FUEL TECHNOLOGY Liquid fuels: The liquid fuels are characterized by low flash point, high calorific value, low viscosity at ordinary temperatures and low moisture and sulphur content. Petroleum is one of the most important liquid fuels in the world. Gasoline or petrol is the main product obtained from a naturally occurring crude oil, petroleum. Other petroleum products are motor spirit, diesel, kerosene oil etc., The word Petroleum is comes out from the words Petra = rock and Oleum =Oil. And hence, petroleum is also known as rock oil or mineral oil. It is dark-greenish brown, viscous liquid and occurs below the surface of the earth at a depth of 500 1500 ft at various places. It is generally present with water and natural gas. ORIGIN OF PETROLEUM (CRUDE OIL) - MODERN THEORY: Petroleum occurs at different depths at different places varying from 500ft to 1,500 ft. It is brought to the surface by drilling holes in the earth s crust in the oil deposits and sinking pipes until the drill penetrates the oil deposits. Since the oil inside the earth is enveloped by natural gas, sometimes it comes out very rapidly through the pipes due to the pressure exerted by the natural gas, when the pressure of the gas diminishes, the crude oil is pumped out using lift pumps. In this method, two co-axial pipes are lowered into the earths crust in oil deposits compressed air is blown through the outer pipe. Due to the pressure of compressed air the crude oil gushes into the inner pipe and comes out. This is stored in big steel tanks from where it is taken to refineries through pipeline for refining. CLASSIFICATION OF CRUDE OIL / PETROLEUM : The chemical nature of the crude petroleum varies with the part of the world in which it is found. It is divided into the following three classes called bases. (i) Paraffin Base: If the residue obtained after the removal of volatile compounds of crude is rich in paraffins or waxes, the petroleum is called paraffin base. It is mainly composed of saturated hydrocarbons from CH 4 to C 35 H 72 and a little of naphthalenes and aromatics. The hydrocarbons from C 18 H 38 to C 35 H 72 are semi-solids called waxes. (ii) Asphalt or Naphthalene Base: If the residue obtained after removing volatile compounds of crude oil is rich in naphthenes, it is classified as asphalt or naphthene base. This crude contains high percentage of cyclic compounds with smaller amounts of paraffins and aromatic compounds. (iii) Mixed Base: If the residue left after removing volatile compounds of crude oil contains both paraffins and naphthene, the petroleum is called mixed base.
REFINING OF CRUDE OIL OR PETROLEUM: Crude oil coming out during the mining process cannot be used as such unless it is refined. First impurities like dirt, water, sulphur etc. are removed and then the crude oil is subjected to fractional distillation in which the crude oil is vaporised and the vapours are passed through a fractionating column in which various fractions are condensed based on the boiling points. This process of fractionation of crude oil into various desired products and removal of undesirable substances from the petroleum is known as refining of petroleum and the plants setup for this purpose are known as oil refineries. The refining of petroleum involves the following steps: Step 1: De-emulsification or Separation of Water: The crude oil which is pumped out is in the form of stable emulsions of oil-in-water type and waterin-oil type which are yellow to dark brown in colour. The electrical method is widely used at the petroleum processing plants in which the crude oil is subjected to an electrical field formed by a high voltage alternating current which destroys the emulsion films. Droplets of the dispersed phase coalesce to form bigger drops, which separate out from the oil. This process is called as Cottrell s process. Step 2: Removal of Harmful Sulphur Compounds: The crude oil is treated with copper oxide, the sulphur compounds present in the crude oil react with copper oxide resulting in theformation of copper suiphide, a solid, which is then removed by filteration. Step 3: Fractional Distillation: In an oil refinery the crude oil is fractionally distilled and separated into groups of compounds with specific boiling point ranges (fractions) and a residue of higher boiling constituents (bottom). Some of these can be directly used after purification. The bottoms are subjected to cracking operations in which chemical degradation of oil fraction into lower boiling and useful fractions is effected. The distillation is now a days a continuous process in which the crude oil is preheated to 350 380 C is fed continuously in fractionating column containing a large number of trays each fitted with bubble caps (loose cap). The tower is heated by superheated steam sent in from below, so as to create a temperature gradient in the fractionating column of about 300 C at the bottom anddecreased gradually to 125 C at the top. The operation of the fractionating column is based on the fact that, the hydrocarbons of petroleum, boil at different temperatures according to their size and molecular weight. Due to this intense heat, all volatile constituents except the residue are evaporated. The hot vapours are then passed up to the fractionating column. The vapours ascending the column are exposed to progressively lower temperature as they pass from one tray to another resulting in fractional condensation at different heights of column. Higher boiling fractions condense first while the lower boiling fractions turn by turn. Any uncondensed vapour escaping from the top of the column is condensed to form straight run gasoline a part of which is returned to the column as a reflux liquid. The various primary fractions collected, their boiling ranges and uses are summarised in the table below:
CRACKING: Chemical conversions of petroleum fractions obtained by physical processing is necessary to make better use of heavier, less-volatile fractions and to meet the growing demand for gasoline. The yield of petrol obtained by the direct distillation of crude oil called straight run motor spirit, being only 20%, is hardly sufficient to meet the demand. The techniques available for increasing the yield of petrol from petroleum fractions are called cracking and reforming. Cracking is the process of decomposition of less volatile higher hydrocarbons into more volatile lower hydrocarbons by application of heat, pressure and catalysts. Cracking, besides increasing the yield of petrol also tends to improve the quality of gasoline in terms of antiknock properties. There are two methods of cracking. They are a) Thermal cracking b) Catalytic cracking THERMAL CRACKING: This method is an old method and is now being replaced by other methods. In thisthe heavy oil is subjected to high temperature and pressure, when the higher hydrocarbonsget converted into the lower hydrocarbons. The liquid after being cracked passes throughthe fractionating column and gets
separated into different fractions. Thermal cracking takes place by two ways. They are i) Liquid phase cracking and ii) Vapour phase cracking. (i) Liquid Phase Thermal Cracking: In this process the cracking of heavy oil or residual oil is carried out at a temperature of 475 530 C under high pressure (usually 7-70 atms) to keep the reaction product in liquid state. The yield of petrol or fractionation is nearly 60-65% and the octane number is in the range of 65-70. (ii) Vapour Phase Thermal Cracking: In this process the heavy oil or residual oil is first converted to vapour state by heating to about 600 C and subjected to 3.5-10.5 atmospheres pressure for a very short time. The yields have been exceptionally good (around 70%), but the octane number is somewhat below 70. However it is disadvantageous to subject heavy oil or residual oil t this process because they fail to vapourise completely. CATALYTIC CRACKING: A better quality and yield is obtained by the process of catalytic cracking than the thermal cracking. The liquid is cracked at a temperature of 400-450 C in the presence of a catalyst. Usually aluminium silicates [Al 2 (Si0 3 ) 3 ] or alumina [Al 2 0 3 ] are employed as catalysts. The catalytic cracking possesses some advantages over thermal cracking. There are two methods of catalytic cracking, they are (i) Fixed bed catalytic cracking and (ii) Fluid bed (or) Moving bed catalytic cracking. WORKING OF I.C. ENGINES - KNOCKING: In the working of an internal combustion engine a mixture of petrol vapours and air acts asa fuel. In the down stroke of the piston, a mixture of gasoline or petrol vapour and air is drawn fromthe carburettor into the cylinder, and in the up stroke, the mixture is compressed. The ratio of the original volume of the fuel mixture to that of the final volume obtained after compression is called compressed ratio. Higher the compression ratio, higher is the efficiency of the engine. At the end of the up stroke a spark from the ignition system ignites compressed air gasoline mixture in the immediate vicinity of the spark plug, the gases expand and a flame front travels at regular and orderly rate through the remainder of the fuel mixture and this results in the production of power in right direction. However, when the compression ratio exceeds the limit, the ignition of the fuel takes place before the piston head reaches the end of its stroke, which lowers the ignition temperature of the end gas. As a result, combustion of fuel takes place much rapidly and the end gas burns in an explosive and disorderly fashions producing a sharp metallic sound similar to rattling of hammer called knocking. Thus, there is a huge loss of energy and damage to the piston and cylinder. Antiknock Compounds: Knocking can be decreased by the addition of certain compoundsto the fuel and these compounds are called antiknock compounds. Thus, those compounds whichwhen added to gasoline or petrol increase its octane number and decrease knocking are known asantiknock compounds for e.g., tetra ethyl lead (TEL) is a very important alkylated product and theprincipal anti knock compound for gasolines. It is added upto 0.01 percent in gasoline. It is believed that tetra ethyl lead dissociates into ethyl radicajs which may combine with some of the straight chain hydrocarbons forming branched chain hydrocarbons and thus lowering knocking property of fuel. In order to prevent deposition of lead in the engine, ethylene dibromide is added to the gasoline, which breaks down in the engine to ethylene and bromine. Lead combines with bromine to form volatile lead dibromide which leaves the engine along with the exhaust gases.
Note: The knocking tendency decreases with increase in the compactness of the molecules, double bonds and cyclic structure with normal paraffins. The anti knock properties decrease with increase in the length of the hydrocarbon chain. Branched chain paraffins have higher anti knock properties than their normal isomers. The resistance to knock increases with the number of branches and their position. Thus 2-methyl hexane has an octane number of 55, while 2,2-dimethyl pentane has an octane number of 80.Knocking order of various hydrocarbons is as follows: Straight chain hydrocarbons >branched chain hydrocarbons > olefins > cycloparaffins > aromatics. OCTANE NUMBER: The concept of octane number was developed by Edger in 1927 for the evaluation of gasolines. He found that n-heptane produces maximum knocking and has been assigned an octanenumber of zero where as 2,2,4-trimethyl pentane produces minimum knocking and is the best fuelfor spark-ignition engines, and has been assigned an octane number of 100.Thus, the octane number of the given fuel is the percentage of iso-octane in a mixture ofiso-octane and n-heptane that gives the same knocking properties as the fuel under test, under similar experimental conditions. If we say that the octane number of the fuel is 70, it means that this hasthe same combustion characteristics as a mixture of 70% iso-octane and 30% n-heptane. CETANE NUMBER: Diesel engines differ from gasoline engines. In diesel engine, a mixture of diesel and air isignited by high temperature generated by compression inside the cylinder instead of using a spark.for diesel engines, straight chain hydrocarbons are considered to be superior to branched chainhydrocarbons. Usually the straight chain hydrocarbons ignite readily and n-hexadecane or cetane which ignites more rapidly than any other commercial fuel is given a rating of 100.The aromatic compound, naphthalene which ignites slowly than any other fuel has been given arating of zero. Hence, cetane number of a diesel fuel is the percentage of cetane by volume in a mixture of cetane and 2-methyl naphthalene which has the same ignition qualities as the sample fuel undersimilar conditions in a test engine. 2-methyl naphthalene (cetane number = 0)
MANUFACTURING OF SYNTHETIC GASOLINE OR PETROL: As there are limited sources for the supply of petrol, which might be exhausted very soon because of increased consumption, attempts are being made to synthesise petroleum from alternate sources. Coal is one such source. Three processes are mainly used for the production of synthetic petrol. They are 1) Fischer-Tropsch s Method 2) Bergius Process. FISCHER-TROPSCH S SYNTHESIS: Water gas (CO + H 2 ) produced by passing steam over heated coke is mixed with hydrogen. It is purified by passing through Fe 2 0 3 and then into a mixture of Fe 2 0 3 + Na 2 CO 3 to remove organic sulphur compounds. The purified gas is compressed to 5-25 atmospheres. The compressed gas is lead through a convertor containing a catalyst, consisting of a mixture of 100 parts of cobalt, 5 parts of thoria, 8 parts of magnesia and 200 parts of kiesel guhar earth maintained at about200 300 C. A mixture of saturated and unsaturated hydrocarbons are formed. The out coming hot gaseous mixture is led to a cooler, where a liquid, resembling crude oil is obtained. The crude obtained is fractionated to give gasoline and heavy oil. The heavy oil is cracked to get more gasoline. BERGIUS PROCESS: In this process, low ash coal pulverised to a fine powder is converted into a paste with a heavy oil and a catalyst nickel oleate is mixed with it. This mixture is sent to the convertor maintained at 350 500 C and a pressure of 200-250 atmospheres, where it meets the hydrogen gas. The hydrogen combines with carbon of the coal giving various hydrocarbons from waxes to gases including a liquid resembling crude oil. Reaction is carried for 1 hour. The gases coming out of the convertor are passed into a fractionators to get gasoline, middle and heavy oil. The middle oil fraction is subjected to hydrogenation in presence of a catalyst to produce more gasoline. The heavy oil fraction is recycled to make a paste with fresh batch of coal powder.
REFINING (OR) PURIFICATION OF GASOLINE: Gasoline obtained from straight distillation, or by cracking or by synthesis contains many undesirable substances, colouring matter, sulphur compounds, gums etc., which have to be removed. 1) Removal of sulphur: Gasoline containing complex sulphurous compounds called mercaptans(rsh), H 2 S and S is purified by the OXIDATION methods: a) Oxidation process: Oxidation of impurities especially mercaptans is done by the following methods. i) H 2 S0 4 treatment: Mercaptans on treatment with H 2 S0 4 get oxidized into insoluble R 2 S 3. S + 2RSH + H 2 S0 4 > R 2 S 3 + 2H 2 0 + S0 2 ii) Doctor s treatment or sweetening of petrol : This consists of the treatment of mercaptans with sodium plumbite (Na 2 PbO 2 ) which removes them in the form of lead mercaptide [Pb (RS) 2 ]. 2RSH + Na 2 PbO 2 ----> Pb(RS) 2 + 2NaOH Treatment with sulphur (sweetening) converts the mercaptides to sulphides while the lead is removed as PbS. Pb (RS) 2 + S > R 2 S 3 + PbS The sulphides so obtained are soluble in gasoline but do not have bad odours. iii) Catalytic Separation: PbS acts as a catalyst in the presence of NaOH and oxidizes the mercaptans to sulphides. 2 RSH + 2S + 2 NaOH > R 2 S 3 + Na 2 S + 2H 2 0 iv) Cupric chloride treatment: The mercaptans get oxidized to R 2 S 3 in the presence of CuCl 2 2RSH + CuCl 2 + S > R 2 S 3 + 2HCl + Cu 2) Removal of gum: The gum is a sticky undesirable material formed in the gasoline when diolefines present in the oil undergo polymerization. The treatment with sulphuric acid removes gum, but not completely. The gum formation can be minimized by the use of some inhibitors like benzyl aminophenols, butyl aminophenols etc. 3) Removal of colour: The colour of gasoline is due to the presence of S, N and gum etc., This can be removed by treatment with H 2 S0 4 and subsequent treatment with NaOH and washing with water or passing through a bed of Fuller s earth. REFORMING OF PETROL: Reforming is the process of bringing about structural modification in the components of straight run gasoline prepared by the fractional distillation of the crude oil. The prime objective of reformig is to improve the antiknocking characteristics of gasoline. This is usually carried out either thermaly or in the presence of a catalyst. i) Thermal reforming : This is carried out in a reactor maintained at a temperature of 500-600 C and a pressure of 85 atm. The feed stock in this process is straight-run gasoline. To prevent the gas formation at the expense of gasoline the process of quenching i.e., rapid cooling of the products are then fractionated to remove residual gas (stabilizing). During thermal reforming, cracking also occurs to yield alkanes and alkenes, which might undergo dehydrogenation, followed by cyclization (dehydrocyclization) to yield napthalenes. Conversion of n-alkanes to branched-chain alkanes also takes place during the process of thermal reforming.
ii) Catalytic reforming:catalytic reforming is carried out to get a better grade and yield of gasoline by using either a fixed bed or fluidized bed of Pt (0.75%) supported on alumina at 460-530 C and 35-50 atmospheric pressure. The reactions occurring during catalytic reforming process are:
LUBRICANTS In all types of machines, the surfaces of moving or sliding or rolling parts rub against each other. This mutual rubbing of one part over the other leads to resistance of movement, which is termed as friction. Friction usually causes the wear and tear of the machinery, and since heat is generated in this process, it also reduces the efficiency of the machinery. Therefore to overcome the problems created due to friction we employ a substance coined as lubricant. Thus a lubricant may be defined as a substance which reduces the friction when introduced between two surfaces and the phenomenon is known as lubrication. MECHANISM OF LUBRICATION: There are mainly three types of mechanisms by which lubrication takes place. They are: 1) Fluid film lubrication or hydrodynamic lubrication or thick film lubrication 2) Boundary lubrication or thin film lubrication and 3) Extreme-pressure lubrication. Fluid Film Lubrication or Hydrodynamic Lubrication: In this type of lubrication, the moving or sliding surfaces are separated from one another by a thick film of lubricant (i.e. 1000 A 0 in thickness), so that there is no direct contact between them. This film also results in covering the irregularities on the moving or sliding surfaces, thereby reducing friction and wear and tear. Further the coefficient of friction which is a ratio of force required to cause motion to the applied load is as low as 0.001 to 0.03. Based on all the above points it is clear that the fluid film lubrication is based on the properties of the lubricant particularly on its viscosity. This lubrication is satisfactorily done by hydrocarbon oils. These are generally blended with selected long chain polymers in order to maintain the viscosity of the oil as constant in all the seasons. Light machines like sewing machines, watches, clocks, delicate and scientific instruments are provided with fluid film lubrication. Boundary Lubrication or Thin Film Lubrication: A thin layer of lubricant is adsorbed on the metallic surfaces due to physical or chemical forces. This adsorbed layer helps to avoid a direct metal to metal contact between the rubbing surfaces. The load is carried by the layer of the adsorbed lubricant on both the metal surfaces. This type of lubrication operates when a continuous film of lubricant cannot persist. The coefficient of friction in this case varies from 0.05 to 0.15. Solid lubricants, greases and oils with proper additives function as lubricants in this type of lubrication. For example, graphite, MoS 2, mineral oils with additives of fatty acids or fatty oils and vegetable and animal oils and their soaps.
Extreme Pressure Lubrication: Normally under heavy load and high speed operating conditions, a special type of lubricants called high-pressure lubricants are to be employed in order to withstand the high temperatures generated due to frictional heat. In such applications liquid lubricants fail to stick and may decompose and even vaporize. Chlorinated esters, sulphurized oils and tri-cresylphosphate are examples of such additives. These additives react with metallic surfaces, at prevailing high temperatures, to form metallic chlorides, sulphides or phosphides, in the form of durable films. These films can withstand very high load and high temperatures (because of their high melting points). Hence, they serve as good lubricant under extreme-pressure and extreme-temperature conditions. CLASSIFICATION OF LUBRICANTS: On the basis of their physical state, lubricants can be classified as: a) liquid lubricants (or) lubricating oils b) semi-solid lubricants (or) greases and c) solid lubricants Lubricating Oils: These lubricating oils provide a continuous fluid film over the moving or sliding surfaces. They also act as cooling and sealing agents, and prevent corrosion. For eg: animal and vegetable oils, mineral or petroleum oils, and blended oils. Animal and Vegetable oils possess good oiliness, they are costly and undergo oxidation easily in the presence of moist air or aqueous medium. They are also useful in the preparation of greases and used as additives to improve lubricating characteristics of petroleum oils. However, these are now less preferred. Examples of vegetable oils: Olive oil, palm oil, castor oil, rape seed oil and hazel nut oil. Examples of animal oils: Whale oil, lard oil and tallow oil. Mineral oils obtained by fractional distillation of petroleum are cheap, quite stable under operating conditions and abundantly available and these replaced the utility of animal and vegetable oils. Further, since single oil does not possess all the good qualities of lubrication, certain specific substances (i.e. additives) have to be added to achieve the desirable characteristics. This resulted in blended oils. b) SEMI SOLID LUBRICANT (OR) GREASES: Lubricating grease is a semi solid. It consists of a soap being dispersed throughout liquid lubricating oil [liquid lubricant may be petroleum oil (or) synthetic oil and it contain additives for specific requirements]. Greases are prepared by saponification of fat with alkali, followed by adding hot lubricating oil while under agitation. The soap component forms the gel structure (interconnected network) within which the oil component resides. To improve the heat resistance of grease inorganic solid thickening agents are added. Ex: - carbon black, colloidal silica etc. Greases have higher frictional resistance than oils and can support much heavier loads at lower speeds. Greases are used - where the lubricating oil cannot serve as a lubricating film due to high load, low speed, sudden jerks etc. (Ex: - rail axle boxes)
SOLID LUBRICANTS: These are used either as a dry powder form or mixed with oil or water. The commonly used solid lubricants are graphite and molybdenum disulphide. Molybdenum disuiphide has a sandwich like structure in which layer of molybdenum atoms lie between two layers of sulphur atoms. This is effective upto 800 C whereas graphite is effective upto 370 C. Mixture of graphite (7%) and molybdenum disuiphide (70%) bonded with silicates (23%) are capable of withstanding high temperatures, low pressures and nuclear radiations and hence used in space vehicles. PROPERTIES OF LUBRICANTS: 1) VISCOSITY: Viscosity is the property of a fluid that determines its resistance to flow. It is an indicatorof flow ability of a lubricating oil, i.e., the lower the viscosity, greater the flow ability.viscosity is also coined with 2 other terms i.e., a) Absolute viscosity : This is defined as the tangential force per unit area which is required to maintain a unit velocity gradient between two parallel layers. It is denoted by q and its unit in C.G.S. system is poise. b) Kinematic Viscosity : It is the ratio of absolute viscosity to density for any fluid. It is denoted by y and its unit in C.G.S. system is stokes. VISCOSITY INDEX: Further the variation of viscosity of a liquid with temperature is called viscosity index. Generally for every 1 C rise in temperature the viscosity index decreases by 2%. However, the viscosity of good lubricating oil should not change much with a change in temperature. The mathematical expression for viscosity index (VI) is where, U = viscosity of test oil at 38 C L = viscosity of the standard oil at 38 C having a VI of zero H = viscosity of the standard oil at 38 C having a VI of 100 Oils of gulf origin are assigned an arbitrary VI of zero, while those of Pennsylvania are assigned as VI value of 100. Any test oil is compared at 38 C with zero VI and 100 VI oils. 2) FLASH AND FIRE POINT: Flash Point: The flash point of an oil may be defined as the lowest temperature at which it gives off vapours, which will flash if brought into contact with a flame. Fire Point: The fire point of an oil is defined as the lowest temperature at which it will give enough vapours, which on rising will begin to produce a continuous flame above the oil for at least 5 seconds. Significance: In industries oils are generally used for combustion, lubricant and cooling purposes. To use oil for any purpose, we are required to know the flash point and fire point, to eliminate the fire hazards and to set working temperature conditions. Flash point is also used as a means to identify the presence of impurities in the lubricant oil.
3) CLOUD AND POUR POINT: Cloud point: The cloud-point is the temperature at which crystallization of solids in the form of a cloud or haze first becomes noticeable, when the oil is cooled in a standard apparatus at a standard rate. Pour Point:The pour point is the temperature at which the oil just ceases to flow when cooled at a standard rate in a standard apparatus. Significance: In order to understand the suitability of lubricants in cold conditions, we make use of cloud point and pour points. Usually lubricating oils derived from petroleum contains dissolved paraffin wax and other asphaltic or resinous impurities. These impurities tend to separate out of the oil at lower temperatures. Further solidification of lubricant normally causes jamming of the machine. Cloud point is useful for estimating the temperature at which filter screens in the fuel intake system of diesel engines might become clogged because of separation of wax. Pour point values of petroleum and non-petroleum lubricants are necessary when functioning under sub-freezing conditions. The pour point has a greater significance for lubricating oils. It determines the suitability of a lubricant or a hydraulic oil for low temperature installations. Important examples are refrigerator plants and aircraft engines, which may be required to start and operate at sub-zero temperatures. 4) ANILINE POINT: The tendency of a lubricant to mix with aniline is expressed in terms of aniline point. Significance: This point gives an indication of the possible deterioration of oil in contact with rubber sealings, packings etc. to prevent leakage since aromatic hydrocarbons have a tendency to dissolve natural rubber and certain synthetic rubbers. Aniline point of a lubricant is a measure of its aromatic content. A lubricant with a low aniline point has high aromatic content and consequently low aromatic content in the lubricants is desirable. Aniline point is determined by mixing equal volumes of the oil sample and aniline in a test tube. The mixture is heated till a homogeneous solution is obtained. The test tube is then allowed to cool under controlled conditions. The temperature at which the two phases (oil and aniline) separate out as indicated by the sudden appearance of cloudiness throughout the medium, is recorded as the aniline point of the sample. 5) ACID VALUE (OR) NEUTRALIZATION NUMBER: Acid value is defined as the number of milligrams of KOH required to neutralize the acidic constituents in 1 gm of the oil. Acidity in lubricating oil may be due to oxidation products of oil, additives used to improve processing of the lubricating oil. Generally lubricating oils have acid values of less than 0.1. Significance: This is a measure of acidic or basic impurities present in the lubricating oil. Determination of acidic impurities is most common and is expressed as acid value or number. Values greater than 0.1 indicate oxidation of oil leading to gum formation and corrosion of the equipment. Therefore determination of the acid value is useful for determining whether the lubricating oil has to be replaced.
6) MECHANICAL STABILITY: To know the suitability of a lubricant under conditions of very high pressure, different mechanical tests are carried out. One such test is four-ball extreme-pressure lubrication test in which the lubricant under test is poured in a machine containing four balls. The lower three balls is stationery while upper ball is rotated. Load is gradually increased and the balls are examined at specific intervals for scale formation etc. on them. If the lubricant is satisfactory it will not form any scales otherwise the heat generated may weld the balls and lead to scratches or scale formation. Thus, this test enables us to determine the maximum load that can be carried safely by a lubricant.