8 compression ignition engines

7/25/2019 8 Compression Ignition Engines

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88TOPICTOPIC

Compression IgnitionCompression IgnitionEnginesEngines

Sections 10.1, 10.2.1-10.2.3, 10.3.2Sections 10.1, 10.2.1-10.2.3, 10.3.2


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Combustion in CI enginesCombustion in CI engines

Fuel injected into cylinder toward the end of the

compression stroke atomizes, vaporizes and mixes with high

temperature air

Since the air temperature and pressure are above the fuelsignition point, spontaneous ignition of portions of already

mixed fuel and air occurs after a delay period

he cylinder pressure increases as combustion occurs

!njection continues until desired amount of fuel has entered

the cylinder

"ombustion continues well into expansion stroke until all

fuel is burned


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Combustion in CI enginesCombustion in CI engines

!t is unsteady, heterogeneous, three#dimensional

process

$etails of combustion process depend on%

&"haracteristics of the fuel

&$esign of combustion chamber and fuel#injectionsystem

&'ngines operating conditions


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CI combustion vs. SI combustionCI combustion vs. SI combustion

here is no knock limit as in S! engines & higher

compression ratios can be used

or(ue is varied by varying the amount of fuelinjected & the engine can be operated unthrottled

)lack smoke *excessive soot+ in the exhaust

constrains the air#fuel ratio & mixture composition isalways lean of stoichiometric *-. or more+


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Fuel conversion efficiencyFuel conversion efficiency

$iesel engines have a higher maximum efficiency

than S! engines due to following reasons

&"ompression ratio is higher

&/ir#fuel mixture is always lean of stoichiometric


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he fall in part load efficiency is moderated by%

&he absence of throttling

&he leaner air#fuel mixture

&he shorter duration combustion

Efficiency at partial loaEfficiency at partial loa


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he major problem in diesel combustion chamber

design is achieving sufficiently rapid air#fuel mixing

to complete combustion in the interval close to $"

$iesel engines are divided into two basic categories

according to their combustion chamber design

&$irect#injection *$!+ engines

&!ndirect injection *!$!+ engines

!ypes of iesel combustion systems!ypes of iesel combustion systems


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"irect-In#ection systems"irect-In#ection systems

(a) quiescent chamber with multihole nozzle; (b) bowl-in-piston chamber with swirl

and multihole nozzle; (c) bowl-in-piston chamber with swirl and single-hole nozzle


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Inirect-In#ection systemsInirect-In#ection systems

(a) swirl prechamber; (b) turbulent prechamber


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Stages of combustionStages of combustion

he following stages of diesel combustion processcan be defined%

&!gnition delay

&0remixed or rapid combustion

&1ixing#controlled combustion

&2ate combustion phase3 /s with controlled

combustion the rate of combustion is governed bydiffusion until all the fuel is utilized

hese stages are identified on the pressure and heat#

release diagrams


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IgnitionIgnition

elayelay

/fter injection there is initially no apparent deviation from the

unfired cycle3 $uring this period the fuel is breaking up into

droplets, being vaporized and mixing with air3


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$remi%e$remi%e

combustioncombustion

/ very rapid rise in pressure caused by ignition of the fuel#air

mixture prepared during the ignition delay period


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&i%ing-&i%ing-

controllecontrolle


Several processes occurs simultaneously & li(uid fuel

atomization, vaporization, mixing of fuel vapor with air,

preflame chemical reactions3 )urning rate is controlled

primarily by the fuel#vapor mixing process


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'ate'ate


/s with controlled combustion the rate of combustion is governed

by diffusion until all the fuel is utilized


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Stages of combustionStages of combustion


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Ignition elayIgnition elay

Ignition dela period controls !alue o" pea# pressureand its position relati!e to T$C% thus it a""ects

engine e""icienc

&actor e""ecting dela period are'

& Fuel properties

& !njection timing

& 'ngine load

& 'ngine speed

& 0arameters of fuel injection e(uipment

& !ntake air pressure and temperature

& Swirl rate


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Factors affecting elay(Factors affecting elay(Fuel cetane numberFuel cetane number

Fuel ignition (uality is defined by its cetane number *"4+

"4 is defined by blends of n-cetane *"4 of 5--+ and

isocetane *"4 of 56+ and is given by

"4 7 percent n#cetane 8 -356 percent isocetane

"etane number of commercial diesel fuel is normally in

the range 9- to 66

he higher "4 the shorter ignition delay period


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Factors affecting elay(Factors affecting elay(In#ection timingIn#ection timing

/t normal engine conditions minimum delay occurs withthe start of injection at about 5- to 56: before $"

!f injection starts earlier, the initial air temperature and

pressure are lower so the delay will increase

!f injection starts later *closer to $"+ the temperature

and pressure are initially slightly higher but then decrease

as the delay proceeds

he most favorable conditions for ignition lie in between


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Factors affecting elay(Factors affecting elay(Engine loa an speeEngine loa an spee

/s engine loadincreasesthe residual gas

temperature and the wall

temperature increase

his result in higher charge

temperature at injection,

thus shortening the delay

period

/s engine speedincreases

*at a constant load+ the

delay period increases


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Factors affecting elay(Factors affecting elay(Inlet airInlet air TTanan pp

Intake air temperatureand pressurewill affect the

delay via their effect on

charge conditions during the

delay period !ncrease in inlet air

temperature and pressure

results in shorter delay

period /lso increase in

compression ratiowill

decrease the ignition delay

Ignition dela !s charge temperature

8 compression ignition engines

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