Recently, there are many cases where the high-speed trunk engine is employed on board high-speed ferries, etc., and the use of synthetic lubricating oil of longer service life (several times longer than that of the mineral oil) is increased from the viewpoint of the resources saving and the ecology (waste oil treatment, etc.) in Europe while the use of the mineral oil is popular in Japan. The synthetic lubricating oil suppresses carbon generation at piston ring zone and generation of lacquer at the liner, and emission from the engine can be reduced by the lower consumption of the lubricating oil.
In the medium-speed trunk engine, the use of the low grade residual fuel is common, and the problem of migration of the unburnt fuel into the lubricating oil becomes serious. Because the compatibility of the highly refined lubricating base oil with the asphaltene in the residual heavy oil is generally bad, the oil companies have developed and provided on the market the lubricating oil with excellent compatibility so that the asphaltene content is not turned into sludge in the lubrication system.
However, only excellent compatibility is just passing the entrance to solve the problem. The asphaltene content in the oil causes degradation of the function specific to the lubricating oil, and also increases the risk of rapid degradation. The medium-speed trunk engine oil having excellent purifying performance of the asphaltene in the system oil, while keeping various conventional performances and excellent compatibility, is now under development, and it is told that the product will soon be put into the market.
Though a little apart from the intention of the marine lubricating oil in this paper, the lubricating oil for the medium-speed trunk engine for IPP (Independent Power Producer) is explained.
In this engine, the severity to the lubricating oil is high attributable to the facts that, in comparison with the marine engines, 1) the normal engine load factor is higher, 2) plant managers familiar to the control of the lubricating oil are insufficient in number, and 3) the cooling capacity is likely to be insufficient, different from the ships where the engine cooling system can make sufficient use of sea water, and the lubricating oil for the marine trunk diesel engines for IPP is required to be examined including the ideal way of its control.
9.2.2 Lubricating Oils for Crosshead Engine
* Cylinder Oil
With the progress of the engine development, the load condition (factors of temperature, pressure, time) for the cylinder oil has been greatly changed. There are some situations incapable of being coped with by the present cylinder oil to the changes in the combustion condition due to fluctuation of the operating conditions and the inconsistent fuel oil quality. Temperature： The liner temperature (the temperature at the position of the top ring at TDC) rises up to 235-240℃.
Though the effect of the thermal stress on the engine design has been sufficiently taken into consideration regarding the temperature rise, the thermal load to the cylinder oil is greatly increased. If only the viscosity change is compared (neglecting the evaporation of the light composition), the following results can be obtained.
(Example)： VI = 100, Viscosity @ 100℃ = 20.61 cSt (SAE50)
If the cylinder oil in the above-mentioned example is required to have the viscosity at 180℃ equivalent to that at 240℃ of the liner, the cylinder oil viscosity grade will not be less than SAE60.
Because the main base oil viscosity of the cylinder oil is of SAE30 class, and at most SAE40 even when the additive is added, it is necessary to use the bright stock or the alternative heavy base material in order to manufacture the cylinder oil of SAE50 class. The product using only the base oil of SAE40 will be strongly requested if high evaporation loss of the low viscosity base oil of the Dump-Bell blend cylinder oil and the low thermal stability of the heavy base material are taken into consideration.
Pressure： Though Pmax was increased to improve the thermal efficiency of the engine, appropriate measures are taken on the design of the engine. On the other hand, the viscosity of the cylinder oil was dropped due to the temperature rise of the liner, and maintenance of the oil film under high pressure becomes difficult. In addition, the increase in Pmax remarkably increases the gas leakage from the ring gap as the wear of the top ring is progressed, forming the scuffing environment.
Time： In the low-speed engine, the fuel oil which is long in combustion time is often burned, and the liner is exposed to the flame or hot gas for a long time. In particular, in the high S/B ratio engine, the temperature control of the liner is difficult and the condition where the cylinder oil is exposed becomes severer. Though the temperature drop of the upper part of the liner is effective for the mechanical wear, a middle to lower part is likely to be supercooled (temperature in the vicinity of the dew point of sulfuric acid), promoting generation of the chemical wear. Today, the condition to use the cylinder oil becomes severer, and it is necessary to review the base oil viscosity and TBN (additive, etc.)under the condition to meet the present situation.
