Lubricants used in automobiles form a unique class by themselves. They have to perform in different
types of vehicles under a wide variety of operating conditions. In the last two decades, the design, efficiency and
manufacture of automotive vehicles have undergone a radical change all over the world. This resulted in a variety of vehicles, both petrol and diesel, which are modern,
fuel efficient, comfortable and high performers.These vehicles would have fount it difficult to operate without progressive advancements in lubricants performance standards
Performance standards for Automotive lubricating oils are established by numerous bodies around the world. In USA, the Society of Automotive Engineers (SAE), the American Petroleum Institute (API) and US military, issue lubricant specifications which are very popular and internationally accepted
In India Lubricant specifications are issued by the Bureau of Indian Standards, which are generally in line with the above International standards.
LUBRICATION OF REQUIREMENT OF INTERNAL COMBUSTION ENGINES
AND BUSH BEARING
Internal combustion engines are two types. Four stroke or Two stroke, petrol and diesel engines. Petrol engines are spark ignited and Diesel engines are of compression ignition type. Depending on the type of engine, methods employed for lubrication vary considerably.
FOUR STROKE ENGINES
For four stroke engines the arrangement most widely applied is pressure circulating system for the main and big end bearings, with a by-pass to the camshafts and other components. Cylinders are lubricated by oil thrown from the big ends, by direct feed from mechanical lubricators, or by a combination of two. Gudgeon pins are lubricated by means of oil leads taken from the big ends or by oil throw. These arrangements apply both to wet and dry sump systems, the former being usually employed in engines of the high speed type, whereas the latter are used for large stationary sets and normally incorporate an oil cooler.
Notable exceptions to these generalizations are horizontal engines of the open-crankcase type and vertical two-stroke crank case compression scavenging engines, both of which usually have ring oiled main bearings and banjo feed to the big end.
BEARING LUBRICATION
Bearing lubrication presents no special difficulties in itself as long as an adequate supply of oil reaches the bearings; this is because the rotation of the journal tends mechanically to build up and maintain an oil film between the bearing and journal surfaces. Bearing failures are due generally, either to the restriction in the supply of oil as the result of mechanical faults or blocked oil-ways, or to break-up of the bearing metal under the repeated pounding to which it is subjected by the power strokes. This latter effect is most likely to occur if there is an abnormally high rate of pressure rise caused by faulty ignition, the use of low grade fuel, incorrect timing, etc.
Circulating systems i internal combustion engines are necessarily enclosed and the oil is exposed to air at moderately high temperatures in the crankcase. Heated air has a very definite oxidizing effect on all oils, an effect which is most pronounced when the oil is in a fine state of dispersion, as it is in the crankcase of an engine during operation. It is therefore desirable that the oil should have high resistance to oxidation, otherwise rapid deterioration of the oil may take place with consequent thickening, sludging and production of acids. With the wider spread use of certain alloy bearing metals which are more susceptible to corrosion than white metal, the need for oils having high oxidation stability has increased, especially in the case of engines running under heavy duty conditions
The more common troubles in the bearing lubrication systems of I.C. engines are that the oil thickens up, thins out, emulsifies or throws down black deposits. The last, by lodging in oil-ways and filters, may interfere with the circulation of the oil and in extreme cases, cause bearing failure. Troubles of this nature can occur, irrespective of the quality of oil used, through contamination of various kinds. Thus thinning of oil is caused by unburned fuel reaching the relatively cool cylinder walls and then findings its way into the crankcase. Partially burnt fuel and carbons, which are the results of imperfect combustion, increase the viscosity of the oil and in time are deposited as sludge. Water may be present as a result of a leak in a gasket or condensation of combustion products due to low operating temperatures; this may lead to sludging through formation of oil-and-water emulsions, particularly if dust or other solid foreign matter is present, even in minute quantities.
To get the maximum life out of a good lubricating oil, it is necessary to pay great attention to conditions of combustion and to prevent the entry of impurities.
CYLINDER LUBRICATION
The reciprocating movement of a piston, wherein it is momentarily stationary at the top and bottom of the stroke, and the high temperature and pressure conditions in the combustion chamber, provide circumstances particularly unfavourable to the maintenance of a continuous oil film between the piston rings and the walls of the cylinder. Cylinder lubrication is therefore a far more difficult problem than that of lubricating the bearings.
For cylinder lubrication the oil must be fluid enough to reach the cylinder walls without delay on starting up from cold and spread over the surfaces rapidly. Thick oils may fail in this respect; moreover they have a greater tendency to form carbon. The oil must be chemically stable to avoid the formation of gummy deposits and should burn once it has reached the combustion chamber.
Many of the operating troubles with the cylinders of internal combustion engines are, however, connected not with the lubricating oil itself but with the combustion of the fuel. Thus, in gasoline engines, the formation of carbon and other deposits may often be traced to faults such as incorrect carburetors adjustment and faulty ignition timing.