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"Fuels"

It is a source of heat energy, which may be burnt in atmospheric air in such a manner that heatevolved is capable of being economically used for domestic and industrial purposes for heatingand generation of power. Combustion converts chemical energy of fuel into useful work byengine.

Classification of fuels:SOLID FUELS LIQUID FUELS GASEOUS FUELS

i. Primary fuelsWood, peat, lignite,bituminous coal,

Anthracite, oil shales, tarsands, bitumen

ii. Secondary fuelsSemi coke, cokecharcoal, briquettes

petroleum coke,pulverized coal, solidrocket fuels (Thiokol,hydrazine nitrocellulose).

i. Primary fuelsCrude oil or petroleum

ii. Secondary fuelsGasoline/motor spirit,diesel oil, kerosene,fuel oil, coal tar,alcohols, syntheticspirit.

i. Primary fuelsNatural gas

ii. Secondary fuelsCoal gas, water gas,producer gas, oil gas,acetylene gas, liquefiedpetroleum gas.

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COMBUSTION :

It is an exothermic oxidation reaction, in which a fuel usually oxidized into water and CO 2 gases with

release of heat energy.

Fuel + Air + Ea CO2 + H2O -H

For a single pure hydrocarbon, combustion graph can be plotted against enthalpy as

Ea= Activation energy

= Difference of energy between ground and excited state of reactant= Amount of energy required to bring up one mole of reactant into excited state.

This energy is required to make all C__

C bonds in fragile state therefore Ea depends upon nature of C__

C

bond. If Ea is higher then it difficult to combust and if lower then it is easy to combust.

Hcombustion =Difference of energy between reactant and product

=Amount of heat energy released by complete combustion of one moleof hydrocarbon.It depends upon number of carbons per hydrocarbon molecule.

But fuels are not pure compounds.

LPG=C3 to C4, Petrol = C5 to C10, Diesel = C12 to C18

So these terminologies are replaced as

Pure Hydrocarbon Hcombustion , Ea (per mole)Fuel (Mixture of Hydrocarbons)Calorific Value , Combustibility (per unit mass)

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Parameters of fuel quality1- Flash pointLowest temperature at which a fuel gives off enough vapours which are sufficient to form anignitable mixture with air in such a way that application of test flame will produce spark or flashwithin the test flame chamber. It is inverse measurement of volatility of fuel. Low flash pointfuel would easily atomized by carburetor and vice versa.

2- Adiabatic Flame TemperatureThe highest possible temperature which can be achieved by the complete combustion of fueunder the conditions that the burning occurs in an adiabatic vessel and no dissociation occur.

There are two types of adiabatic flame temperature depending on how the process iscompleted, constant volume and constant pressure.The constant volume adiabatic flame temperature is the temperature that results from acomplete combustion process that occurs without any work, heat transfer or changes in kineticor potential energy. The constant pressure adiabatic flame temperature is the temperature thatresults from a complete combustion process that occurs without any heat transfer or changesin potential energy. Its temperature is lower than the constant volume process because someof the energy is utilized to change the volume of the system (i.e., generate work).

3- Calorific Value:Amount of heat evolved by complete combustion of a unit mass of fuel.

In CGS system cal/gIn SI system Kcal/Kg

4- Octane numberIt indicates antiknock properties of fuel. Standard reference fuels for octane number below 100are iso-octane and n-heptane with assigned values 100 and 0 respectively. So octane numberdefined as,Volume percentage of iso-octane in a blend with n-heptane which produce equal knockintensity as test fuel under standard conditions of single-cylinder engine.So a fuel of 87 octane number means its knock intensity matched with knock intensity of a

blend of87 volume percent ofiso-octane and 13 volume percent ofn-heptane.

Octane number higher than 100 is called Performance number. Standard reference fuel forperformance number is blend of tetraethyl lead (TEL) with iso-octane. Maximum performancenumber is 125 e.g Aviation gasoline.

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5- Cetane numberCetane number show how rapidly combustion begins after injection of fuel into combustionchamber i.e it is inversely related to ignition delay of a fuel in engine. Reference fuels areHexadecane (C16H34 = cetane) having low ignition delay time and 2,2,4,4,6,8,8-heptamethynonane (C16H34) have long ignition delay time, they are assigned value 100 and 0 respectively

on cetane scale. Cetane number defined as, Percentage volume of hexadecane in a blendwith heptamethyl nonane which have equal ignition time delay as test fuel under standardcondition of single cylinder engine. Structural features like long chain, saturated, un-branchedand chain increase cetane number while branching & un-saturation decrease cetane number.High cetane number fuel will facilitate easy starting of engine in cold weather, faster warm upincrease engine efficiency & power output, and reduce exhaust smoke, odor & combustionnoise.

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Parameters of lubricants quality

1. Dropping Point of grease:Dropping point is the temperature at which grease passes from a semisolid to a liquid state sothat it becomes substantially fluid under the test condition.

2. Viscosity Index (VI)In many applications a lubricating oil will have to function in a machinery over a considerablewide range of operating temperature while oils generally have low viscosity at high temperatureand high viscosity at low temperature. Although change in viscosity could not be avoided butgood oil should have minimum variation in viscosity along with temperature.Viscosity index show effect of change of temperature on the viscosity of an oil. VI is thenumerical inverse relationship expression of average slope of the viscosity temperature curve oflubricating oil b/w 40 0C to 100 0C temperature. In this concept, an oil whose viscosity changerapidly with change in temperature should have high slope value & low VI while an oil with aminimum change in viscosity with change in temperature has a low slope value & high VI.

Pennsylvanian oil show small change in viscosity with temperature so choose as high viscosityindex standard with VI value 100 while Gulf oil show large change in viscosity withtemperature so choose as low viscosity index standard with VI value zero. VI of oil underexamination is calculated by comparing its viscosity with these two standard oils b/w 40 0C to100

0C temperatures by following formula.

VI = VL - VS x 100VL - VH

VL = Viscosity of low standard oil at 40 0C

VS = Viscosity of sample oil at 400C

VH = Viscosity of high standard oil at 400C