Friction and lubrication
Here is an introduction to engine lubrication
Lubrication is the introduction of a substance, called a lubricant (e.g. oil to create an oil fi lm), between two moving contact surfaces, to reduce friction. This reduction of friction greatly reduces the wear of the surfaces and thus lengthens their service life. It also reduces the energy required for the movement. Lubrication is important in all moving parts of the vehicle but the engine has the greatest need.
Under a microscope, even the smoothest engine components have a surface that looks very rough. If these surfaces made contact they would rub together, overheat and destroy themselves. To prevent this happening, engines have a lubrication system that pumps or drips a constant supply of oil on all the moving metal components.
Although the basic function of a lubricant is to reduce friction and wear, it may also perform a number of other functions. It carries off generated heat and it helps to form a gas-tight seal between piston rings and cylinders. It also carries away harmful combustion waste products. Lubrication helps to control corrosion by coating parts with a protective film. A detergent added to the lubricant helps to removes sludge deposits.
Some engine oils are refi ned from crude oil to which are added viscosity index enhancers, reduced friction enhancers, antioxygenates, sludge, lacquer and corrosion inhibitors, and cleaning agents for carbon, acids and water. Synthetic and semi-synthetic oils have improved performance for environmental or special purposes. Multigrade oils have been developed to modify the viscosity index and give thin oils at low temperatures that do not become excessively thin at higher temperatures .
Viscosity is a measure of an oil’s resistance to flow ( Fig. 2.115 ). Thin oil will flow more easily than thicker oil. A viscosity index is the measure of a change in an oil’s fl ow rate with a rise in temperature. The higher the viscosity index, the smaller the change in viscosity. Manufacturers’ recommended viscosity ratings generally refl ect the lowest temperature at which the vehicle is being used and may be different for summer and winter use. The viscosity rating is an indicator not of oil quality but of oil fl ow under particular conditions. Some low-grade oils carry recommendations that limit the use of the vehicle, particularly for high engine speeds, heavy loads and long journeys. Engine oils are not normally biodegradable and should not be allowed to enter the environment as either vapour or liquid. Total loss lubrication systems used on small two-stroke engines, such as those on motorbikes and outboard motors, use a ‘petroil’ mixture of petrol and a specially formulated biodegradable oil. Other types of oil should not be used.
Several international standards (described in a later section) describe the properties of an oil, but some original equipment manufacturers (OEMs) are now specifying their own. The safest advice on what oil to use is always to follow manufacturers’ recommendations.
Several international standards (described in a later section) describe the properties of an oil, but some original equipment manufacturers (OEMs) are now specifying their own. The safest advice on what oil to use is always to follow manufacturers’ recommendations.
Boundary lubrication also commonly occurs on some bearings during starting of the engine. Pressure is generated once the engine is moving as a result of the shape and motion of the surfaces. This is known as hydrodynamic lubrication. For example, the rotation of the crankshaft in a bearing forces lubricant into the wedge-shaped space between the shaft and the bearing ( Fig. 2.117 ). The clearance between the bearing and the shaft, the load on the shaft, the speed of rotation and the viscosity of the lubricant have a marked effect on this process.
Figure 2.117 Con rod big end lubrication: 1, bearing shell; 2, load; 3, shaft; 4, lubrication pressure; 5, oil supply.
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