1.Combustion Process in ICENew Microsoft Office PowerPoint Presentation

8/13/2019 1.Combustion Process in ICENew Microsoft Office PowerPoint Presentation

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Combustion

rocess in InternalCombustion

Engines


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General Process:

The combustion process

generally takes place eithein a homogeneous or a

heterogeneous fuel vapoair mixture.


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Heterogeneous mixture :

s of gaseous fuel or atomized liquid f

plets are injected in air and they buair in which the fuel is injected may

stationary or moving. In both casesombustion takes place in the gaseou

ase. Therefore when liquid droplet ajected they must vaporize and form

por air mixture before combustion m


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he combustion process depe

on:* the nature and velocity of th

chemical reactionheat and mass transfer in the fla

zone and heat transfer to thesurroundings


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In heterogeneous gas mixture the

ombustion rate is determined by th

velocity of mutual diffusion of fuelvapors and air. The rate of chemical

reaction comes in second. Theiffusion rate is usually higher than th

evaporation rate and therefore theombustion rate is determined by th

rate of evaporation.


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As soon as a homogeneous fuel vapor ai

mixture is ignited, a flame appears and i

gradually spreads in the mixture. Flamepropagation is caused by heat transfer

and diffusion of burning fuel moleculesrom the combustion zone to the adjace

layer of fresh mixture. The adjacent laye

is thereby heated above the self ignitio

temperature and consequently a flame

appears in the layer.


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The flame front is a narrow zon

separating the fresh mixture forthe combustion products. The

velocity with which the flamefront moves with respect to the

unburned mixture is in a directionormal to its surface and is calle

the normal flame velocity


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The flame velocity is dependent

on the rate of chemical reactionin the flame front, the rate of

heat transfer and diffusion ofburning gas mixture from the

flame front to the adjacent

layer.


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The maximum flame velocity is

obtained when the mixture is slightly

rich in fuel (=0.85 to 0.9)The flame velocity at atmospheric

pressure and room temperature iswithin 30 -50 cm/sec. when the

mixture is made comparatively leaner

or richer in fuel the flame velocity

decreases until reaching a minimum

value of 810 cm/sec


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When the flame velocity reaches

these low valuese heat loss from the combustion zon

at release amount due to combustiThe flame will extinguish at this

limiting condition. These mixturstrength give the flame blow out

conditions


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The mechanism of combustion of

heterogeneous mixtures, such as jets

of gaseous fuel or droplets of

tomized fuel injected in stationary o

moving stream of air, is quite

different from that of homogeneous

mixture. The rate of combustiondepends on the rate of diffusive

mixing of fuel with air


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uel vapor formed subsequently from th

ame droplet takes part in combustion a

it passes through the flame zone.eaction rate have less influence becaus

combustion mainly depends onevaporation rate. Self ignition or

pontaneous ignitionof fuel air mixture,

t the high temperature produced due t

ompression, is of primary importance i

determining combustion.


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ombustion can take place with very

lean overall mixtures (air coefficient>4.0) when the mixture is

heterogeneous. This type has localones where the air coefficient has

values between 0.85 and 0.9

corresponding to maximum rate of

chemical reactions


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Ignition usually starts at this zone and

the flame produced helps to burn the

fuel in the adjoining leaner mixture. Ifthe overall mixture is not sufficiently

lean (


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igh temperature produced a

hese zones due to combustion o part of fuel will cause fue

olecules in these zones to cracithout oxidation and for

arbon particles. This is theason for smoky exhaust o

iesel engines at full load


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Self

Ignition


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Spontaneous ignition of gaseous fuel jet

or droplets of atomized liquid fuel

njected into stationary or moving heateir, is called self ignition. Self ignition ma

also take place in homogeneous fuelvapor air mixture if its temperature is

raised to a high value. In both cases the

emperature of the fuel vapor air mixtur

must reach a value where the mixture

burn spontaneously


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aseous or liquid fuels injected i

hot air whose temperature is eqor higher than the self ignition

temperature, do not burn as soas they are injected. Similarly

combustion does not take placesoon as a homogeneous mixture

eated to self ignition temperatu


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There is always a time lag

between the moment the fuel is

injected or the homogeneous

mixture is heated to self ignitionemperature and the moment th

flame appears. This time lag iscalled the ignition lag or

ignition delay i


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In case of liquid fuel the ignition lag

consists of:1. the time taken in forming the droplet

vaporating the droplets and preparing t

ombustible fuel vapor air mixture. This icalled the physical lag or physical delay

. the time taken by reacting gases beforactual combustion starts. This called th

chemical lag or chemical delay.


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2. Mixture strength

hen a combustible mixture is diluted wit

ess air (> 1), the rate of reaction diminis

cause the concentration of molecules in t

ixture is reduced and some heat producering combustion is spent in heating the ex

air; therefore the ignition lag increases.

Enrichment of the mixture (< 1) results i

igher fuel concentration in the mixture an

therefore, decreases the ignition lag.


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The reaction rate is reducedafter certain degree of

enrichment (< 1) due to non-availability of sufficient oxygen

molecules. Therefore, theignition lag again increases


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3. Additive compoundsSome compounds added

with fuel in small quantities,

influence the ignition lag ofthe fuel vapor air mixture


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COMBUSTION IN

SPARK IGNITION

ENGINES


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1. Phases of Combustion

In spark ignition engines ahomogeneous mixture of vaporized

liquid fuel (or gaseous fuel), air and

residual gases is ignited by an electric

spark. The charge near the spark gas

burns soon after the spark is applied. Aflame develops and spreads over the

entire mixture.


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3 4

1 2

1spark point1-2 First phase

2-3 second phase

3-4 third phase


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From a flame trace (doneby photography) it can be

observed that flame

travel can be divided intothree periods.


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The First Phase

The Second Phase

The Third Phase


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1. Composition of mixture

2. Compression ratio

3. Load

4. Speed

5. Shape of combustion chamber


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Abnormal

Combustion inSpark Ignition

Engines


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Combustion

Chamber for Spark

Ignition Engines


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Ignition

System


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Phases of Combustion


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Ignition lag phase or preparatorphase of combustion

*Rapid combustion phase*Controlled combustion phase

*After burning phase


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Factors affecting

ombustion in compressioignition engines


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Abnormal Combustion in

Compression IgnitionEngine


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* Knocking*Reduction of diesel knock


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ombustion Chamber fo

Compression Ignition

Engines

Documents

1.Combustion Process in ICENew Microsoft Office PowerPoint Presentation