Lubricating oil is often called the blood of an engine. Proper lubrication of all moving parts is essential for the operation of an IC engine. The lubrication primarily decreases the power required to overcome friction and reduces wear between the rubbings and bearing surfaces, thereby increases the power output and the engine service life, and avoids seizure and serious damage of the components.
Additionally the lubricant acts as a coolant, carrying heat away from the bearings, cylinders and pistons. The lubricating film on the cylinder wall acts as a seal to prevent the gases of combustion from blowing by the piston rings and entering the crankcase. Thus the effectiveness of engine lubrication plays an important role in determining the service life and the performance characteristics of an engine.
The lubrication system is consisted of oil pan, oil pump, oil filter, and oil passages. The chapter starts with purpose of lubricating oil and the lubricating system, and the principle of lubrication. Oil additives, deterioration, ratings, properties and classifications have been discussed before discussing the lubrication system and their components. The chapter ends with the lubrication system service, followed by oil pressure and level indicators.
Purposes of Lubricating Oil
An important purpose of engine oil is to lubricate engine parts so that friction and wear are reduced. Lubrication between two moving surfaces results from oil film that builds up to separate the surface and support the load. The lubrication system must provide a continuous flow of oil to all the engine bearings and other lubricated surfaces so that the oil film on each component is maintained to minimize wear. The correct oil viscosity is also essential for reducing friction.
The lubricating oil carries heat away from the component which is lubricated. The oil is returned to the oil pan. Some engines incorporate external oil coolers to assist in oil cooling in the oil pan. It is necessary to keep the oil temperature below the flash point of the oil. An engine oil must also have a high heat resistance so that carbon formation due to break down is kept to a minimum.
The engine oil has an ability to clean all the engine components, which are in contact with it. Additives in the engine oil help the oil to perform its cleaning operation. Carbon formation is cleaned from the pistons and rings by engine oil. Also other engine components, such as valve stems, valve lifters, rocker arms, and camshafts are also cleaned.
The engine oil helps the piston rings to form a tight seal between the rings and cylinder walls. Microscopic irregularities in the piston rings or cylinder walls are filled by the oil film, preventing the escape of combustion chamber gases. The engine oil clings to the metal surfaces and resists the tendency of combustion chamber gases to “blow by” the piston rings.
The oil film also provides lubrication between the rings and the piston ring grooves thus allowing the rings to move freely and therefore to have continuous contact between the rings and cylinder walls. Oil between the engine parts cushions the parts from the shock as the combustion charge forces the piston down. With the additives the oil has ability to minimise scuffing, reduce rusting, resist oxidation, and maintain the oil’s viscosity characteristics. If the oil is too thin it rapidly leaks from the clearances, thus allowing the parts to come in contact, resulting in scoring of the parts. When the thickness is too large, the oil requires excessive power to overcome drag between the rubbing surfaces.
The purpose of the lubrication of engine, in brief are :
(i) It reduces friction and prevents metal-to-metal contact between the working parts of the engine.
(ii) It carries away a considerable amount of heat from the underside of the piston crowns, the valve stems and connecting rod main bearings.
(Hi) It forms a sealing medium between the piston rings and the cylinder walls preventing loss of compression.
(iv) It protects working surface against corrosion.
(v) It removes gritty and carbonaceous deposits of the working surface.
(vi) It cushions the parts against impact and vibration.
(vii) It reduces operation noise.