Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University
ICAT 08, Istanbul, Turkey
ANALYSIS OF COMBUSTION
CHARACTERISTICS OF CAI-ENGINE WITH
VARIOUS VALVE STRATEGIES
Jin Nam KIM, Ho Young KIM, Sam S. Yoon, Woo Tae KIM and Sang Dong SA
Korea University, Hyundai Motor Company
2008. 11. 13.
Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University
ICAT 08, Istanbul, Turkey
ContentsContents
Introduction
Theoretical Modeling
Numerical Details
Results and Discussion
Concluding Remark
Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University
ICAT 08, Istanbul, Turkey
IntroductionIntroduction
Homogeneous mixture of air, fuel and residual gases is compressed until
auto-ignition occurs at sites distributed throughout the combustion chamber.
CAI Engine operation
CAI combustion in IC engine provides better performance in various aspects
compared with both SI and CI combustion.
- Operate unthrottled at part load and therefore reduce pumping losses
- Overall combustion temperature is significantly reduced by the presence of excess air or internal EGR and produces ultra-low NO emission
Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University
ICAT 08, Istanbul, Turkey
There is no direct method to control auto-ignition timing !!!!!!
Fuel : High volatile fuel, Dual fuel supply
Intake air : Intake charge heating auto ignition timing is advanced
Compression ratio : Low smoke and NOx emission with high compression
ratio and low intake charge temperature.
Injection time : Achieve the combustion stability by injection timing and
multi injection timing.
Recirculation or Trapping of burned gases : Exhaust Gas Recirculation,
Residual gas trapping with variable valve timing
Control the auto-ignition of CAI Engine
Uniform mixture formation
Stable ignition
Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University
ICAT 08, Istanbul, Turkey
Trapping of residual gas – Internal EGR in CAI
Trapping of exhaust gases by closing exhaust
valves relatively early and opening intake valve late.
Intake valves are opened and fresh charge drawn
into the cylinder.
Fresh charge and exhaust mixture is then
compressed in the next compression stroke.
The auto-ignition occurs in the final stage of
compression stroke.
Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University
ICAT 08, Istanbul, Turkey
Objective of this study
Poor mixing or stratification between fuel and intake gases often
poses
a great technical challenge (incomplete combustion, high emission
level)
The distribution of the internal EGR affects the mixture formation as well
as the combustion characteristics of CAI engineNumerically examine the effects of various valve strategies on the overall performance for the CAIengine
To Obtain - Flow and Mixing Characteristics
- Combustion Characteristics
- Emission Characteristics
Various Valve timing and Lift Strategies are applied
Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University
ICAT 08, Istanbul, Turkey
Compression Ratio 10.5 : 1
Bores X Stroke 88 mm X 97 mm
Connecting Rod Length 143.75 mm
Intake Valve Seat Diameter 32.8 mm
Exhaust Valve Seat Diameter 26 mm
ModelingModeling Engine Specification
STL format CAD data
Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University
ICAT 08, Istanbul, Turkey
EquationsEquationsEquationsEquations
ModelingModeling
jj j j
u St x x x
Generalized equation for continuous phase
2
, ; ;ji ik t e t t
j i j
u c ku uwhere G
x x x
Continuity 1 0
Momentum
Turbulent kinetic energy
Species
Enthalpy
Dissipation rate
SS
iu e
k
sY
h
e
k
e
e
Y
e
h
,
2
3 i
j ke e ij d u
i j i k
u upk S
x x x x
kG
1 2kc G ck
,d mS
,d jS
,d hS
Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University
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mass transfer
fuel droplet
heat transfer
ls m
dmA F
dt ,
,
ln t vm g t
t v s
p pF K P
p p
1
2d
d d d d d d
dum C A u u u u V p
dt
0.687 3
3
24 1 0.15Re Re 10
0.44 Re 10
d dd
d
C
Where
Where
, "p d d dd s d fg
d C T dmm A q h
dt dt
Where "d dq h T T
By Ranz correlation
By Yuen & Chen correlation
By El Wakil formulation & Ranz-Marshall correlation
Fuel injection
Droplet Break-up
Spray injection with atomization Reitz and Diwakar model
Reitz and Diwakar model
Generalized equation for Dispersed phase
didi
dxu
dt
ModelingModeling
Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University
ICAT 08, Istanbul, Turkey
ReactantsReactantsReactantsReactants ProductProductProductProductEquilibrium ConstantEquilibrium ConstantEquilibrium ConstantEquilibrium Constant Reaction TypeReaction TypeReaction TypeReaction Type
Ignition and Combustion model
Shell auto ignition model – Halstead et al., 1977
Simplified multi-step reaction mechanism to predict the spontaneous auto-ignition
EBU (Eddy Break UP) Combustion model – Magnussen 1981
Initiation
Propagate
Form B
Form Q
Form B
Degenerate
Out Terminate
Out Terminate
2RH O
*R
*R
*R
*R Q
B
*R
*2R
qK
pK
1 pf K
4 pf K
2 pf K
bK
3 pf K
tK
* HeatR P
*2R
*R B*R Q
*R B
*2R
min , , pOF ebu F ebu
O P
YYR A Y B
k s s
ModelingModeling
Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University
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Time step size – crank angle 0.2°
Time marching calculation – fully implicit scheme
PISO Algorithm
Differencing scheme – MARS scheme
Turbulence model - high Reynolds modelk
Numerical SchemeNumerical Scheme
Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University
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3 - Dimensional Grid Generation Section of valve center
Exhaust valve
Intake valve
Negative Valve Overlap !!!
Negative Valve Overlap !!!
Mesh GenerationMesh GenerationAt TDC : 320,000 cells
At BDC : 700,000 cells
By Pro-am, Es-ice
Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University
ICAT 08, Istanbul, Turkey
Exhaust Valve TimingA1 A2 A3 A4 A5 A6
EVO 140 130 120 140 140 140
EVC 290 290 290 300 310 320
IVO 440
IVC 590
NVO 150 150 150 140 130 120
Intake Valve TimingA1 B1 B2 B3 B4 B5 B6 B7
EVO 140
EVC 290
IVO 440 430 420 440 440 440 420 460
IVC 590 590 590 600 610 620 570 610
NVO 150 140 130 150 150 150 130 170
Parametric Studies The maximum valve lift of intake and exhaust valve is 2 mm.
Case A1 is the benchmarking case
Case A1,A2,A3 are EVO advanced
Case A4,A5,A6 are EVC retarded
Case B1 and B2 are IVO advanced
Case B3,B4 and B5 are IVC retarded
Case B6 andB7 are intake stroke
shifted of 20 CAD
(advanced and retarded, respectively)
Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University
ICAT 08, Istanbul, Turkey
Intake valve lift (mm)A1 C1 C2 C3
Intake valve 1&2
2 3 4 5
Intake valve lift strategies (mm)A1 D1 D2 D3 D4
Intake valve 1
2 0
Intake valve 2
2 4 6 4 6
Intake Valve 1
Intake Valve 2
Exhaust Valve 2
Exhaust Valve 1
Parametric Studies
Case C1, C2 and C3 are increasing both intake valve lift.
Case D1,D2,D3 and D4 are various intake valve lift scenario considering swirling motion
in cylinder.
Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University
ICAT 08, Istanbul, Turkey
Results and DiscussionResults and Discussion
Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University
ICAT 08, Istanbul, Turkey
0 200 400 600 800 10000
5
10
15
20
25
30
35
40
Cy
lind
er
Pre
ss
ure
(b
ar)
CAD (Crank Angle Degree)
1D Pressure 3D Pressure
The results of 3D simulation is analogous to those of 1D simulation. The maximum pressure difference is about 5%
1D Gas Dynamic Engine System Simulation (Ricardo WAVE)
Comparison of in-cylinder pressure in 1D and 3D simulation result
Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University
ICAT 08, Istanbul, Turkey
Additional pressure phase of residual gas re-
compression
compared with a standard 4-stroke engine cycle due to
the negative valve overlap for all cases.
When EVC is retarded (Case A4,A5 and A6),
under-lap period is decreased and less amount of the
residual
gases remains.
Pressure inside the cylinder
The pressure at recompression stroke is decreased
12
r
1
n
ii
n
2i
i
C C
C
1
1
n
i iin
ii
CVC
V
The Case A6 has the largest uniformity index because of
30 CAD retarded EVC, resulting in a larger amount of
intake air and vigorous mixing
Uniformity Index
Various Exhaust Valve Timing
Uniformity Index ( Weltens, 1993)
-considering EGR mass fraction and volume at each cell
Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University
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Case A1(EVO : 140 °CA)
Case A2(EVO : 130 °CA)
Case A3(EVO : 120 °CA)
Mass fraction of internal EGR at the valve center
- EVO advanced (Case A1, A2 and A3)
The mass fraction of internal EGR is analogous to the result of case A1.
The improvement of the mixing characteristics has not occurred even though the EVO is
advanced.
Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University
ICAT 08, Istanbul, Turkey
Case A1 (EVC : 290 °CA)
Case A4 (EVC : 300 °CA)
Case A5 (EVC : 310 °CA)
Case A6 (EVC : 320 °CA)
Mass fraction of internal EGR at the valve center
- EVC retarded (Case A1, A4, A5 and A6)
The highly accelerated intake flow penetrates the piston head and causes a vigorous mixing in the case A6, unlike the case in A2 and A3.
The case A6 has the best mixing characteristics between the internal EGR and thefresh air at the end of compression stroke.
Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University
ICAT 08, Istanbul, Turkey
Various Intake Valve Timing
Intake valve timing had practically no effect on the
pressure during negative valve overlap
All other mixing characteristics such as cylinder temperature and flow fields also remained unchanged.
Two dominant temporal increases in the intake port
gas concentration, commonly referred to as the
“early backflow”, “late backflow”.
The most advanced IVO timing, Case B2 and B6 were
shown to the highest values for early backflow.
The most retarded IVC timing, Case B5 was the
largest value for late backflow.
However, various backflow patterns were observed !!!
Case B6 presents the most optimal operating condition in terms of improved thermal efficiency
The heat loss due to late backflow is unfavorable because of the longer residence period of the hot EGR in the intake port;
B2,B6
B5
B6
Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University
ICAT 08, Istanbul, Turkey
Distribution of Scalars - Increasing Intake Valve Lift (Valve 1 & 2)
Internal EGRInternal EGRInternal EGRInternal EGR FuelFuelFuelFuel O2 at 710 CADO2 at 710 CADO2 at 710 CADO2 at 710 CADTemperatureTemperatureTemperatureTemperature O2 at 715 CADO2 at 715 CADO2 at 715 CADO2 at 715 CAD
1000 1200 0.5 0.8 0 0.07 0.04 0.090.04 0.09
Case C1Case C1
Case C2Case C2
Case C3Case C3
•The areas with high internal EGR mass fraction correspond to relatively high temperature areas.
•Auto-ignition areas correspond to the fuel concentration from fuel and oxygen mass concentration
Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University
ICAT 08, Istanbul, Turkey
As the intake valve lift increase, more cool fresh air could be induced
Combustion and Emission - Increasing Intake Valve Lift (Valve 1 & 2)
Lower pressure and temperature
NO formation is modeled by Zeldovich’s reaction mechanism
Around1800 K
Case C1, yielding the highest temperature (2083 K)
- The largest NO emission
When temperature greater than1800 K, - NO formation increased.
Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University
ICAT 08, Istanbul, Turkey
Intake valve strategy (different valve lift 1 & 2)
Intake Valve 1
Intake Valve 2Case
Intake
Valve 1
Intake
Valve 2
Case BM 2 mm 2 mm
Case D1 2 mm 4 mm
Case D2 2 mm 6 mm
Case D3 0 4 mm
Case D4 0 6 mm
To relate the intake valve lift profile to the flow structure
Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University
ICAT 08, Istanbul, Turkey
Intake valve strategy (different valve lift 1 & 2)
0 01
2 20 0
1
( ) ( )
( ) ( )
n
i i i i ii
s n
i i ii
m y y u x x v
m x x y y
0 01
2 20 0
1
( ) ( )
( ) ( )
n
i i i i ii
tx n
i i ii
m y y w z z v
m z z y y
0 01
2 20 0
1
( ) ( )
( ) ( )
n
i i i i ii
ty n
i i ii
m z z u x x w
m x x z z
Equation by mattarelli et al. X Y
Z
440 460 480 500 520 540 560 580
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
NT
x
Crank Angle [deg]
Case BM Case D1 Case D2 Case D3 Case D4
440 460 480 500 520 540 560 580
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
NT
y
Crank Angle [deg]
Case BM Case D1 Case D2 Case D3 Case D4
With Intake Vale Lift 1=0 as in Case D3 and Case D4, the swirl intensity substantially increases at
the
end of the intake stroke.
The x-axis tumble intensity increases as the valve lift increases but gradually loses
The flow direction (from positive, negative value) of y-axis tumble gradually changes the intake stroke.
Intake valve 1 lift =0Case D3 and D4
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Internal EGRInternal EGRInternal EGRInternal EGR FuelFuelFuelFuel O2 at 710 CADO2 at 710 CADO2 at 710 CADO2 at 710 CADTemperatureTemperatureTemperatureTemperature O2 at 715 CADO2 at 715 CADO2 at 715 CADO2 at 715 CAD
1000
1200
0.5 0.8 0 0.07 0.04
0.09
0.04 0.09
Case BMCase BM
•The areas with high internal EGR mass fraction correspond to relatively high temperature areas.
•Auto-ignition areas correspond to the fuel concentration from fuel and oxygen mass concentration.
•A larger auto-ignition spot in the reaction zones appeared for the most homogeneous mixture as shown in Case D4.
Distribution of Scalars - Intake valve strategy (different valve lift 1 & 2)
Case D1Case D1
Case D2Case D2
Case D3Case D3
Case D4Case D4
Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University
ICAT 08, Istanbul, Turkey
Concluding RemarkConcluding Remark
To investigate the effects of various valve strategy and its subsequent
combustion for a CAI engine, parametric studies were conducted
The results represent that
When the EVC motion was retarded, the less residual gas was remained and more
intake air was supplied
The mixing characteristics were improved with the high momentum of the intake air
The advancing EVO motion virtually had no effect on the mixing characteristics
Some areas with high internal EGR mass fraction inside the cylinder correspond to
relatively high temperature areas.
High fuel concentration and the auto-ignition spots were affecting each other and
Case D4 with the most homogeneous mixture yielded the best combustion efficiency
Combustion & Aerosol Science Laboratory, Korea University Combustion & Aerosol Science Laboratory, Korea University
ICAT 08, Istanbul, Turkey
Thank you
For your attention