11.7.
Properties of Lubricating Oil
The quality of a lubricating oil is tested for the following various properties to evaluate its suitability and merits for certain service conditions.
(a) Viscosity. Viscosity is a measures of the flow ability of an oil under a particular temperature and pressure, (refer section 11.6)
(6) Flash Point or Fire Point. The lowest temperatures at which the oil flashes and fires, known as flash and fire points. These two temperatures must be sufficiently high for any lubricating oil to avoid flash or burn during use.
(c) Cloud. The low temperature at which the lubricant changes from liquid state to a plastic or solid state is called cloud point. In some cases the oil appears to be cloudy at the start of solidification.
(d) Carbon Residue. Lubricating oils being the chemical compounds of carbon and hydrogen, when burnt deposit carbon on the engine parts. This should be as low as possible for lubricating oil.
(e) Corrosion. A lubricant should not corrode the working parts.
(/) Pour Point. The lowest temperature at which the oil pours is called its pour point. Below this temperature the oil becomes plastic, so it does not produce hydrodynamic lubrication and therefore cannot be used below this temperature.
(g) Colour. This test is not so important except for checking the uniformity of any given grade of oil.
(h) Dilution of Crankcase Oil. Petrol vapour may escape past the piston rings during the compression stroke, which mixes with oil and affects its lubricating property. The test, which determines the amount of dilution in crankcase oil, indicates the suitability of such oil.
(i) Emulsification. A lubricant when mixed with water tends to separate. The emulsifica-tion number is an index of the tendency of any oil to emulsify with water.
(J) Oxidation at High Temperature. Lubricating oils may break down at high temperature due to oxidation producing hard carbon and varnish, which deposits on the engine parts. Therefore, lubricants must resist oxidation.
(k) Evaporation. Evaporation test is conducted to find the quantity of oil that may evaporate at high temperatures. Lubricating oil should have a low evaporation characteristic.
(/) Sulphur Content. Sulphur in a corrosive form is detrimental in lubricating oil. Thus its presence should be avoided.
(m) Specific Gravity. Specific gravity of lubricating oil varies considerably and hence should not be regarded as the main indication of its lubricating property.
(n) Neutralisation Number. Oil may contain impurities, if not removed during refining, which have deleterious effect on the properties of the oil.
