LUBRICATION SYSTEM
As you know that our body requires
fluids like water and also oil in the form of fats like ghee, butter,
cooking oil for maintenance of our system. Similarly lubrication is
required for maintenance of engine. Lubrication circuit is one of the
most important ones in the engine. The engine cannot run smoothly for
more than a few minutes without the lubricating oil.
Whenever two metallic surfaces move
over each other under direct contact, dry or solid friction is
produced. This is due to the irregularities on the two surfaces
interlocking each other. The dry friction thus created produces a lot
of heat and results in wear of the metal surface.
Lubrication system delivers oil to the moving parts of the engine to reduce friction and to assist
in keeping the parts cool.
Objective of Lubrication
- To reduce friction between moving
parts to its minimum value so that power loss is minimized.
- To reduce wear of the moving
parts as far as possible. Apart from these primary objectives,
lubrication also serves other important purposes, which may be
called secondary.
These are as follows:
- To provide cooling effect –
The lubricating oil takes heat from the hot moving parts during its
circulation and delivers it to the surrounding air through the crankcase.
- To provide cushioning effect – The
lubricating oil serves also as a good cushion against the shocks
experienced by the engine. For example, instant combustion of the
fuel in the combustion chamber produces a sudden pressure rise in the
cylinder and the shock goes to the bearings through the piston,
gudgeon pin and the connecting rod. This shock is then absorbed by
the layer of oil present in the main bearings.
- To provide cleaning action –
The lubricating oil serves another useful purpose in providing a
cleaning action. During its circulation it carries away many
impurities, e.g. carbon particles.
- To provide a sealing action – The
lubricating oil also helps the piston rings to maintain an effective
seal against the high pressure gases in the cylinder from leaking out
toward the crankcase side. Other than engine, lubricants are used
for protecting following components also: Gearbox Differential
Steering gear box In different joints grease is used as lubricant.
The main parts of an automotive engine which require lubrication are:
- Main crankshaft bearings
- Big end bearings
- Gudgeon pin bearings
- Piston rings and cylinder walls
- Timing Gears
- Camshaft and camshaft bearings
Properties of a good lubricant
As we understand that the lubricant is a very important component of an engine. We must know
some of the properties.
1. Viscosity: In simple language, Viscosity may be considered as the resistance of the
lubricating oil to flow. It is this property alone, due to which, the bearing surfaces are kept
apart, i.e. hydrodynamic lubrication is maintained.
The viscosity of the lubricating oil should be just sufficient to ensure hydrodynamic
lubrication. A higher value than this would be of no use since it will involve higher power
losses due to the increased oil resistance.
The viscosity of the lubricating oil, at the time of starting the engine, should be low
otherwise the engine may not start. On the other hand with the engine running, the oil
viscosity decreases due to increase of temperature, which is contrary to what is desirable,
since at all operating temperatures, the viscosity should not fall below the minimum value
required to maintain hydrodynamic lubrication. Therefore, the viscosity should ideally
remain the same at all temperatures. Relative change of viscosity with temperature is
called viscosity index.
However, the viscosity of all the oils does vary with temperature
and, therefore, the oil with minimum variation is preferred.
Viscosity is the most important property of lubricants and they are mostly selected on the
basis of their viscosity and temperature coefficient of viscosity (viscosity index).
2. Physical Stability: The lubricating oil must be stable physically at the lowest and the
highest temperatures encountered in practice. There should not be any separation of
solids at the lower temperatures and at higher temperatures it should not vapourise
beyond a certain limit.
3. Chemical Stability:At higher temperature the oil should remain chemically stable. There
should not be any tendency for oxide formation, many of the oxidation products being
sticky substances clog the lines and cause faulty piston rings and valve action.
The oil should also not decompose at high temperatures to form carbon. Carbon thus
formed, lowers rings efficiency, thus reducing engine compression. The spark plugs and
the valves also do not function efficiently due to sticking of carbon particles.
4. Resistance against Corrosion: The oil should not have any tendency to corrode the
pipe lines, crankcase and other engine parts with which it comes into contact.
5. Pour Point: The minimum temperature at which the oil will pour is called its pour point.
Obviously, since the oil will not be able to flow below the pour point, it cannot be used
below this temperature for lubrication. Thus, the pour point of the oil should be less than
the lowest temperature encountered in the engine.
The pour point is the temperature at which a lubricant just ceases to flow when cooled
under standard conditions. The lower the pour point, the better is the lubricant.
6. Flash Point: The flash point of the oil should be sufficiently high so as to avoid flashing of
Automobile Technology Level-2 17
oil vapours at the temperatures occurring in common use. A flash point higher than the
minimum desired value will not serve any useful purpose.
Flash point of a lubricating oil is the minimum temperature at which it gives off sufficient
vapour so as to form an explosive mixture with air. If the oil is heated further a stage will
reach when it will begin to burn continuously on applying a flame to it. This temperature is
called the fire point.
7. Cleanliness: The oil should be sufficiently clean and stable itself so that the crankcase
and oil lines are kept clean. Further it must contain agents, called detergents, which
remove the impurities from the engine parts during oil circulation. These impurities may
either be filtered out or removed with the change of oil at periodic intervals.
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