Cylinder liners are, almost without exception, cast components which at first sight appear to be cylindrical units of no great complexity. However, even though their shape is simple the materials from which they are made are not quite so basic. For many years a good quality cast iron was used in their production. More recently, the worsening quality of fuels has given rise to greater wear rates. This has led to improvements in the liner material quality, to resist wear and to provide the liner with as long a life as possible. In alloying any material there is such a quantum leap in costs that no-operator owner must consider the benefit worth while before he buys in such components.
The self-lubricating property of liners is a result of the ‘graphite’ in the matrix (structure) of the cast iron. The graphite is available in two board groups; flake, and nodular or spheroidal graphite. Both forms provide the iron with self-lubricating properties, although the flake form tends to have slightly better capabilities than the nodular in that respect.
A self-lubricating material is needed because of the difficulty in lubricating the ring pack as it moves up and down the liner surface. The flash point of lubricating oil, broadly speaking, is around 210 deg. The temperature at the opening of the exhaust is frequently at least twice this, so any oil exposed to this temperature will, at the very least, begin to carbonise. The lighter elements will start to voporise, leaving the carbon base, which does not lubricate the ring pack as effectively. It is evident that the lubrication of the ring pack over the TDC of the unit will be particularly difficult, due to the higher pressures as well as the elevated temperatures in that region. These pressures force the rings out onto the liner surface so there is little likelihood of there being an adequate oil film between the ring and the liner. Even under the best conditions, the oil film in a liner is rarely thick enough to ensure total separation of the two materials. Over TDC, therefore, the self-lubricating properties of the liner compensate for the reduced effectiveness of the normal cylinder oil supply. This indicates why the greatest wear can be expected over the top part of the cylinder liner. Lower in the stroke, not only does the temperature fall, but the ring to liner pressure reduces with the reduction in cylinder pressure.
One of the main forms of wear is the so called ‘microseizure’ (mz). Another form of wear is generated by acidic corrosion of the liner surface. This can usually be traced to an attack by sulphuric acid. This acid is formed from the sulphur dioxide generated during combustion. The amount of acid formed is therefore directly related to the amount of sulphur present in the fuel during combustion. In an attempt to combat this in the early days of diesel engines, the jacket temperatures were lifted, the idea being to reduce the amount of acid condensing out at the ‘dewpoint’. Effective as this was, it did not eliminate the problem and, over the intervening years, cylinder lubricants have been developed with elevated reserves of alkaline to combat the acid attack. One of the major problems is the difficulty in matching the alkaline reserve to the sulphur content in the fuel. Sulphur content varies according to the field the oil came from, and will vary from bunker station to bunker station, and quite frequently vary at one terminal, as supplies come from different refineries. A quantifying scale has been developed to indicate the reserve of alkalinity within an oil, called the ‘total base number’ (TBN) or, more frequently now, the ‘base number’. A high number (70) indicates a high alkaline reserve so a fuel with a sulphur content of 3% is best matched by a cylinder oil with a TBN of 70. At the other end of the scale a TBN of 10 may be considered adequate for a sulphur content of 1%. The ‘matching’ of TBN to sulphur content of the fuel being used, as it may have been loaded on top of previous bunkers, and economics make it impossible to carry a wide selection of TBN cylinder oils. It is up to the operators of the vessel to provide an oil that has a TBN roughly compatible with the average sulphur content found in the bunkering stations that the ship visits.
Where the rings cannot seal against the liner (in the heavily corroded areas) blow past will occur, such that:
- the rings are locally overheated and begin to lose their tension;
- the liner surface is overheated locally and begins to lose its strength (wear more easily);
- cylinder lubricating oil is burned off the liner wall and wear rates go up.
Wear rates are difficult to specify. Much depends on:
- the quality of the materials and their compatibility with one another;
- the quality of fuel being used and, equally importantly, the quality of combustion (atomisation, penetration);
- the effective distribution of the correct quality and quantity of cylinder lubricating oil;
- the loading of the engine running temperatures and pressures (even today, raising the jacket water temperatures ‘slightly’ can reduce the amount of sulphuric acid attack).