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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: Fuel + (Containing C, H, S, etc) O2 atmospheric Oxygen Product (Fuel gases CO2, H2O, SO2, etc.) + Heat energy Classification: Chemical fuels can be classified as follows on the basis of their state & source. State Solid fuel Natural Or primary fuels Wood, coal, Dung, etc. Artificial or Secondary fuels Coke, charcoal, Pulverized Diesel, Kerosene, coal, etc Liquid fuel Crude oil Or Petroleum Petrol, Gasoline, Alcohols, Etc. Gaseous fuel Natural gas Coal gas, Water gas, Producer gas, etc. Comparison Between solid, Liquid & gaseous fuels: OR Advantages & Disadvantages of solid, Liquid & gaseous fuels: A) Solid fuels : Advantages: 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. Disadvantages: 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. B) Liquid fuels : Advantages: 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. Disadvantages: 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 fuel. C) Gaseous fuel : Advantages: 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. Disadvantages: 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. : 1) 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 i) Cals/gm 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 determinations:- 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 % basis. % 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 coal. 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 coal. 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. 1) 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 2 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 14 100 wt . of coal sample (1 lit. 1 N H2SO4 solution = 14 gm of Nitrogen) 3) Sulphare: 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. S Combustion 𝑆𝑂42− BaCl2 soln BaSO4 (ppt) (From coal) This ppt is filtered, washed, heated to constant weight. Thus % S = 4) Oxygen 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. Cracking: Definition: Process of decomposition of high boiling bigger hydrocarbon molecules into simpler low boiling hydrocarbons of lower molecular weight. C10H22 Cracking Decane B.Pt. = 1740C C 5H12 + Pentane C 5H10 Pentene 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: i) 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. ii) 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 : i) Fixed bed catalytic cracking. ii) Moving bed or fluid bed catalytic cracking. i) 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. ii) 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 place. 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 6000C. In a regenerator carbon is burnt. Thus catalyst can be again reused by circulating in a reactor. Fig – Moving bed type catalytic cracking. Advantages of catalytic cracking over thermal cracking : 1) Catalytic cracking is more selective. Hence yield of gasoling is higher. 2) Gasoline contains more amount of aromatics, isomerised olefins & isoporaffins. 3) Lower pressure is required. 4) The cracking process can be easily controlled. Hence desired products can be obtained. 5) 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 V2 V1 Knocking 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 ratio. 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 decreases. 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 gases. 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 engine. 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. Sr.No. Hydrocarbon Octane number Cetane number 1. Iso-octane 100 22 2. n-heptane 0 64 Part – B Lubricants 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 : i) Heavy machinery working on a crade job at very high loads and slow speeds. ii) Where a liquid or semi solid lubricants film cannot be maintained. iii) Where parts to be lubricated are not easily accessible. iv) 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.: i) When a machine is worked at slow speeds & high pressures. ii) In situations where oil can not be maintained in position due to bad seal. iii) 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. i) 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. ii) 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. iii) 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. iv) Blendedd oil : It is known as compounded oils. v) 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 emulsion. Two types of emulsions are used for lubricating jobs : i) 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. ii) 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 mechanism. 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 temperature. 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 L x 100 L H V V VL = S.U.V. of standard low viscosity oil at 1000F Where, 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 jet. 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 noted.