Page 154 - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

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138 Automobile mechanical and electrical systems

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 ( Fig. 2.114 ).
Viscosity is a measure of an oil’s resistance to fl ow ( Fig. 2.115 ). Thin oil will fl ow
Defi nition 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
Viscosity smaller the change in viscosity. Manufacturers’ recommended viscosity ratings
A measure of resistance to fl ow.
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

Figure 2.114 Oils

Figure 2.115 Viscosity is the resistance to fl ow