Cylinder liners form an integral part of the marine engine. They are fabricated from a cast iron alloy before being machined internally and externally to the required dimensions. Various types of liner wear happen in ships engines, mostly occurring between the piston rings and the liner.
Marine diesel cylinder liners will wear through time, but this wear can be minimized, for example by using the correct grade of high alkalinity cylinder lube oil to combat the acidic sulfur residues produced from the combustion of heavy fuel oil.
The following sections examine the types of liner wear and their causes, with the first section giving an overview of the liner construction and installation to the block.
Overview of a Marine Two-stroke Cylinder Liner
The cylinder liners used in modern marine two-stroke engines are cast from a cast iron based alloy of chromium, molybdenum, and vanadium to increase the wear characteristics. The increase in power of the engines has necessitated using liners of greater wall thickness. However this required the adaptation a different cooling method for the thicker upper section of the liner. (The lower sections required little or no additional cooling.)
Lubrication methods have also advanced, with the new method of lube oil injection being controlled by the engine management system. This ensures the correct amount of oil is injected for optimum efficient lubrication as the piston rings are passing the injection points.
Finally, even modern liners are susceptible to unnecessary wear due to high temperatures, overloading, and use incorrect grade of lube oils.
However even a well maintained liner can be expected to have a wear rate of up to 0.1mm per 1000 hours of service. The maximum allowable wear before replacement is necessary is 1% of the liner diameter.
The causes of excessive liner wear are explained fully in the following sections.
Example of Wartsila Cylinder Liner
Typical MAN B & W Cylinder Liner
Frictional wear takes place between the sliding surfaces between the cylinder liner and piston rings, and can somewhat controlled by adequate cylinder lubrication, but is also affected by:
- Materials of construction
- Surface finish
- Choice of cylinder lubrication
- Piston speed
- Engine load
Other factors that can either increase or reduce the rate of wear include the cylinder working pressure, temperature, and maintenance of piston rings, combustion efficiency, and any contamination from air or fuel.
Corrosion occurs mainly in engines burning heavy fuels, particularly those with a high sulphur content.
It is caused by the acid formed during combustion, and this may be neutralized by the use of high alkaline cylinder oil.
Sulphuric acid corrosion may be caused in the lower part of the liner if the jacket cooling water temperature is too low. This may allow vapors present after combustion to condense. The moisture formed absorbs any sulphur present to form sulphuric acid. This can be prevented by maintaining jacket temperature above the corresponding dew point.
Water vapor will be present from the combustion of hydrogen together with any water present in the fuel. It may be increased if water passes from the charged air cooler.
Abrasion is caused from metal particles, both from the splintering of piston rings and fuel oil, ash being present in some heavy fuels, along with catalytic fines that will act as an abrasive.
These hard particles act as an abrasive material between the piston rings and the cylinder liner causing liner abrasive wear.
Adhesion or scuffing is a form of local welding between particles of the piston rings and liner rubbing surface, resulting in a rapid wear.
This is exacerbated if the lubrication oil film between piston rings and liner is reduced due to excessive temperature, insufficient supply or incorrect distribution of oil, and piston blow-by.
Engines operating on some low sulphur grades of fuel oil may also be prone to scuffing damage.
Clover leafing is a form of wear on cylinder liners due to high sulphur content in the fuel oil. Clover leafing takes place between each pair of lubricating quills.
Cylinder oil is injected with maximum alkalinity from the quills and as it passes down, the alkalinity reduces and acidity increases. This results in acidic etching on the liner surface in the form of leaf scales along the sides of lubricating quills.
- Appearance: micro seizure resembles abrasive wear since the characteristic marks run axially on the liner surface.
- Causes: this is due to irregularities in the liner and piston rings coming into contact during operation. They occur as a result of breakdown of lubrication due to an insufficient quantity of lube oil, insufficient viscosity, or excessive loading.
- Effects: this results in instantaneous seizure and tearing takes place on the liner surface and on the piston rings surface.
Micro-seizure may not be destructive; indeed it often occurs during a running in period. However, it does become destructive if it is allowed to continue.