I.C. engine is made of many moving parts. Due to continuous movement of two metallic surfaces over each other, there is wearing of moving parts, generation of heat and loss of power in the engine. Lubrication of moving parts is essential to prevent all these harmful effects.
Purpose of Lubrication in an I.C. Engine:
Lubrication produces the following effects:
(a) Reducing frictional effect
(b) Cooling effect
(c) Sealing effect, and
(d) Cleaning effect
(a) Reducing Frictional Effect:
The primary purpose of the lubrication is to reduce friction and wear between two rubbing surfaces. Two rubbing surfaces always produce friction. The continuous friction produces heat which causes wearing of parts and loss of power. In order to avoid friction, the contact of two sliding surfaces must be reduced as far as possible. This can be done by proper lubrication only. Lubrication forms an oil film between two moving surfaces. Lubrication also reduces noise, produced by the movement of two metal surfaces over each other.
(b) Cooling Effect:
The heat, generated by piston, cylinder, and bearings is removed by lubrication to a great extent. Lubrication creates cooling effect on the engine parts.
(c) Sealing Effect:
The lubricant enters into the gap between the cylinder liner, piston and piston rings. Thus, it prevents leakage of gases from the engine cylinder.
(d) Cleaning Effect:
Lubrication keeps the engine clean by removing dirt or carbon from inside of the engine along with the oil.
Lubrication Theory:
There are two theories in existence regarding the application of lubricants on a surface:
(а) Fluid film theory and
(b) Boundary layer theory
(a) Fluid Film Theory:
According to this theory, the lubricant is supposed to act like mass of globules, rolling in between two surfaces. It produces a rolling effect which reduces friction.
(b) Boundary Layer Theory:
According to this theory, the lubricant is soaked in rubbing surfaces and forms oily surface over it. Thus the sliding surfaces are kept apart from each other, thereby reducing friction.
Types of Lubricant for an I.C. Engine:
Lubricants are obtained from animal fat, vegetables and minerals. Lubricants made of animal fat, does not stand much heat. It becomes waxy and gummy which is not very suitable for machines.
Vegetable lubricants are obtained from seeds, fruits and plants. Cotton seed oil, olive oil, linseed oil and castor oil are used as lubricant in small simple machines.
Mineral lubricants are most popular for engines and machines. It is obtained from crude petroleum found in nature. Petroleum lubricants are less expensive and suitable for internal combustion engines.
A good lubricant should have the following qualities:
(i) It should have sufficient viscosity to keep the rubbing surfaces apart.
(ii) It should remain stable under changing temperatures.
(iii) It should keep lubricated parts clean.
(iv) It should not corrode metallic surfaces.
Lubricating Oil Tests in an I.C. Engine:
There are number of tests for lubricating oil.
Some important tests are given below:
1. Viscosity Test:
The viscosity of a fluid is the measure of its resistance to flow. The oil should be viscous enough to maintain a fluid film between the bearing surfaces. In order to measure viscosity of lubricating oil, an instrument called Saybolt viscometer is used. To express relative weight of oils, the term light, medium and heavy are generally replaced by viscosity number, developed as standards by the American Society of Automotive Engineers.
2. Flash Fire Test:
When oil is heated to its approximate boiling point, vapour is given off fast enough to burn when contacted with a spark of a flame. The temperature at which vapour flashes momentarily is called Flash point and that at which a flame persists approximately for five seconds is called Fire point. Flash point test is done to indicate the fire hazards of petroleum products as well as for purpose of identification and classification.
3. Pour Point Test:
The temperature at which the oil just flows under prescribed conditions is known as Pour point.
4. Gravity Test:
It is determined by using a hydrometer, calibrated to read in API degree. The property of gravity is of importance in the control of refinery operations.
5. Colour Test:
Colour test is described in terms of colour by reflected light or transmitted light. Colour test does not indicate much about quality of the oil. It is not a reliable test.
6. Carbon Residue Test:
Method of finding the amount of carbon residue when a given sample of oil is heated and evaporated under predetermined conditions is called Carbon residue test.
Engine Lubricating System of an I.C. Engine:
The lubricating system of an engine is an arrangement of mechanism and devices which maintains supply of lubricating oil to the rubbing surface of an engine at correct pressure and temperature.
The parts which require lubrication are:
1. Cylinder walls and piston
2. Piston pin
3. Crankshaft and connecting rod bearings
4. Camshaft bearings
5. Valve operating mechanism
6. Cooling fan
7. Water pump and
8. Ignition mechanism
There are three common systems of lubrication used on stationary engines, tractor engines and automobiles:
1. Splash system
2. Forced feed system and
3. Combination of splash and forced feed system.
1. Splash System:
In this system, there is an oil trough, provided below the connecting rod. Oil is maintained at an uniform level in the oil trough. This is obtained by maintaining a continuous flow of oil from the oil sump or reservoir into a splash pan which has a depression or a trough like arrangement under each connecting rod.
This pan receives its oil supply from the oil sump either by means of a gear pump or by gravity. A dipper is provided at the lower end of the connecting rod. This dipper dips into the oil trough and splashes oil out of the pan. The splashing action of oil maintains a fog or mist of oil that drenches the inner parts of the engine such as bearings, cylinder walls, pistons, piston pins, timing gears etc.
This system is usually used on single cylinder engine with closed crankcase. For effective functioning of the engine, proper level of oil is maintained in the oil pan. Lubrication depends largely upon the size of oil holes and clearances. This system is very effective if the oil is clean and undiluted.
Its disadvantages are that lubrication is not very uniform and when the rings are worn, the oil passes the piston into combustion chamber, causing carbon deposition, blue smoke and spoiling the plugs. There is every possibility that oil may become very thin through crankcase dilution. The worn metal, dust and carbon may be collected in the oil chamber and be carried to different parts of the engine, causing wear and tear.
2. Forced Feed System:
In this system, the oil is pumped directly to the crankshaft, connecting rod, piston pin, timing gears and camshaft of the engine through suitable paths of oil. Usually the oil first enters the main gallery, which may be a pipe or a channel in the crankcase casting. From this pipe, it goes to each of the main bearings through holes.
From main bearings, it goes to big end bearings of connecting rod through drilled holes in the crankshaft. From there, it goes to lubricate the walls, pistons and rings. There is separate oil gallery to lubricate timing gears. Lubricating oil pump is a positive displacement pump, usually gear type or vane type.
The oil also goes to valve stem and rocker arm shaft under pressure through an oil gallery. The excess oil comes back from the cylinder head to the crankcase. The pump discharges oil into oil pipes, oil galleries or ducts, leading to different parts of the engine. This system is commonly used on high speed multi cylinder engine in tractors, trucks and automobiles.
3. Combination of Splash and Forced Feed System:
In this system, the engine component, which are subjected to very heavy load are lubricated under forced pressure, such as main bearing, connecting rod bearing and camshaft bearing. The rest of the parts like cylinder liners, cams, tappets etc. are lubricated by splashed oil.
Oil pump – Oil pump is usually a gear type pump, used to force oil into the oil pipe. The pump is driven by the camshaft of the engine. The lower end of the pump extends down into the crankcase which is covered with a screen to check foreign particles. A portion of the oil is forced to the oil filter and the remaining oil goes to lubricate various parts of the engine.
An oil pressure gauge fitted in the line, indicates the oil pressure in the lubricating system. About 3 kg/sq. cm pressure is developed in the lubrication system of a tractor engine. If the oil pressure gauge indicates no pressure in the line, there is some defect in the system which must be checked immediately. Lubricating oil pump is a positive displacement pump.
Oil Filters and Other Accessories Used in an I.C. Engine:
Oil Filter:
Lubricating oil in an engine becomes contaminated with various materials such as dirt, metal particles and carbon. An oil filter removes all the dirty elements of the oil in an effective way. It is a type of strainer using cloth, paper, felt, wire screen or similar elements.
Some oil filter can be cleaned by washing, but in general old filters are replaced by new filters at specified interval of time prescribed by manufacturers. Wearing of parts, oil consumption and operating cost of an engine can be considerably reduced by proper maintenance of oil filters.
Oil filters are of following two types:
i. Full-flow filter and
ii. By-pass filter
i. Full Flow Filter:
In this filter, the entire quantity of oil is forced to circulate through it before it enters the engine. A spring loaded valve is usually fitted in the filter as a protection device against oil starvation in case of filter getting clogged. Filter element consists of felt, cloth, paper and plastic. All these elements are replaceable and should be changed after the recommended period.
ii. By-Pass Filter:
In this type of filter, the supply lines are from the pump and are connected to permit only a part of the oil through the filter, the balance oil reaches directly to the engine parts. Over a period of operation, all the oil in the crankcase passes through the filter.
Other Accessories Used in an I.C. Engine:
I. Oil Pressure Gauge:
Oil pressure gauge is used to indicate the oil pressure in the oil lines. It serves to warn the operator of any irregularity in the system.
II. Crankcase Breather:
The engine crankcase is always fitted with some kind of breather, connecting the space above the oil level with the outside atmosphere. The purpose of the breather is to prevent building up pressure in the crankcase. During operation of the engine, the crankcase oil reaches a temperature of 160°F to 180°F or even more and simultaneously the air above it gets heated up. Consequently the air is likely to expand and cause pressure rise if it were unable to escape.
The breather serves as ventilating passage of air. It enables the water of combustion to escape as vapour. It prevents excessive accumulation of volatile fuel or unburnt fuel. It also reduces the corrosion and fuel dilution. The simplest type of breather consists of a pipe from the upper part of the crankcase to the outside atmosphere. In practice, an air filter is mounted for breathing purpose.
III. Relief Valve:
Relief valve is provided to control the quantity of oil circulation and to maintain correct pressure in the lubricating system. The valve consists of a ball, held in place over an opening by an adjustable spring.
This valve operates in such a way as to permit a certain amount of oil to by-pass back to the reservoir as and when required. If the spring tension is decreased, more oil will by-pass and less oil will be forced through the system. If the tension is increased, less oil will go by the valve and more will be forced through the system.
IV. Grease:
Grease is a semi fluid lubricant, used for slow moving parts. It is useful for those parts, which are connected and are not easily accessible, such as universal joints, water pumps, wheels and axle bearings. Grease consists of a mixture of mineral oil and metallic soap along with a few other chemicals. Soap works as a thickening agent for a lubricant at a certain temperature. Grease is classified according to consistency of the fluid as No. 0, No. 1, No. 2, No. 3, and No. 4 specifying the condition as fluid, very soft, soft, medium and hard respectively.
Troubles in Lubrication System of an I.C. Engine:
There are a few common troubles in lubrication system such as:
1. Excessive oil consumption
2. Low oil pressure and
3. Excessive oil pressure
1. Excessive Oil Consumption:
When there is excessive oil consumption in the engine, the reasons are:
(а) More oil goes to combustion chamber and gets burnt
(b) Some leakage occurs in some part of the line and
(c) Loss of oil in form of vapour through ventilating system.
Oil can enter the combustion chamber through rings and cylinder walls, worn piston rings and worn bearings.
2. Low Oil Pressure:
Low oil pressure can result due to:
(a) Weak relief valve spring
(b) Worn oil pump
(c) Cracked oil line
(d) Obstruction in the oil lines
(c) Very thin oil and
(f) Worn-out bearings.
Care should be taken to remove these defects as far as possible to increase the oil pressure in the lubricating system. Sometimes defective oil pressure indicator shows low oil pressure. This should be checked.
3. Excessive Oil Pressure:
Excessive oil pressure may result due to:
(a) Stuck relief valve
(b) Strong valve spring
(c) Clogged oil line and
(d) Very heavy oil.
These defects should be removed to reduce the excessive oil pressure in the lubricating system. Sometimes defective oil pressure indicator records high oil pressure. Care should be taken to check this defect.
Care and Maintenance of Lubrication System in an I.C. Engine:
The following are few suggestions for good lubrication system:
1. A good design of oil circulation system should be chosen.
2. Correct grade of lubricant ensures long and trouble free service.
3. Oil should be maintained at desired level in the oil chamber.
4. Oil should be cleaned regularly and after specified period of use, old filters should be replaced by new filters.
5. Connections, pipings, valves and pressure gauge should be checked regularly.
6. Oil should be changed regularly after specified interval of time. Before putting the new oil, the crankcase should be cleaned and flushed well with flushing oil.
7. Precautions should be taken to keep the oil free from dust and water.