricating oil
• Breakdown of the oil film due to high temperatures

Effects

The effects of micro-seizure include:

• Roughening of the liner surface
• Damage to piston rings
• If not corrected, it may develop into major seizure or severe liner damage

Large two-stroke marine engines operating on heavy residual fuel oil (HFO) require cylinder lubricating oil with high alkalinity, commonly expressed as a high Base Number (BN), in order to neutralize the acidic products formed during combustion.

The typical composition of such cylinder oil includes the following components:

1. Base Oil

• The main component is a high-viscosity mineral base oil.
• This base oil provides the primary lubricating film strength required to protect the piston rings and cylinder liner.

2. Alkaline Detergents (High BN Additives)

• Cylinder oils contain calcium-based alkaline detergents.
• These additives neutralize sulphuric acid formed during fuel combustion and help maintain the cleanliness of engine components.
• Typical cylinder oil Base Number (BN) ranges from 40 to 100, depending on the sulphur content of the fuel used.

3. Dispersants

• Dispersants help keep carbon particles and combustion residues suspended in the oil, preventing them from forming harmful deposits on engine components.

4. Anti-Wear Additives

• Anti-wear additives reduce direct metal-to-metal contact between moving parts, thereby minimizing wear of the piston rings and cylinder liner.

5. Antioxidants

• Antioxidants prevent oxidation of the lubricating oil at high temperatures, thereby extending the service life of the oil.

6. Corrosion Inhibitors

• These additives protect metal surfaces from acidic corrosion, particularly the cylinder liner, which is exposed to sulphurous combustion products.

7. Thermal Stability Improvers

• Thermal stability additives ensure that the lubricating oil maintains its film strength and stability at high operating temperatures, which is essential for reliable engine operation.