COMMERCIAL ASPECTS :

CIMAC's working group on Lubricants, has carried out studies on oil consumption of medium speed engines. Medium speed engines are usually lubricated through the so-called 'dry-sump' system. This means that the oil charge, which has been typically between 1-1.3 kg/kw, is set in a tank external to the engine. With such an installation, the oil flow is such that the whole charge of oil passes through the engine every 5-6 mins. Usually the specific lube oil consumption has been about 1g/kwh.

But in today's marine diesel industry there is a tendency to reduce both the quantity of oil in service as well as lube oil consumption.

It is now not unusual to see oil charge at 0.3 kg/kw and specific lube oil consumption as low as 0.2 g/kwh. In such a condition, the whole charge goes through the engine every 1.5-2 mins, but unfortunately topping up may not be sufficient to maintain the proper condition of oil.

CIMAC's working group is studying this problem in depth by examining the parameters affecting most on the oil consumption. They have been found to be :

•              Engine design and conditions

•              Oil quality

•              Fuel quality

•              Oil system quality

•              Operating  Condition

•              Service effect on BN depletion

•              System losses

•              System maintenance.

Under all these eight conditions, it has been found that the oil was subject to degradation caused by the following key factors :

•              Oxidative and thermal stresses of the oil

•              Product of Combustion '

•              Metallic particles as a result of controlled wear at ring/liner surface.

•              Scraps left in the engine during maintenance.

•              Water

•              Corrosion.

The primary concerns about oil degradation relate to alkalinity reserve depletion, viscosity and insolubles. The purpose of the alkalinity reserve (or base number BN) depletion is to project the engine from -the adverse effects caused by the sulphur contents of the fuel. In service, this alkalinity reserve depletes eventually, theoretically, to a stabilished level.

On .the concern regarding viscosity, it appears that due to higher thermal stresses induced by these new trends on lube circuits, viscosity is increasing. Another parameter contributing to viscosity increase is the raw fuel dilution.

With regard to insolubles, it seems that because of combustion of low grade heavy oils and lower quantity of oil in service, the level of insoluble may rise quickly.

The CIMAC working group recommends the careful charting of oil conditions by monitoring  oil  quality.

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