8 compression ignition engines
TRANSCRIPT
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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$remi%e
combustioncombustion
/ very rapid rise in pressure caused by ignition of the fuel#air
mixture prepared during the ignition delay period
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&i%ing-&i%ing-
controllecontrolle
combustioncombustion
Several processes occurs simultaneously & li(uid fuel
atomization, vaporization, mixing of fuel vapor with air,
preflame chemical reactions3 )urning rate is controlled
primarily by the fuel#vapor mixing process
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'ate'ate
combustioncombustion
/s with controlled combustion the rate of combustion is governed
by diffusion until all the fuel is utilized
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Stages of combustionStages of combustion
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Ignition elayIgnition elay
Ignition dela period controls !alue o" pea# pressureand its position relati!e to T$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