1.Lec1 Fuels for Internal Combustion Engines

7/28/2019 1.Lec1 Fuels for Internal Combustion Engines

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Fuels for Internal

Combustion

Engines


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Introduction.

Heat required in I.C.E. is generally

produced by chemical reaction of fueland air inside the engine cylinder. The

reaction time is of the order ofhundreds to thousands of a second


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In engines where the fuel air

mixture is formed outside thecylinder ( carburetor engines )

the fuel must evaporate easily

at atmospheric temperature

and form an inflammable

mixture with incoming air.


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In engines where the fuel is

directly injected into thecylinder (diesel engines ), the

atomized fuel must evaporatequickly and must be well mixed

with air so that he combustionprocess may be completed

within reasonable time.


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The reaction characteristics of the

fuel should be such that the rate ofpressure and temperature rise

inside the cylinder due tocombustion should be moderate;

this decreases the possibility oflarge mechanical and thermal

stresses


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The fuel at the completion of

combustion should not leave

appreciable carbon, coke, sulfur or

gum deposits which may cause

excessive wear and corrosion on

cylinder and piston rings. The

composition of the fuel should notlead to the evolution of harmful

combustion products.


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Influence of Fuel

on Engine

Performance


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1. Output:.

The output break h.p. Nbr of an ICE isexpressed as follows

)4500427(

)1(thcabr Q

FFVN


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Qv is a fuel characteristic and affects

the engine output so does the thermalefficiency. The inlet temperature of

mixture which also vary with fuel

characteristics affects the engine

output

cav QF

FQ

)1(


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2. Thermal efficiencyDue to combustion of a certain fuel the

change of number of moles nm

=nmproducts -nmmixture affects thepressure of the products as follows:

mixture

products

mixture

products

mixtureproductsnm

nm

T

Tpp


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Hence an increase in the number of moles

increases the pressures of the product for

the same value of TP which is dependenton Qc . Therefore, for fuels where there is

an increase in the number of moles due to

combustion the thermal efficiency will behigher than for fuels where there is no

such increase.

mixture

products

mixture

products

mixtureproductsnm

nm

T

Tpp


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3. Inlet temperatureIf the prevailing atmospheric temperature

is higher than that necessary for efficientevaporation and distribution of fuel, inlet

temperature of the mixture in carburetor

engines will be reduced due to the

evaporation of fuel. Fuels with higher

evaporation heat needs will reduce theinlet temperature to a greater extent.

Using such fuels will increase the engine

output due to the increase in inlet density.


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4. Specific Fuel consumption

in case of petroleum fuels the heat of combustiondoes not vary very much. Therefore SFC mainly

depends on the thermal efficiency. For alcohols or

benzols whose heat of combustion is much lower

than those of Petroleum fuels the SFC will be much

higher compared to petroleum fuels for the same

power output and thermal efficiency

cthermal QCFS

1

...


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Properties of

Internal CombustionEngine Fuels


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.1. Volatility

.

For engines with carburetor the liquid fuelmust be volatile enough to produce a

combustible fuel vapor air mixture at

intake temperature and to producecomplete fuel vapor air mixture inside the

cylinder before combustion. In case of

diesel fuels the evaporation rate at the

average gas temperature should be

sufficiently high.


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2. Anti-knock value

and ignitability.

.

.


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2.1 . Spark ignition engine

fuel.In SIE fuel air mixture is taken inside

the cylinder where aftercompression, combustion is

initiated in this mixture. Thus thefuel must resist abnormal burning

or detonation Ab l b i d i


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Abnormal burning or detonation

can cause:

*too rapid rate of energy release*excessive temperature and pressure

inside the cylinder*decreases the efficiency of

conversion of heat into useful work

**this will adversely affect the

efficient and smooth operation of the

engine


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The fuels used for petrol

engines must offer sufficientresistance to detonation, i.e.

it must have a suitable anti-

knock property. The anti-knock property is expressed

by the Octane number


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2.2. Compression ignition engine

fuelIn case of diesel engine the fuel is

injected into compressed air with

increased temperature. There is a time

interval between the moment the fuel

is injected to the moment it is ignited.This interval is known as the ignition

lag or ignition delay


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A long period of ignition lag will

result in a high rate of pressurerise in the cylinder and this will

result in rough running of theengine. Ignition lag depends on

the operating conditions as wellas on the physical and chemical

properties of the fuel


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3. Heat of Combustion..The quality of a fuel in

general is determined by the

amount of heat evolvedupon complete combustion


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4. ChemicalComposition and

Molecular Structure

of Fuels


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5. Anti-knock

Rating of Fuel

5 1 F l f S k I i i E i


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5.1. Fuel for Spark Ignition Enginesresistance to detonation is an extremely

important characteristic of the S.I. fuel. Inaddition to the effect of the chemical

characteristic of the hydrocarbon, other

factors such as fuel-air ratio, ignition

timing, dilution, effectiveness of jacket

cooling, atmospheric conditions,compression ratio, etc. , affect the

tendency of the fuel to detonate in engine

cylinder.


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Hence a scale of detonation

intensity is necessary and a

particular intensity denotedby a point on the scale must

be selected for comparisonof fuels being rated.


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The anti-knock value of a fuel is

determined by comparing its anti-

knock property with a mixture of tw

reference fuels. The iso-octane

(C8H18) is observed to have very highresistance to detonation. On the

other hand normal heptane(C7H16)was found to have extremely

poor resistance to detonation.


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A suitable scale known asthe Octane number scale

was evolved for specifying

the knock resistanceproperty of fuels.


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Iso-octane was given an octane

number of 100, and normalheptane was rated at 0 octane

number. The octane number of a

fuel is defined as the percentage by

volume of iso-octane in a mixture

of iso-octane and normal heptane,

which will match the detonation

intensity of the fuel.


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Knock InhibitorsChemical compounds like tetra-ethyl

lead Pb(C2H5)4 , iron carbonyl, and

nickel carbonyl appreciably increasesthe knock resistance of spark ignition

engine fuel when a small quantity isadded to the fuel (0.5 ml to 1.5 ml per

liter of gasoline)


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A few milliliters of tetra-ethyl lead

Pb(C2H5)4 added per liter of

gasoline increase the octane

number. The tetra-ethyl lead

Pb(C2H5)4 is widely used but it alsoproduces cylinder deposits and

poisonous emissions. Therefore theamount of compound added is

restricted


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5.2 Fuels for compression

ignition engines.In the case of diesel fuels the

knock resistance depends on

chemical characteristics as well

as on operating conditions and

engine design


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Cetane number of a fuel is the

percentage by volume of normalcetane in a mixture of normal

cetane and alpha methylnapthelene, which has the same

ignition characteristics (ignitiondelay) as the test fuel when

comparison is carried out


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Knock InhibitorsKnock resistance property of diesel fuel can be

improved by using small quantities of

compounds like amyl nitrite, ethyl nitrate, ethyl

nitrite or ether. These compounds are generally

introduced in small amounts with the intake air.

They ignite early in the compression stroke and

thereby increase the temperature of

compressed air at the time when fuel injection

starts. The ignition delay of the fuel is thereby

reduced


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h l


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Chemical

Reactions in

Fuel

Combustion


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Analysis ofProducts ofCombustion

Documents

1.Lec1 Fuels for Internal Combustion Engines