Possible Sources of Lube Oil Contamination

Possible Sources of Lube Oil Contamination
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We all know that lubrication is the most important factor, whether it may be any engine or any moving parts, where there is enormous friction. Therefore proper lubrication should be done so that the heat produced due to friction is reduced. So, the lubricant used should possess all the desirable properties and there should not be any deterioration (which means a gradual decline in its quality).

Analysis of lubricating oil is a part of the routine maintenance program in which a sample of the lubricating oil will be sent to a laboratory for analysis. The oil may be subjected to tests such as viscosity, fuel oil dilution, water, solid impurities, etc. The detailed analysis of the lube oil is one of the valuable preventive maintenance tools which enable us to identify the problems before a major repair is necessary.

Oil analysis may be done in three parts. They are:

  1. Properties of the lubricating oil
  2. Condition of the lubricating oil (to tell whether it can be used further or not)
  3. Fuel dilution, dirt contamination, excessive bearing wear, etc.,

The oil analysis can increase the machinery’s life, decrease failure, and reduce money paid for repair. (See image below for an example of an oil analysis report. Click the image to enlarge.)

Due to the severe operating load, speed, temperature effect and the introduction of foreign substances into the lube oil system, there are chances that the system gradually lowers the properties and hence it becomes harmful for the engine operation. Therefore it is always good to identify and analyze the possible sources of contamination and monitor them from time to time.

Possible sources of contamination are by the unburnt fuel, carbon, water, acid, solid impurities, etc.

Sources of Lube Oil Contamination


  • In case of a trunk piston engine, the unburnt fuel may leak into the crankcase due to blow-by, because of which there is a reduction in flashpoint, viscosity, and load carrying capacity of the oil.


  • There are minute carbon particles present. Due to the local overheating on the bearing surfaces, carbon particles are formed. Carbon may also be due to blow past gasses leaking past the piston into the crank case. These carbon particles also increase the viscosity, flash point, and affect the load carrying capacity of the lube oil.


  • This is a common problem encountered as a result of leakage from the glands, seawater or freshwater leak from the coolers, from the piston cooling water system, etc. The presence of water thickens the lube oil and there are chances of formation of emulsion.
  • If it is seawater, because of presence of the salts, the bearing surfaces and journal pins get roughened.
  • The corrosion due to water reduces its load carrying capacity and lube oil properties.


  • Sulphur from the fuel oil upon oxidation forms sulphur trioxide and mixes with any water, forming sulphuric acid upon oxidation.
  • Sulphuric acid is extremely corrosive. This acid mixes with excess lube oil and finds access to the crankcase through a defective gland. This occurs in the case of a two stroke engine.
  • In case of a trunk piston engine, due to leaky piston rings and scrapper rings, the combustion gasses bearing sulphur derivatives may contaminate the lube oil.


  • The lube oil may be contaminated by the solid impurities. These may be due to the wear down of the white metal bearing, journal surfaces, corroded parts, sludge from the lube oil storage tanks, and ash from the products of combustion due to the blow past.
  • To remove these solid impurities the lube oil is purified by a method called continuous purification.


Though there are more chances of contamination, modern methods are adopted to avoid contamination, and they are monitored from time to time to avoid contamination.