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Unit – IV
Fuels & Lubricants
Part- A: - Fuels
Definition of chemical fuel:
It is a combustible substance containing carbon as main constituents & on proper burning
gives large amount of heat, which can be used economically for domestic & industrial purposes.
The general reaction for fuel is represented as:
C, H, S, etc)
(Fuel gases CO2,
H2O, SO2, etc.)
Heat energy
Chemical fuels can be classified as follows on the basis of their state & source.
Solid fuel
Natural Or primary fuels
Wood, coal, Dung, etc.
Artificial or Secondary fuels
coal, etc
Liquid fuel
Crude oil Or Petroleum
Gasoline, Alcohols, Etc.
Gaseous fuel
Natural gas
Coal gas, Water gas, Producer
gas, etc.
Comparison Between solid, Liquid & gaseous fuels:
Advantages & Disadvantages of solid, Liquid & gaseous fuels:
A) Solid fuels :
1) They are easy to transport.
2) They are convenient to store.
3) Their cost of production is low.
4) They possess moderate ignition temp.
1) Their ash content is high.
2) Their cost of handling is high.
3) They require large excess of air for complete combustion.
4) They can not be used as internal combustion engine fuels.
Liquid fuels :
1) They burn without forming dust, ash, clinkers, etc.
2) They are easy to transport through pipes.
3) They require less excess of air for complete combustion.
4) They can be used as internal combustion engine fuels.
1) The cost of liquid fuel is relatively much higher as compared to solid fuels.
2) They give bad o dour.
3) Costly & special storage tanks are required for storing liquid fuels.
4) There is a greater risk of fire hazards in case of highly inflammable & volatile liquid
Gaseous fuel :
1) They can be conveyed easily through pipelines.
2) They are clean in use.
3) Complete combustion without pollution is possible.
4) They can also be used as internal combustion engine fuels.
1) Very large storage tanks are needed for them.
2) There is a greater risk of fire hazards in case of highly inflammable gases.
3) They are more costly as compared to solid & liquid fuels.
Characteristics of good fuel:
1) It should have high calorific value.
2) It should have low moisture content.
3) The content of non combustible matters like ash, clinkers, etc. should be low.
4) The products of combustion should not be harmful such as CO, SO2, etc.
5) It should be easy to store & transport.
6) It should be available in plenty at lower cost.
7) Combustion of the fuel should be easy to control i.e., start or stop.
Calorific Value:
It is the amount of heat liberated when unit mass or volume of a fuel is completely burnt,
in excess of air.
Calorific Value having two types. :
Gross Or Higher colorific value : (HCV)
It is the total amount of heat produced from the complete combustion of unit mass or
volume of the fuel, when the products of combustion have been cooled to room
temperature. Thus HCV includes the heat of condensation of products (Steam).
2) Net or Lower calorific value: (LCV)
The net amount of heat produced when unit mass or volume of the fuel burns completely
& the products of combustion escape outside.
Thus LCV value does not include the heat due to condensation of products.
The relation between HCV & LCV can be written as,
LCV = HCV – (% of hydrogen X 9 X latent heat of water vapour)
The latent heat of water vapour is 587 kcal/Kg at room temperature (i.e., 250c)
Or LCV = HCV – 0.09 H x latent heat of water vapour.
(Where H is % of hydrogen in fuel)
Units of calorific value:
A) For solid & liquid fuels
ii) Kcals/Kg
iii) B.T.U./lb
B) For Gaseous fuels.
i) K.Cals/m3
ii) B.T.U./ft3
Analysis of coal:
In order to check the quality of coal, analysis of coal is necessary.
The analysis of coal is of two types:
1) Proximate analysis
2) Ultimate analysis
1) Proximate analysis :
It is useful to decide the practical utility of the coal. It involves following
i) Moisture
ii) volatile matter
iii) Ash
iv) Fixed carbon
i) Moisture :
1 gm of finely powdered coal is heated in a silica crucible at 100 to 1100C for 1 hour.
Evaporation of moisture occurs. Then it is cooled in a desiccator & again weighed. Loss in mass
is reported on % basis.
% of Moisture 
Mass of moisture
 100
Mass of sample
ii) Volatile Matter :
Above moisture free sample is then heated in a covered crucible at
950  200C for
exactly 7 minutes. Then it cooled in desiccator & weighed. The loss in mass is reported on %
% of volatile matter 
Mass of volatile Matter
 100
Mass of sample
iii) Ash :
1 gm of dried coal is burnt in an open crucible at 700 to 7500C in a Muffle furnace until a
constant mass residue is obtained. Then it is cooled in a desiccator & weighed. The residue is
reported as ash on the % basis.
% of Ash 
Mass of Ash
 100
Mass of sample
iv) Fixed carbon :
It is estimated indirectly by difference.
% of fixed carbon = 100 - % of (moisture + volatile Matter + ash)
Significance of proximate analysis:
Proximate analysis provides following valuable in formation in assessing the quality of
1) Moisture: Moisture lowers the effective calorific value of coal. Hence lesser the
moisture content, better the quality of coal.
2) Volatile Matter: Coal containing High % of volatile matter burns with a long flame,
high smoke & low calorific value. Hence, lesser the volatile matter, better the rank of
3) Ash: Ash is useless non combustible matter, which reduces the calorific value of coal. It
causes hindrance to air & heat. Hence lower the ash content, better the quality of coal.
4) Fixed carbon: Higher the % of fixed carbon, greater is its calorific value & hence better
the quality of coal.
Ultimate analysis:
It involves determination of the % of C, H, N, S & O present in coal sample.
Carbon & hydrogen:
The known accurately weighed (1-2 gm) coal sample is burnt in presence of
oxygen gas in a combustion apparatus. Carbon & hydrogen in the coal sample are
converted into CO2 & H2O respectively.
CO2 is absorbed in KOH tube while H2O is absorbed in CaCl2 tube. The % of C & H are
found by knowing the increase in weight of tubes.
% of C 
Increase in wt.of KOH tube 12 100
weight of coal sample  44
% of H 
Increase in wt.of CaCl tube  2  100
weight of coal sample  18
2) Nitrogen:
In Kjeldal’s flask, 1 gm of coal sample is heated with conc. H2SO4 in presence of
catalyst K2SO4. The clear solution is obtained. It is treated with excess of KOH, then
NH3 gas is liberated.
Nitrogen in coal + conc. H2SO4
∆, catalyst K2SO4 Ammonium sulphate
(Clear solution)
+ KOH solution (Excess)
NH3 gas.
Then NH3 gas is absorbed in known volume of standard acid solution. The unused
acid is titrated with std. NaOH solution,
Thus, % of N 
volume of acid used  Normality 14
wt . of coal sample
(1 lit. 1 N H2SO4 solution = 14 gm of Nitrogen)
In a Bomb’s colorimeter sulphur present in coal is converted into sulphate during
combustion. Washings obtained from bomb colorimeter contain equivalent amount of
sulphate. It is treated with Bacl2 solution.
𝑆𝑂42− BaCl2 soln
BaSO4 (ppt)
(From coal)
This ppt is filtered, washed, heated to constant weight.
Thus % S =
wt.of Baso 4 ppt  32 100
wt. of coal  233
: Oxygen is determined by difference i.e.,
% of oxygen = 100 - % of (C + H + S + N + ash)
(Ash is determined as given in proximate analysis)
Significance of Ultimate analysis:
1) Carbon & Hydrogen:
Calorific value of the coal increases due to C & H. Hence greater the % of C & H
in coal better is quality & calorific value of coal.
2) Nitrogen:
Nitrogen in coal does not contribute to its calorific value Hence lesser the
nitrogen better is the quality of coal.
3) Sulphur:
Sulphur containing coal is not suitable for metallurgical process as it affects the
properties of metals. Moreover, sulphur is oxidized into SO2 & SO3 which causes
atmospheric pollution. Hence sulphur is not desirable.
4) Oxygen:
Calorific value of coal decreases due to presence of oxygen with moisture. Hence
good quality of coal should have low percentage of oxygen.
Definition: Process of decomposition of high boiling bigger hydrocarbon molecules into
simpler low boiling hydrocarbons of lower molecular weight.
B.Pt. = 1740C
C 5H12
C 5H10
B.Pt. = 360C
The petrol obtained by cracking is superior quality because such petrol has higher octane
number of the petrol and less knocking properties.
There are mainly two methods of cracking:
1) Thermal Cracking
2) Catalytic Cracking
1)Thermal Cracking :
In this method the heavy oil is subjected to high temperature & pressure. The higher
molecular weight hydrocarbons are decomposed to lower mole weight hydrocarbon such as
paraffins, olefins & some hydrogen.
Thermal cracking are of two types:
Liquid phase cracking : In this type, oil is maintained in liquid state. The cracking is
carried out by heating heavy oil at a high temperature of 475 to 5300C & under pressure
of 100 kg/cm2 .The cracked products are separated by fractional distillation. The yield
is about 60%.The octane rating of the gasoline is 65-70.
Vapour phase Cracking :
In this method, generally kerosene oil or gas oil is first vapourised & then cracked
at high temperature (600-6500C). Heavy oil cannot be cracked by this method. Only the
oils which can be vapourised, are cracked. e.g. kerosene oil.
It requires lesser time than liquid phase cracking. The octane rating of gasoline is
more than 70, but stability is poor.
Fig : Thermal Cracking
2) Catalytic Cracking :
In this method, the cracking is carried out at much lower temperature (300-4500C) &
pressure.The catalysts used are crystalline aluminium silicates or Alumina.
Cracking results into the formation of gas, gasoline coke & other liquid products due
to various reactions such as dehydrogenation, isomerization, hydrogenation &
polymerization. The gasoline produced by this method is of better quality.
Catalytic cracking is also of two types :
Fixed bed catalytic cracking.
ii) Moving bed or fluid bed catalytic cracking.
Fixed bed catalytic cracking.
In this process, the heavy oil charge is passed through a preheater, where the oil
is vaporized & heated to cracking temperature (420-4500C).
The hot vapour are passed to the catalytic chamber containing catalyst.
In a catalytic chamber also the temp. is maintained at 420 to 4500C & a pressure
of 1.5 kg/cm2 .
Here the cracking of about 40% of charge converts into gasoline. About 4% of
carbon is formed. It gets, deposited on catalyst bed.
The cracked vapours from the catalyst chamber are then passed through
fractionating column where heavy oil condensed & collected at the bottom. The
vapours are then led through cooler, where some of the gases are condensed along
with gasoline and uncondensed gases move on. The gasoline containing some
dissolved gasses is then sent to a stabilizer, where the dissolve gases are removed and
pure gasoline is obtained.The yield is about 30-40 %. The octane rating of the gasoline
is 70-80.
Fig. : Fixed bed catalytic cracking.
Moving bed or fluid bed catalytic cracking :
In this type of catalytic cracking, the oil vapours are cracked at high temperature &
pressure in presence of fine powdered catalyst.
The heavy oil ar gasoline is heated in preheater. These vapour of oil & fine
powdered catalyst together are forced into the reactor. The reactor is maintained at the
temperature of about 5300C & pressure of 3-5 Kg/Cm2.
In the reactor, cracking of bigger hydrocarbons into smaller (lighter) molecules takes
The lighter hydrocarbons move to the top of the reactor. They are separated from the
catalyst powder by cyclone separators & enter into the fractionating column. Catalyst
retains in the reactors.
The gasoline vapours & gases moves at the top of the column. They are passed into
the cooler, where gasoline gets condensed. This gasoline is further stabilized into
stabilizer by removing the other gases.
The catalyst powder becomes heavier due to deposition of carbon. It settles at the
bottom of reactor. It is forced by air blast into regenerator for burning of carbon at about
In a regenerator carbon is burnt. Thus catalyst can be again reused by circulating in a
Fig – Moving bed type catalytic cracking.
Advantages of catalytic cracking over thermal cracking :
Catalytic cracking is more selective. Hence yield of gasoling is higher.
Gasoline contains more amount of aromatics, isomerised olefins & isoporaffins.
Lower pressure is required.
The cracking process can be easily controlled. Hence desired products can be obtained.
External fuel is not required. The heat is obtained by burning the carbon coated on catalyst.
Use of Gasoline in Internal Combustion Engine:
In an IC engine, a mixture of gasoline vapour and air is used as a fule.
Functioning of Engine :
In an IC Engine, a mixture of air and petrol vapour is compressed and ignited by an
electric spark, which causes oxidation of Hydrocarbon (HC) molecule i.e. combustion reaction
take place.
After the combustion reaction is initiated by spark in the cylinder, the flame spreads
pressure increases rapidly and smoothly through the gas mixture. The expanding gas drives the
piston down the cylinder this is called Suction stroke.
Fig:-Compression ratio
When the combustion complete, exhaust gases go out through silancer the pressure is
decreases. Therefore the piston moves upword direction and this is called Compression stroke.
The rate of oxidation of a HC molecule depends on the (i) Number of carbon atoms in the
molecule, (ii) Structure of fuel, (iii) Temperature.
The temperature depends upon the compression ratio.
∴ Compression ratio =
∴ Compression ratio =
Gaseous volume at the end of suction stroke
Gaseous volume at the end of compression stroke
It is due to some irregularities develop in the combustion of fuel (air + gasoline vapour
mixture) inside the cylinder. So some sound is heared inside the engine known as Knocking.
First portion of the fuel burns in a normal manner but the last portion burns so
spontaneous thereby creating a large momentary pressure in balance in the combustion chamber
given knocking. These noise explosive voilence is called knocking or pinging or detonation. The
tendency of knocking depends upon (1) Nature of fuel engine, (2) Engine speed and (3) Air fuel
eg – Straight chain paraffins > Branched chain paraffins > Olefines >
Napthalene > Aromatic Hydrocarbon (HC)
Straight chain paraffins have poor antiknock property and it improves with increasing length of
hydrocarbon. Due to presence of olefins, iso-paraffins and aromatic hydrocarbon the antiknocking property is increases.
Octane Number (Octane Rating of Fuel)
The knocking characteristics of fuel are usually expressed by octane number. The octane
number of fuel introduced Edger in 1927, is measured of its tendency to knock when burnt in
spark ignition engine.
2, 4, 4 trimethyl pentane have very good combustion characteristics i.e. It has very less knocking
property when mixed with air, hence, its octance number is taken as 100 (Less knocking property
– more octane number)
The HC n – heptanes (C7H16) has more knocking property. So, its octane number is taken
is zero. (More knocking property – Less octane number)
Therefore the octane number of gasoline is defined as the percentage of iso-octane in a
mixture of iso-octane and n-heptane, which match the fuel under test in knocking characteristics.
eg – The sample as gasoline is said to have an octane number 80, if it matches in
knocking properties in the test engine to mixture of 80% iso-octane and 20% n-heptance.
The octane number has been increased by a mixture of iso-octane and n-heptane with
addition of tetra ethyl lead (TEL) (C2H5)4 Pb.
Relation between Knocking and chemical structure of Fuel
1) Knocking tendency of fuel decreases, compactness of fuel molecule and double bond and
cyclic structure increases.
2) As the length of the alkane increases, knocking property increases i.e. octance number
eg – octance number of n-butane, n-pentane, n-hexane and n-heptane are 90, 60, 30 and
40 respectively.
3) As the branching in HC increases knocking property decreases because branches
increases resistance to knock.
4) Alkene has less knocking property than its corresponding alkane.
5) Aromatic hydrocarbon take benzene, tolune have less knocking property.
6) Aliphatic hydrocarbon like alcohol has less knocking property.
Antiknocking agents
The compounds which reduces knocking properties of gasoline, called as antiknocking
agent.Adding of antiknocking agent in petrol called as doping.
eg- Tetraethyl lead (TEL) Pb(C2H5)4.
Role of TEL: When TEL is added an antiknocking agent in petrol, during burning of
pertrol in IC engine, TEL is converted into cloud of fine particles of lead oxide, which reacts
with hydrocarbon to form hydrocarbon peroxide, thereby reducing ratio of oxidation of
petrol. The unreacted lead oxidation particles produces air pollution in the form of exhaust
To remove harmful effect of lead oxidation particles, ethylene dibromide is added
(C2H4Br2) in petrol. Ethylene dibromide removes PbO in the form of gas of lead bromide
along with other exhaust gas. The presence of sulphur compounds in petrol reduces the effect
of TEL.
Use of Diesel in IC engines
Diesel oil is used as fuel in the IC engines of compression ignition type.
Functioning of engine: In diesel engine, air only is first forced into the cylinder and
compressed at about 30-50 kg/cm2. As a result of compression the air temperature is raised to
500-6000C. At this stage diesel is ingected in the form of spray into the very hot air. Then the
fuel droplets vapourise and get heated to the temperature at which spontaneous ignition takes
place. This causes pressure to increase further to about 70kg/cm2, due to which piston
reaches bottom dead centre, fuel ingection stops and exhaust gases are driven out pressure
begins to fall and piston moves up.
Diesel knock: the diesel knock is caused by a retarded ignition called as ignition lag.
When a fuel has a long ignition lag, a large portion of the oil gets injected and accumulated
into the cylinder even before the ignition is initiated. This results in a violent combustion and
a sudden increase in pressure.
The greater the ignition lag, the higher is the diesel knock. The ignition lag and diesel
knock depends on
1. The engine design
2. Types of the injectors
3. The size of oil droplets
4. The chemical nature of fuel
Paraffins gives less diesel knock than the aromatic hydrocarbon. Hence diesel fuel should
contains mostly straight chain hydrocarbons with minimum percentage of aromatic and
branched chain hydrocarbon molecule.
Cetane Number
The knocking characteristics of a diesel oil are expressed in terms of cetane numbers.
Definition : The percentage by volume of cetane, in a mixture of cetane and ∝ - Methyl
naphthalene which exactly matches in it’s knocking characteristic with the oil under test is called
as cetane number of diesel.
Cetane (C16H34) is a saturated hydrocarbon which has very short ignition lag, as
compared to any commercial diesel fuel, Hence its catane number is taken as 100. However, ∝methyl naphthalene (C11H10) (aromatic HC) has very long ignition lag as compared to any
commercial diesel oil. Hence its cetane number is taken as zero.
eg – When diesel oil has cetane number 40, means that the spontaneous ignition
temperature oil is just same as that of a mixture of 40% cetane and 50% ∝-methyl naphthalene.
Improvement of Cetane number of Diesel oil :
The cetane number of an diesel oil can be improved by using additive like acetylene,
ethyl nitrate, acetone, di-ethyl ether, etc. In addition to dopes, other compound (inhibitor) are
also added to prevent the gum formation and to reduce surface tension for the formation of fine
spray of diesel oil during injection.
Relation between Knocking property of Diesel and Petrol :
In petrol engine, knocking is caused due to sudden combustion of fuel before sparking of
spark plug. In diesel engine knocking is caused due to delay in combustion of fuel in diesel
To reduce the knocking due to fuel in diesel engine, we have to increase rate of
combustion of fuel in diesel engine i.e. we have to increase cetane number of corresponding fuel.
Octane number
Cetane number
Part – B Lubricants
When two metal surfaces are moving or sliding over each other causing friction and wear
and tear of the surface resulting in the loss of energy which appear in the form of heat. As the
machine parts get heated up. They are damaged and results in welding of surface.
“Any substance introduced between two moving or sliding surfaces in order to reduce the
frictional resistance is known as a Lubricant.”
Function of Lubricant :
1) It reduces surface deformation. Wear and tear because the direct contact between the
rubbing surface is avoided.
2) It reduces loss of energy in the form of heat.
3) It reduces the expansion of metal by local frictional heat.
4) It reduces the maintenance and running cost of machine.
5) It Prevents the entry of moisture, dust between the moving part and thus as seal.
6) It acts as a hydraulic fluid in aircrafts.
7) Lubricant acts as a seal/gasket between piston rings and cylinder will in I.C. engine. This
prevents the leakage of gases at high pressure in combustion chamber. Thus it reduces the
power loss.
Classification of Lubricants :
Lubricants may be broadly classified as follows :
1) Solid lubricants :
A solid lubricant is that material which separates two moving surfaces under
ordinary conditions and drease friction and weor.
Solid lubricants are used in situations such as :
Heavy machinery working on a crade job at very high loads and slow
Where a liquid or semi solid lubricants film cannot be maintained.
Where parts to be lubricated are not easily accessible.
Where the operating temperatures and pressures are too high.
e.g – Soap, Stone, Graphite, Talc, Chalk, Mica, Teflon, Molybdenum
disulfide, etc.
2) Semi – Solid lubricants :
The block of soap/Waxes are fixed at a point hear the moving parts with the
results that some of it gets powdered and is carried into the moving surfaces. The most
important semi-solid lubricants are greases and vaselines.
Lubricating greases are employed in the following situations.:
When a machine is worked at slow speeds & high pressures.
In situations where oil can not be maintained in position due to bad seal.
In situations where the bearing has to be sealed against entry of dirt, water, dust & grit.
e.g. : Greases, vaselines, etc.
3) Liquid lubricants :
There are mainly lubricating oils. The main function of lubricating oils is to reduce
friction and wear between two sliding or moving surface by maintaining a continuous
fluid film in between them. They oilnen vijwary liquid lubricants are classified on the
basis of their origin.
Vegetable oil : Oils of vegetable animal origin contain glycerides of higher fatty
acids. As they decompose on heating but do not distil they are called “fixed oils”.
Fixed oils have a propertd called oiliness by virtue of which they are aclsorbed on the
metal surface. Some of the important vegetable oils having potential use as lubricants
are given below. : Oilve oil, Castor oil, Rope oil, Palm oil, Cotton seed oil, Etc.
Animal oils : Animal fats and oils are extracted from the crude fat by a process called
“rendring” in which the enclosing tissue is broken by treatment with steam or with the
combined action of steam and water.
e.g.: whale oil, Lard oil, Tallow oil.
Mineral oil : Mineral oils are prepared from the heavier fractions obtained from
fractionation of crude oil at atmospheric pressare.
e.g : - Dewaxing, acid refining, solvent refining.
Blendedd oil : It is known as compounded oils.
Synthetic oil : In certain machines operating conditions lubricants fail to serve the
purpose. In such cases synthetic lubricating oil are used e.g. Polyglycols, silicon frids.
4) Emulsion :
It is a dispersion system consisting of two immiscible liquids is called an
Two types of emulsions are used for lubricating jobs :
Oil in water types emulsions : There are prepared by mixing together an oil
containing about 3 to 2% of water soluble emulsifying agent. e.g – Water soluble
soap, alkyl solphonate, alkyl sulfates etc.
Water in oil type emulsions : Which are prepared by mixing together water and oil
containing 1 to 10% of water insoluble emulsifiers.
e.g. : alkaline earth metal soaps.
Mechanism Of Lubrication
The purpose of lubrication is to form a film of lubricant between the moving or
sliding material surfaces, thereby reducing frictional resistance and even reducing
wear and damages of the rubbing surfaces. The lubrication is affected by three main
1) Fluid Film or Hydrodynamic Lubrication
2) Boundary Lubrication or Thin Film Lubrication
1) Fluid Film Lubrication :
It is also called as hydrodynamic lubrication. In this type of lubrication. The sliding
surfaces of the metals are completely seprated by applying thin film of liquid lubricants in
between them. The thickness of film upto 100A0 in hydrodynamic lubrication. So that there is no
direct contact between the metal surfaces.
Under hydrodynamic lubrication metal-to-metal contact is prevented and the friction
between the layer of the fluid is low. The liquid lubricants do not have any affinity to the
metal surface and it sticks to it only due to viscosity or stickness. So, this condition is known
as Fluid-film lubrication or hydrodynamic lubrication.
Uses : Delicates machine, watches, clocks, scientific instruments.
2) Boundary Lubrication :
This type of lubrication is applied when continuous fluid film cannot be
maintained and direct metal-to-metal contact is possible due to certain reasons. This
happens when (1) Shaft come into action from rest. (2) Shaft come into action from res.
(3) The speed is very low or (4) The load is very high or (4) If the viscosity of the oil is
very low.
In such lubrication it is necessary that a thin layer of oil be adsorbed on rubbing
surface by physical or chemical or both forces on the metal surfaces. “This property
which is responsible for maintaining a boundary film of oil by adsorption is known as
Oiliness”. A oil which has more oiliness than another oil of the same viscosity, gives the
low coefficient of friction at low speed or high loads.
The load is carried by layers of lubricant which have been adsorbed on metal
surface forming thin-film of metal soap which acts as lubricants. This kind of lubrication
is known as boundary Or thin film lubrication.
Boundary lubrication is not effective at high temperature as the fatty acids break
down at high temperature. eg- Vegetable oil Animal oils have great oiliness
(adsorption/attachment power) than the mineral oils, because of their chemical
constitutents. Mineral oils have a symmetrical molecule with a – CH3 grp. At each end
but fatty acid RCOOH (Steric acid C17H35COOH, Oleic acid C17H35COOH) have Polar
carboxyl group (COOH) at one end. Such – COOH group reacts with metal surface with
formation of mono-molecular layers. Where as long HC chain (R-) is oriented outwards
in an perpendicular direction (Fig. 7.3 (a)). The adsorbed molecule forms a continuous
film on the metal surfaces. The film so produced reduces the wear and friction.
However in case of vegetable/animal oil as the operating temperature is raised,
remarkable increase in friction and wear.
Extreme Pressure Lubrication :
When the moving or sliding surfaces are under very high pressure and
speed, excessive frictional heat will be generated. The high local temperature thus
produced at the surfaces and the ordinary lubricant fail to stick and may even get
vapourised. To meet these extreme pressure and high temperature condition special
additives are added to mineral oil called “Extreme pressure additives”. These additives
form a more durable film on the metal surface which is capable of standing high
The additives used in extreme pressure lubrication are organic compounds.
Containing certain active groups or radicals i.e. chlorine, sulphur, phosphorus. These
reacts with metallic surfaces at high temperature to form metallic chlorides, sulphides or
phosphides having high melting point and as good lubricants under high pressure and
temperature conditions, and reduces wear and tear of metal surface.
Properties or Testing of Lubricating Oil
To assess the suitability of lubricant for a particular use, the actual behavior of the
lubricant have to be studied. A through knowledge of the properties of lubricants can
avoid an defects caused by their use in different machines. The following properties or
tests must be taken into consideration while selecting lubricant for a particular machine.
1) Viscosity and Viscosity Index :
Viscosity : Viscosity is the property of liquid by virtue of which it offers
resistance to its own flow. eg – It is not possible to maintain a liquid oil film
between two moving or sliding surfaces. If the viscosity of lubricant is too low
and hence excessive wear will occur. If the viscosity of lubricant is too high.
Excessive friction will take place.
Viscosity of the liquid decrease with increasing temperature and so
lubricating oil becomes thinner as the operating temperature is raised the rate
of change of viscosity of an oil with temperature is measured in terms of
viscosity index.
Viscosity index (V.I) :
The rate at which the viscosity of oil changes with temperature is measured by an
arbitrary scale known as viscosity index. (V.I.).
If the viscosity of an oil decreases rapidly with increase in temperature. A oil has low
viscosity index (V.I.) and if the viscosity of an oil is only slightly affected on increasing
temperature, an oil has high viscosity index (V.I.).
Increasing temperature  Viscosity decreasing  Low V.I.
Increasing temperature  Slightly decreasing in viscosity  High V.I.
Determination of V.I.
For this purpose, use of two types of standard oil, Paraffinic-base Penny
sylvanian oil (V.I. = 100) and Napthanic –base Gulf oil (V.I. = 0)
Step I : The viscosities of the oil under test at 1000F and also at 2100F first found out Let, Oil
under test at 1000F = U
Oil under test at 2100F = V
Step II : From the list of H-oils (High viscosity index std.) (V.I. = 100). Oil has the same
viscosity at 2100F as the oil under test is selected and its correction at 1000F is read off.
Step III: From the list of L-oils (i.e. with V.I.= 0). The oil which has same viscosity at 2100F as
the oil under test is selected and its corresponding viscosity at 1000F is read off. Let it be L then,
Viscosity Index (V.I.) X 
V V
x  100
V V
VL = S.U.V. of standard low viscosity oil at 1000F
VH= S.U.V. of standard high viscosity oil at 1000F
Measurement of Viscosity of Lubricating Oil
The apparatus which is used for measurement of viscosity known as Viscometer. In India
Redwood viscometer is generally used to determining the viscosity of a lubricating oil. So,
Redwood Viscometer are two types. :
a) Redwood Viscometer No 1
b) Redwood Viscometer No 2
a) Redwood Viscometer No 1 :
It is commonly used for determining viscosity of thin lubricating oils. It has a jet
or bore of diameter 1.62 mm and length 10 mm.
1) Oil cup : It is silver plated brass cylinder of hight 9 cm and diameter 4.65 cm. The upper
end of cup is open and the bottom of cylinder is fitted with an agate jet with length 10mm
and bore of diameter 1.62 mm. The jet is opened or closed by a ‘valve rod’ which is a
small silver plated brass ball fixed to a stood wire. The lid of the cup is fitted with
thermometer, which indicates the oil temperature.
2) Heating bath : Oil cup is surrounded by a cylindrical copper bath containing water. It is
provided with a tap (for emptying water from it). Oil cup is heated through water bath so
that there should be uniform heating temperature is measured by thermometers.
3) Stirrer : The heating bath containing water contain a stirrer having four blades for
stirring water in bath for keeping temperature of water bath uniformaly.
4) Spirit level : The lid of the cup is provided with spirit level for vertical leveling of the
5) Levelling screws : The entire apparatus roots on three legs, provided their bottom with
leveling screws.
6) Kohlraush flask : Is a specially shaped flask for receiving the oil from the jet outlet. Its
capacity is 50ml upto the mark in its nech.
Redwood Viscometer No 1
Working : The oil cup is leveled by spirit level and leveling screws. Ball valve is placed in
cleaned oil cup. Lubricating oil is placed in the cup upto pointer level. Kohlrausch flask is placed
just below the jet. Water is heated either by burner or electric heater. When the water
temperature is slightly above the required oil temperature heating is stopped.
After stirring, when oil and water attains same and stady temperature ball valve is lifted
and simultaneously stop watch is started. Time in second is noted for the collection of 50 ml oil
(1 second) then the valve is immediately closed, to prevent any overflow of the oil. Hence,
viscosity of oil at T0C Redwood No 1 Seconds at particular temperature. For further readings
water bath is further heated and time flow of 50 ml oil through jet.
b) Redwood viscometer No 2 :
It is generally used for viscosity for determining of thicker oils. The diameter of
jet (orifice) of redwood viscometer No 2 is 3.8 mm and length 5 cm. The time flow is
taken for the collection of 100 ml oil in the Kohlrausch flask.
c) Flash Point and Fire Point :
Flash Point: Flash point is lowest temperature at which the oil gives off enough
vapours, which give momentary flash of light when small flame is brought near to it.
Fire Point :
Fire point is the lowest temperature at which the vapours of oil
burns continuously at last for 5 seconds when tiny flame is brought near to it.
The flash point and fire points are usually determined by using Pensky-Marten’s
apparatus and Abel’s Flash point apparatus.
Abel’s Flash Point Apparatus (Closed Cup) :
It consists of cylindrical oil cup surrounded by a double jacketed copper water
bath mounted on tripoel stand. The oil cup is provided with a brass cover having
opening closing sliding shutter, one for small test flame, for a paddle shutter, for
standard thermometer. It is used for oil having flash point below 1200F.
Working : The oil cup is filled with oil upto indicator level. Water bath is filled
with water. Thermometers are dipped into water and oil to read water temperature and
oil temperature. The water bath is heated which heats oil uniformaly. The heating is
uniform due to air gap between bath and oil cup. The oil is stirred continuously with
the paddle stirrer. Stirrer is discontinued only when test flame is introduced over the
oil surface. At every rise of temperature in ascending order flash point and fire point