CRANKCASE L.O CONTAMINATION

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Low speed engines are particularly at risk from crankcase lubricant contamination caused by cylinder oil drainage past the piston rod gland and combustion products. This can lead to severe damage of engine crankcase components and reduction of life of oil which is normally expected to last the lifetime.

There has been a general increase in the viscosity and Base number of crankcase oils over recent years particularly for engines built since the early 1980's. Increased alkalinity, viscosity and in solubles  fuel derived elements such as vanadium and oil additive derived elements such as calcium, suggest that the contamination is from the cylinder oil drainage.

Deterioration of the crankcase oil has led to the expensive necessity of replacing up to 50% of the sump, this is particularly of concern as it is often only a temporary measure.

Four causes are put forward,

1.     New crankcase oil contaminated with new cylinder oil-unlikely

2.     Cylinder oil draining s being recycled and returned to the sump-very likely as it is a common practice to purify oil leaking through the gland, tests done on this purified oil found high amounts of in solubles

3.     Leakage past rod gland- very likely, high pressure scavenge air can blow cylinder oil and dirt past the top scrapper ring and sealing rings into the piston rod drain tank, and even possibly directly into the sump. A problem that worsens with age and wear.

4.     leakage of exhaust valve lubrication system-unlikely

From above the suggestion is the most likely cause for contamination is leakage past the piston rod. It is seen that maintenance of the stuffing box is of the utmost importance. Tell tales and drainage lines should be proved free and use of oil drained from the uppermost drain should not be allowed even after purification due to the high level of contamination which can destroy the properties of the oil in the sump