COMBUSTION IN CI ENGINES

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Selected parameters in CI Engine

Compression ratio rc

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CI engines (Diesel engine(3

Major process in the combustion mechanism4

Combustion In CI. Engines

Only air is compressed through a high compression ratio (14:1 to 24:1)raising its temperature and pressure to a high value.

Fuel is injected into the cylinders late in compression stroke through oneor more injectors into highly compressed air in the combustion chamber.

Injection time is usually about 20° of crankshaft rotation, starting atabout 15° bTDC and ending about 5° aTDC.

Combustion in a CI engine is an unsteady process occurringsimultaneously at many spots in a very non-homogeneous mixture at arate controlled by fuel injection.

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Combustion In C.I. Engines

In addition to the swirl and turbulence of the air, a highinjection velocity is needed to spread the fuel throughout thecylinder and cause it to mix with the air.

After injection the fuel must go through a series of events toassure the proper combustion process:

I. Atomization. Fuel drops break into very small droplets, thesmaller the original drop size emitted by the injector, thequicker and more efficient will be this atomization process.

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Combustion In C.I. Engines

II. Vaporization. The small droplets of liquid fuel evaporate tovapor. This occurs very quickly due to the hot air temperaturescreated by the high compression of CI engines.

High air temperature needed for this vaporization processrequires a minimum compression ratio in CI engines of about12:1

About 90% of the fuel injected into the cylinder can bevaporized within 0.001 second after injection.

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Combustion In C.I. Engines

III. Mixing. After vaporization, the fuel vapor must mix with air toform a mixture within the AF range which is combustible.

This mixing is formed because of the high fuel injectionvelocity added to the swirl and turbulence in the cylinder air

Combustion can occur within the equivalence ratio limits of= 1.8 (rich) and = 0.8 (lean).

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Combustion In C.I. Engines

The non-homogeneous distribution of air-fuel ratio thatdevelops around the injected fuel jet.

The above figure shows fuel jet of a CI showing air-fuel vapor zones around theinner liquid core zone

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Combustion In C.I. Engines

The liquid core is surrounded by successive zones of vaporwhich are

i. (A) too rich to burnii. (B) rich combustibleiii. (C) stoichiometriciv. (D) Lean Combuatiblev. (E) Too lean to burn

• Self ignition starts mainly in zone B and solid carbon and soot formation occure in A and B

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Combustion In C.I. Engines

Self-Ignition. At about 8° bTDC, 6-8° after the start of injection,the air-fuel mixture starts to self-ignite.

Actual combustion is preceded by secondary reactions,including breakdown of large hydrocarbon molecules intosmaller species and some oxidation.

These reactions caused by the high-temperature air, areexothermic and further raise the air temperature in theimmediate local vicinity. This finally leads to an actualsustained combustion process.

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Combustion In C.I. Engines

Combustion. Combustion starts from self-ignition simultaneouslyat many locations in the slightly rich zone of the fuel jet, wherethe equivalence ratio is = 1 to 1.5 (zone B in the previousfig).

When combustion starts, somewhere between 70% and 95% ofthe fuel in the combustion chamber is in the vapor state.

When combustion starts, multiple flame fronts spreading fromthe many self-ignition sites quickly consume all the gas mixturewhich is in a correct combustible air-fuel ratio, even where self-ignition wouldn't occur.

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Combustion In C.I. Engines

In an SI engine,

The air-fuel ratio remains close to stoichiometric value from noload to full load.

In a CI engine,

irrespective of load, at any given speed, an approximatelyconstant supply of air enters the cylinder.

With change in load, the quantity of fuel injected is changed,varying the air-fuel ratio.

The overall air-fuel ratio thus varies from about 18:1 at full loadto about 80:1 at no load.

The CI engine is always designed to operate with an excess air, of15 to 40% depending upon the application

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Combustion In C.I. Engines

The power output curve for a typical CI engine operating atconstant speed is shown in Fig below.

The approximate region of AI F ratios in which visible black

smoke occurs is indicated by the shaded area.

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In cylinder measurment

This graph shows the fuel injection flow rate, net heat release rate and cylinderpressure for a direct injection CI engine.

Motoring curve is Cylinder pressure Vs crank angle curve, which is observed whenno firing occurs into the cylinder that means the pressure which build inside thecylinder is basically due to the compression of the fresh air charge going into thecylinder

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Stages Of Combustion In CI Engines

The combustion in a CI engine is considered to be taking place

in four stages

Ignition delay period

Rapid combustion

Controlled combustion and

After-burning.

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Ignition Delay Period

Ignition delay (0.7-3ms) period is counted from the start of injection to thepoint where the pressure time curve separates from the compression curveindicated as start of combustion.

The delay period in the CI engine influence both engine design andperformance.

It affects

combustion rate

knocking

engine starting ability

the presence of smoke in the exhaust.

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Ignition Delay Period

Point a represents the time of

injection

Point b represents the time at

which the pressure curve(caused by combustion)first separates from thecompression process

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Ignition Delay Period

The ignition delay period can be divided into two parts,

I. Physical delay

II. Chemical delay.

Physical Delay

The physical delay is the time between the beginning of injectionand the attainment of chemical reaction conditions.

During this period,

the fuel is atomized,

vaporized,

mixed with air and

raised to its self-ignition temperature.

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Ignition Delay Period

The physical delay depends on

The type of fuel,

for light fuel the physical delay is small

for heavy viscous fuels the physical delay is high.

Injection Pressure

The physical delay is greatly reduced by using high injectionpressures

Combustion chamber temperatures and Turbulence to facilitate

breakup of the jet and

improving evaporation.

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Ignition Delay Period

Chemical Delay: During the chemical delay, reactions start slowly and then

accelerated ignition taking place.

Generally, the chemical delay is larger than the physical delay.

Chemical delay depends on the temperature of the surroundings

At high temperatures, the chemical reactions are faster

In most CI engines the ignition lag is shorter than the duration ofinjection.

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Factors Affecting The Delay Period

Many design and operating factors affect the delay period. The important ones are: Cetene number

Ignition timing

Compression ratio

Engine speed

Output

Atomization of fuel and duration of injection

Quality of the fuel

Intake temperature

Intake pressure

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Factors Affecting The Delay Period 23

Period of Rapid Combustion

The period of rapid combustion

also called the uncontrolled

combustion, is that phase in which

the pressure rise is rapid.

The period of rapid combustion is

counted from the beginning of

the combustion to the point of

maximum pressure on the

indicator diagram.

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Period of Controlled Combustion

The temperature and pressure in the second stage is alreadyquite high.

Hence the fuel droplets injected during the second stage burnfaster with reduced ignition delay as soon as they find thenecessary oxygen and any further pressure rise is controlledby the injection rate.

The period of controlled combustion is assumed to end at

maximum cycle temperature.

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Period of After-Burning

Combustion does not cease with the completion of theinjection process.

The unburnt and partially burnt fuel particles left in thecombustion chamber start burning as soon as they come intocontact with the oxygen.

This process continues for a certain duration called the after-burning period.

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Period of After-Burning

Usually this period starts from the point of maximum cycletemperature and continues over a part of the expansionstroke.

Rate of after-burning depends on the velocity of diffusion and turbulent mixing of unburnt and partially burnt fuel with the

air.

The duration of the after-burning phase may correspond to70-80 degrees of crank travel from TDC.

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Block Diagram The sequence of the events in the entire combustion process in a , CI

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KNOCK IN CI ENGINES

If the ignition delay is longer, The actual burning of the first

few droplets is delayed anda greater quantity of fueldroplets gets accumulated inthe chamber.

When the actual burningcommences, the additionalfuel can cause too rapid arate of pressure rise asshown resulting in a jammingof forces against the pistonand rough engine operation

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KNOCK IN CI ENGINES

Engine knock phenomenon is similar to that in the SI engine.

In SI engine,

knocking occurs near the end of combustion

In CI engine, knocking occurs near the beginning of combustion.

In order to decrease the tendency of knock

it is necessary to decrease the ignition delay and

thus decrease the amount of fuel present when the actual burning of thefirst few droplets start.

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Types of diesel engines based on engine speed range

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Types of diesel engines based on engine speed range

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Combustion Chambers For CI Engines

CI engine combustion chambers are classified into two categories

Direct-Injection (DI)

Indirect-Injection (IDI)

Direct-Injection (DI)

This type of combustion chamber is also called an opencombustion chamber.

In this type the entire volume of the combustion chamber islocated in the main cylinder and the fuel is injected into thisvolume

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Combustion Chambers For CI Engines

Direct-Injection (DI) Combustion Chamber

Open combustion chamber.

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Combustion Chambers For CI Engines

Direct-Injection (Open Combustion Chamber)

The main advantages of this type of chambers are:

Minimum heat loss during compression because of lowersurface area to volume ratio and hence, better efficiency.

No cold starting problems.

Fine atomization because of multi hole nozzle.

The drawbacks of these combustion chambers are:

High fuel-injection pressure required and hence complex design of fuel-injection pump.

Necessity of accurate metering of fuel by the injection system, particularly for small engines.

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Combustion Chambers For CI Engines

Indirect-Injection (IDI) Type

In this type of combustion chambers, the combustion space is divided intotwo parts, one part in the main cylinder and the other part in thecylinder head.

The fuel-injection is effected usually into that part of the chamberlocated in the cylinder head.

These chambers are classified further into

Swirl chamber in which compression swirl is generated.

Pre combustion chamber in which combustion swirl is induced.

Air cell chamber in which both compression and combustion swirl are induced

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Combustion Chambers For CI Engines

Indirect-Injection (IDI)Combustion Chamber

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Combustion Chambers For CI Engines

The main advantages of the indirect-injection combustion chambersare:

injection pressure required is low

direction of spraying is not very important.

These chambers have the following serious drawbacks which have

Poor cold starting performance requiring heater plugs.

Specific fuel consumption is high because there is a loss ofpressure due to air motion through the duct and heat loss due tolarge heat transfer area.

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Contents of combustion gases from diesel engines39

Limits of pollutant emissions from CI engines40

Effect of stoichiometry on pollutions emission from CI engines

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Effect of injection pressure on soot emission from CI engines

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Nox emissions versus Cetene number43

Soot removal from combustion gases44

SMF (Sintered Metal Filter)45

DPF (Diesel Particulate Filter)46

Scheme of soot particles Removals47