11.4.
Oil Deterioration
Oil deteriorates with use and requires changing. Deterioration of oil occurs mainly due to contamination and breakdown.
External Contamination.
The hard particles may enter through dust induced into the air-intake-system crankcase breather, and may come from metallic particles worn away from moving parts and casting sand still remaining in the engine after manufacture. Small pieces of dirt may also enter the engine while changing the oil filter, removing the rocker cover when checking tappet clearances, or any other servicing which requires the removal of engine components and exposes the inside of the engine, e.g. removal of the cylinder head, timing gears and chain, distributor drive, etc. Oil may also be contaminated with coolant.
Combustion Contamination.
The most common type of contamination, however, is from the blow-by gases, which work their way past the piston and rings. If crankcase ventilation is poor and the engine is cool, blow-by gases remain in the crankcase and mix with the oil. The constitutes of blow-by gas primarily are partially burned fuel and water vapour. Unburned fuel dilutes the oil. The highly acidic water formed due to combustion causes rusting and corrosion. Both combine with polymerised and oxidised hydrocarbons, produced during combustion, undergo further change in the oil and form sludge, binders that hold organic solids, inorganic salts, wear particles, and fuel soot together. When these particles accumulate sufficiently large, they drop out of the oil and deposit in the engine as a cold-engine sludge.
In diesel engines, the sulphur in the fuel while burning with oxygen produces sulphureous acids. Small amounts of these acids may drain down the cylinder walls, mix with the oil, and attack the bearing materials.
Internal Contamination.
Contaminants may be formed within the engine itself. The oil oxidises over long periods of time, due to high working temperature of the oil in the piston region. Oxidation is the process of the oil combining with the oxygen in the air at elevated temperatures. Higher the oil temperature, the more rapid is the rate of oxidation. Oxidation causes thickening of the oil and formation of varnishes sometimes known as lacquer deposits. These deposits may occur on the piston rings and in the grooves, eventually causing the rings to stick so that compression pressure is lost. This causes a loss of power and an increase in fuel consumption. Since some of the products of oil oxidation are acidic it may corrode journal bearing materials.
Operating Conditions.
The amounts of sludge and deposits to a large extent depend on the mode of vehicle operation. Long journey keeps the engine working over a narrow and designed temperature range. This produces efficient combustion, due to which by-products from burning and water formation are at a minimum. Short running produces high levels of unburnt hydrocarbon products and moisture, and crankcase condensation, which dilute the oil and destroy the lubricating properties of the oil.
The normal maximum engine oil temperature is 394 K. Like engine oil, its additives also tend to deteriorate and are used up along with the oil. When they can no longer do their designed job, the oil loses some of its necessary properties. Engine oil should be changed before sludge develops, oxidised deposits form, and additives lose their effectiveness.
