investigating knock in a commercial spark-ignition engine
TRANSCRIPT
Investigating Knock in a commercial Spark-ignition Engine
by Large-Eddy Simulation
M. LEGUILLE, O. Colin, C. Angelberger, F. Ravet
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Limit CO2 footprint of gasoline engines
• Engine downsizing
Eichler et al. (2016)
• Thermally more efficient high loads
Severe thermodynamic conditions inside the combustion chamber
Fuel efficiency gain limited by engine Knock
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Introduction to Engine Knock
Knock = uncontrolled phenomenon related to the auto-ignition (AI) in the fresh gas ahead of the spark-triggered premixed flame
Auto-ignition depends on local conditions (Pressure, Temperature, air-fuel ratio, dilution rate)
Spark-triggered premixed flame
Fresh gas compressed between the flame
and the wall
AI
Pressure wave travelling across the
combustion chamber
Pressure oscillations + audible noise
Knock
AI
AI : Auto-ignition
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Work achieved so far on SI-engine LES at IFPEN
Contribute to the characterisation of knock in a commercial Renault engine
Objective of this study:
B. Enaux V. Granet
Ability of LES to predict CCV
S.Richard A. Robert A. Misdariis
Ability of LES to predict knock
CCV : Cyclic Combustion Variability
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Table of content
I. LES of the commercial RENAULT engine
II. Knock related to combustion phasing variability
III. Knock analysis at constant combustion phasing
IV. Conclusions & Perspectives
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Table of content
I. LES of the commercial RENAULT engine
II. Knock related to combustion phasing variability
III. Knock analysis at constant combustion phasing
IV. Conclusions & Perspectives
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Engine characteristics and geometry
Boundary conditions:
Computational domain limited to the 1st cylinder RENAULT 1.2 TCe engine (H5Ft)
Nb of cylinders 4
Bore / Stroke 72.2 mm / 73.2 mm
Compression ratio 9.8
Engine displacem. 1200 cm3
Max Power 85 kW
Direct-injection syst. 6 holes
Reference Operating Point (Expe. knock limit)
Engine speed 2500 rpm
Spark-timing -5.3 °CA
IMEP 23.45 bars
Start of injection -320 °CA
Injection pressure 135 bars
Equiv. ratio 1.05
Inlet / Outlet 0D/1D simulation
Wall temperature distribution
RANS – CHT simulation with
CONVERGE
ROP : Reference Operating Point
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Computational meshes
Mesh characteristics
TDC 2.8 million cells
BDC 12.1 million cells
Intake 0.6 mm – 0.8 mm
Combustion Chamber
0.6 mm
Around spark-plug
0.15 mm
Engine cycle subdivided into 60 meshes
Use of the Lax-Wendroff numerical scheme
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Approach for direct-injection modelling
Spray model based on a Lagrangian formalism (Thesis N. Iafrate)
Real fuel surrogate
Iso-octane
N-heptane
Toluene
Single-component surrogate of equivalent thermodynamic
properties Rebound condition
Lateral direct-injector
Direct-injection syst. 6 holes
Start of injection -320 °CA
Injection pressure 135 bars
Equiv. ratio 1.05
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Approach for combustion modelling
The source term for the progress variable transport equation is split in:
Richard & al., 31st Symp. Comb. 2007
resolved contributions SGS contributions
Transport equation for flame surface density
ISSIM spark-ignition model
Colin & Truffin, Proc. Combust. Inst 2011
De-De-
TKI auto-ignition model
Robert & al., Proc. Comb. Inst, 2015
Tabulated for isobaric
homogeneous reactors
using detailed chemistry
Read during LES from this
table
𝝎𝒄𝚺 = 𝜌𝑢𝑆𝐿𝚺 𝒄
x lf
x 0.15 – 0.6 mm
lf 0.05 mm
Premixed flames not resolved in
practical LES
ECFM-LES: solving for a filtered flame
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LES results at ROP
ST
One curve = One cycle
In-cylinder Pressure signals
Good CCVs prediction
30 consecutives cycles with AVBP code • ~ 2 days per cycle using 256 cores
ROP : Reference Operating Point
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Table of content
I. LES of the commercial RENAULT engine
II. Knock related to combustion phasing variability
III. Knock analysis at constant combustion phasing
IV. Conclusions & Perspectives
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3D-CFD based criterion to quantify auto-ignition
Computational Knock Index (Robert et al. & Chevillard et al.):
𝑪𝑲𝑰 (𝒏) =
𝝎 𝒄𝑨𝑰𝒅𝑽𝒅𝒕𝒏
𝝎 𝒄𝑨𝑰𝒅𝑽𝒅𝒕𝒏 + (𝝎 𝒄
𝚺 +𝝎 𝒄𝒊𝒈𝒏
)𝒅𝑽𝒅𝒕𝒏
Modified expression to get rid of impact of cool flame:
𝑪𝑲𝑰 (𝒏) = 𝝎 𝒄
𝑨𝑰𝒅𝑽𝒅𝒕𝒏𝒄 𝑨𝑰>𝟎.𝟏
𝝎 𝒄𝑨𝑰𝒅𝑽𝒅𝒕𝒏
𝒄 𝑨𝑰>𝟎.𝟏+ (𝝎 𝒄
𝚺 +𝝎 𝒄𝒊𝒈𝒏
)𝒅𝑽𝒅𝒕𝒏
Give CKI > 0 even in non-knocking cycles
Estimates the proportion of fresh gases burned by AI:
• 𝑪𝑲𝑰 𝒏 = 𝟎: No AI
• 𝑪𝑲𝑰 𝒏 = 𝟏: Full combustion by AI
AI : Auto-ignition
cool flame is not knock (Pöschl et al. 2007)
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Correlation between CKI and CA50 at ROP
Knock free
Knock
One point = one cycle
Knock limiting 𝑪𝑨𝟓𝟎= 15 °CA
Correlation between knock & combustion phasing
AI : Auto-ignition
Addressing the knock issue requires to address the question
of combustion variability
Investigate the origins of combustion variability
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𝐂𝐨𝐫𝐫𝐞𝐥𝐚𝐭𝐢𝐨𝐧 𝐛𝐞𝐭𝐰𝐞𝐞𝐧 𝐂𝐀𝟓𝟎 𝐯𝐬. 𝑪𝑨𝟎𝟐
Origins of the combustion variability
𝑪𝑨𝟎𝟐 =
CA at which 2% of the fuel is consumed
Close to linear correlation
Combustion variability appears during the first instants of
combustion
Investigate fluctuation sources at spark-timing
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Methodology: Conditionally ensemble averaged cycles
• Individual cycles do not allow to draw a meaningful conclusion
• Look for differences between 𝑨𝒗. 𝑬𝒂𝒓𝒍𝒚 and 𝑨𝒗. 𝑳𝒂𝒕𝒆 cycles
20% earliest 𝑪𝑨𝟎𝟐 cycles
𝑨𝒗. 𝑬𝒂𝒓𝒍𝒚 𝒄𝒚𝒄𝒍𝒆
20% latest 𝑪𝑨𝟎𝟐 cycles 𝑨𝒗. 𝑳𝒂𝒕𝒆 𝒄𝒚𝒄𝒍𝒆
Analysis of conditionally ensemble averaged cycles
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Comparison of characteristics of early and late cycles
• Larger velocity towards the spark-plug
in the 𝑨𝒗. 𝑬𝒂𝒓𝒍𝒚 cycle
• Internal aerodynamics • Laminar flame speed
• More heterogeneous 𝑺𝑳 field around
the spark-plug in the 𝑨𝒗. 𝑳𝒂𝒕𝒆 cycle
𝑨𝒗. 𝑬𝒂𝒓𝒍𝒚
𝑨𝒗. 𝑳𝒂𝒕𝒆
At Spark-timing Av. cycles allow to identify characteristic differences between early and late cycles
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Extending LES database through spark-timing sweep
• Variations of 4 spark-timings [ -3.3 °CA, -7.3°CA, -9.3 °CA, -11.3°CA]
• Only combustion phases are re-computed (A.Robert, A. Misdariis)
ST = -5.3 °CA
ST = -3.3 °CA
ST = -7.3 °CA ST = -9.3 °CA
ST = -11.3 °CA
150 LES combustion phases
(ROP)
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Pertinence of correlation between CKI and CA50
Knock free
Knock
𝑪𝑲𝑰 and 𝑪𝑨𝟓𝟎 𝒇𝒐𝒓 𝟏𝟓𝟎 𝒄𝒚𝒄𝒍𝒆𝒔 𝑪𝑲𝑰 and spark-timing
CKI variability at similar 𝑪𝑨𝟓𝟎
• 𝑪𝑲𝑰 and 𝑪𝑨𝟓𝟎 allow to compare cycles independently of spark-timing • Combustion phasing is the key parameter for engine knock
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Table of content
I. LES of the commercial RENAULT engine
II. Knock related to combustion phasing variability
III. Knock analysis at constant combustion phasing
IV. Conclusions & Perspectives
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Cyclic variability of end-gas distribution
𝑪𝟏𝟏 𝑪𝟒
• Premixed flame shape for three individual cycles:
𝑪𝟐𝟕
3 cycles = 3 different flame shapes = 3 different end-gas distributions
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Comparison of premixed flame and AI fronts
AI propagates much faster than the premixed flame
End-gas distribution at knock onset is the key parameter to investigate knock
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Characterizing end-gas distribution
• Partitioning of the combustion chamber:
𝒎 𝒊𝒖=
𝝆𝒖 𝟏 − 𝒄 𝚺 𝒅𝑽𝒊
𝜽
0° 360°
90°
180°
270°
𝜽
𝒄 𝚺 = progress varaible for premixed flame
𝝆𝒖 = fresh gases density
𝑽𝒊 = volume in section « 𝒊 »
𝜽 = section angle
𝜽 = 𝟑° 90 sections
Exh
aust
sid
e
Inta
ke s
ide
• Radial distribution of end-gas:
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Application to cycles at iso- 𝒴𝐹𝑢 𝑡 = 𝑡𝐴𝐼
Cycle Spark-Timing CKI Notation
11 -9.3 °CA 3.91 % 𝐶113.91%
5 -9.3 °CA 2.84 % 𝐶52.84%
14 -11.3 °CA 2.44 % 𝐶142.44%
13 -11.3 °CA 2.41 % 𝐶132.41%
5 -7.3 °CA 1.81 % 𝐶51.81%
14 -9.3 °CA 1.64 % 𝐶141.64%
Partitioning at the onset of AI 𝒕 = 𝒕𝑨𝑰
= proportion of the initial fuel mass in the combustion
chamber still unburned at the onset of AI
𝒴𝐹𝑢 𝑡 = 𝑡𝐴𝐼 [%]
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End-gas distribution & auto-ignition: Strong vs. Weak knocking cycles
𝑪𝟏𝟏𝟑.𝟗𝟏% 𝑪𝟓
𝟏.𝟖𝟏% 𝑪𝟒𝟏.𝟔𝟒%
𝒇𝑨𝑰,𝒊
Proportion of end-gas
consumed by auto-ignition
𝝉𝑨𝑰,𝒊
Auto-ignition delay
Strong knocking cycle Weak knocking cycles
• Large end gas pocket • Small ignition delays • More homogeneous distribution
• Larger distribution of delays
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End-gas distribution & auto-ignition: Intermediate knocking cycles
𝑪𝟓𝟐.𝟖𝟏% 𝑪𝟏𝟒
𝟐.𝟒𝟒% 𝑪𝟏𝟑𝟐.𝟒𝟏%
𝝉𝑨𝑰,𝒊
𝒇𝑨𝑰,𝒊
• Quite similar to weak knocking cycles • …but smaller fraction of auto-ignition
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Statistical analysis over 6 cycles at constant 𝒴𝐹𝑢 𝑡 = 𝑡𝐴𝐼
Largest proportion of end-gas consumed by auto-ignition on
the exhaust side
Smallest auto-ignition delays statistically on
the exhaust side
• Auto-ignition delays 𝜏𝐴𝐼 • Fraction of end-gas actually consumed by
auto-ignition 𝑓𝐴𝐼
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Statistical analysis over 6 cycles at constant 𝒴𝐹𝑢 𝑡 = 𝑡𝐴𝐼
Larger auto-ignition delays but substantial proportion of end-
gas consumed by AI
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Link with combustion chamber design
Direct injector cavity
Spark-plug
Direct injector cavity
Slow flame propagation in the cavity More time to AI
Auto-ignition can also take place with larger ID when premixed flame propagation is slow
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Table of content
I. LES of the commercial RENAULT engine
II. Knock related to combustion phasing variability
III. Knock analysis at constant combustion phasing
IV. Conclusions & Perspectives
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Conclusions
Multi-cycle LES of a commercial Renault engine including direct-injection and CHT
Modified expression of CKI to discard impact of cool flame
Cycle to cycle knock (CKI) fluctuations well correlated to combustion phasing fluctuations (CA50)
Strong fluctuations of CKI observed for a given CA50
Analysis of individual cycles by partitioning of the cylinder in sectors
reveals that:
Strong CKI observed when presence of large end gas pockets with small ID
Auto-ignition also possible if larger ID but slow flame propagation
Low/intermediate CKI cycles : more homogeneous distribution of end-gas and larger distribution of ID
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Perspectives
LES tool needs futher improvements on: Spray/wall interaction modelling for better fuel stratification prediction
LES tool can now be used to improve engine design by:
avoiding asymmetrical flame propagation
avoiding locations slowing down the flame propagation
Origin of aerodynamic field variability at spark timing not understood: Pure stochasticity of turbulence ?
Geometrical details (on intake duct etc…) leading to bifurcations in aerodynamics
Thank you
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Radial end-gas distribution: Strong vs. Weak knocking cycles
• Large pocket of end-gas on the exhaust side
• More homogeneous radial end-gas distribution
𝑪𝟏𝟏𝟑.𝟗𝟏% 𝑪𝟓
𝟏.𝟖𝟏% 𝑪𝟒𝟏.𝟔𝟒%
Strong knocking cycle Weak knocking cycles
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Radial end-gas distribution: Intermediate knocking cycles
• Similar CKI values
… but completely different radial distribution of end-gas
𝑪𝟓𝟐.𝟖𝟒% 𝑪𝟏𝟒
𝟐.𝟒𝟒% 𝑪𝟏𝟑𝟐.𝟒𝟏%
Intermediate knocking cycles
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Analysis of source terms of flame surface density
2
(1 ) (1 )cres sgs sgs sgs res res d c L c
b
T T S C C S S n St r
Unresolved strain rate
Resolved strain rate Resolved curvature
Ignition stretch
𝝎𝒄𝚺 = 𝜌𝑢𝑆𝐿𝚺 𝒄
Av. early cycles present larger resolved+sgs strain rates leading to faster combustion
=>Aerodynamics field around spark-plug main contributor of CCVs
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Variability of the radial end-gas distribution
• Averaged mass of end-gas per angle degree in section 𝒊
𝑚 𝑖𝑢 =
1
𝑁𝑐 𝑚 𝑖
𝑢(𝑝)
𝑁𝑐
𝑝=1
𝑁𝑐= number of cycles
• Standard deviation:
𝜎 𝑚 𝑖𝑢 =
𝑚 𝑖𝑢 𝑝 − 𝑚 𝑖
𝑢 2𝑁𝑐𝑝=1
𝑁𝑐
Regions with statistically a large concentration of end-gas
Regions with statistically a small concentration of end-gas
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IN / OUT Boundary Conditions
Relax Coefficient number for: Inlet Outlet
Pressure 3000 5000
Temperature 3000 1000
Species 3000 5000
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Statistical radial end-gas distribution
• Averaged mass of end-gas per angle degree in section 𝒊
𝑚 𝑖𝑢 =
1
𝑁𝑐 𝑚 𝑖
𝑢(𝑝)
𝑁𝑐
𝑝=1
𝑁𝑐= number of cycles
Regions with statistically a large concentration of end-gas
Regions with statistically a small concentration of end-gas
• Statistical analysis with the 6 iso- 𝓨𝑭𝒖 𝒕 = 𝒕𝑨𝑰 cycles
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Conclusions of the individual cycle analysis
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
11 5 14 13 5 4
CK
I [%
]
• Large proportion of the end-gas in a single pocket
• All end-gas pockets in state close to auto-ignition
• More homogeneous radial distribution of end-gas.
• Auto-ignition restrained to small end-gas pockets.
• Different end-gas distribution • Different scenarii of auto-ignition
𝐶113.91% 𝐶5
1.81% 𝐶41.64% 𝐶5
2.81% 𝐶142.44% 𝐶13
2.41% • Provides a global characterization of knock in a cycle • Does not distinguish the multiple AI scenarii in the cycles
𝑪𝑲𝑰 :
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LES modelling for SIE simulation
Modelling
Turbulence Sigma
Spray Lagrangian particles – Rosin-Rammler distribution
Spark Ignition ISSIM-LES
Combustion ECFM-LES
Auto-ignition TKI
Wall treatment Wall law Free Slip
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Computational domain for the LES
• LES of a 4 cylinder engine possible but extremely expensive
• Computational domain limited to the 1st cylinder
• Inlet / Outlet conditions from 0D-1D simulation
• Wall temperature distribution from CHT simulation
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Wall temperature estimation by CHT
Valves bottom
Valves
Cylinder dome
Cylinder head boundary
• Need for accurate wall temperature distribution (A.Misdariis 2015)
• RANS – CHT simulation
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Two peaks in temporal evolution of 𝝎𝒄𝑨𝑰 𝒅𝑽
Auto-ignition is not always a single stage process
Cool flame & main auto-ignition
Cool Flame
• Results from the low-temperature chemistry of hydrocarbons
• Weakly exothermic
Main AI
• Results from the high-temperature chemistry of hydrocarbons
• Highly exothermic
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Local high peak pressure coincides with the onset of main auto-ignition:
Cool flame, main auto-ignition & in-cylinder pressure
Uniform in-cylinder pressure for a cycle without main auto-ignition:
Only the highly exothermic main auto-ignition is responsible for the local and sudden increase of pressure in the cylinder
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Temporal evolution of 𝒄 𝑨𝑰: • Smooth increase of during cool flame period
• Sharp increase at main AI onset
Removing cool flame impact in CKI expression (1/2)
Modified CKI formulation:
𝑪𝑲𝑰 (𝒏) = 𝝎 𝒄
𝑨𝑰𝒅𝑽𝒅𝒕𝒏𝒄 𝑨𝑰>𝟎.𝟏
𝝎 𝒄𝑨𝑰𝒅𝑽𝒅𝒕𝒏
𝒄 𝑨𝑰>𝟎.𝟏+ (𝝎 𝒄
𝚺 +𝝎 𝒄𝒊𝒈𝒏
)𝒅𝑽𝒅𝒕𝒏 ∗ 𝟏𝟎𝟎
𝝎𝒄𝑨𝑰 𝒅𝑽 conditioned to
large 𝑐 𝐴𝐼 values
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Removing cool flame impact in the CKI (1/2)
𝝎𝒄𝑨𝑰 𝒅𝑽 = 𝟎 𝐝𝐮𝐫𝐢𝐧𝐠 𝐜𝐨𝐨𝐥 𝐟𝐥𝐚𝐦𝐞 𝐩𝐞𝐫𝐢𝐨𝐝
𝑪𝑲𝑰(𝟓) = 𝟑. 𝟏 % 𝑪𝑲𝑰(𝟓) = 𝟏. 𝟓 %
𝝎𝒄𝑨𝑰 𝒅𝑽 𝐫𝐞𝐦𝐚𝐢𝐧𝐬 𝟎
𝑪𝑲𝑰(𝟏𝟕) = 𝟎. 𝟗𝟓 % 𝑪𝑲𝑰(𝟏𝟕) = 𝟎 %
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Cyclic variability of CKI
• Initial CKI formulation: All cycles have CKI > 0
• Modified CKI formulation: CKI = 0 cycles CKI > 0 cycles
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ST
Fast cycle
Slow cycle
Cyclic Combustion Variability (CCV) at ROP
CCV are variations of the fuel consumption
Slow cycle
Fast cycle
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Cyclic variability of global operating characteristics
• No significant cycle to cycle variations of global operating characteristics
• No correlation with 𝑪𝑨𝟎𝟐
Look for CCV sources in the local flow variations
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PDF of velocity magnitude
𝑨𝒗. 𝑬𝒂𝒓𝒍𝒚
𝑨𝒗. 𝑳𝒂𝒕𝒆
PDF of laminar flame speed
𝑨𝒗. 𝑬𝒂𝒓𝒍𝒚
𝑨𝒗. 𝑳𝒂𝒕𝒆
Comparison of flow conditions seen by the flame
Spatial probability density functions (PDF)
• 3 °CA after spark-timing
• Conditionned to 10−3 < 𝑐 Σ < 10−2 Right ahead of flame front
Mean 𝑆𝐿: 1.14 m.s-1 / 1.16 m.s-1 Mean 𝑢 : 4.2 m.s-1 / 6.1 m.s-1
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Impact of retarding the spark-timing on CCV
Retarding the spark-timing overally postpones 𝑪𝑨𝟓𝟎
… but it also increases CCV !
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CKI and available fuel mass fraction at auto-ignition onset
𝓨𝑭𝒖 𝒕 = 𝒕𝑨𝑰
=
𝟏 − 𝒀𝑭𝒖 𝒕 = 𝑺𝑻 − 𝒀𝑭
𝒖 𝒕 = 𝒕𝑨𝑰𝒀𝑭𝒖 𝒕 = 𝑺𝑻
∗ 𝟏𝟎𝟎
=
proportion of the initial fuel mass in the combustion chamber still unburned at the onset
of main auto-ignition
𝒕𝑨𝑰 = time at which auto-ignition occurs
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90 sections seems a good compromise
Choice of the number of sections
The section width should be: • Small enough to capture the resolved wrinkling of the flame
• Large enough with respect to the cell characteristic length
48 sections 90 sections
120 sections
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End-gas & auto-ignition distribution
Remaininng time till main auto-ignition in a mesh cell:
𝜏𝐴𝐼 = 𝑡 𝑐 𝐴𝐼 = 0.1 − 𝑡 𝑐 𝐴𝐼
Remaining time till main auto-ignition in section « 𝒊 »:
𝜏𝐴𝐼,𝑖 = 𝑚𝑖𝑛[𝜏𝐴𝐼,𝑖 1 ,… , 𝜏𝐴𝐼,𝑖 𝑗 , … , 𝜏𝐴𝐼,𝑖 𝑛 ]
Mass fraction of end-gas actually consumed by main auto-ignition:
𝑓𝐴𝐼,𝑖 = 𝑚𝑖
𝐴𝐼
𝑚𝑖𝑢
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Engineering criterion to quantify combustion variability at ROP
𝑪𝑨𝟓𝟎 =
CA at which 50% of the fuel is consumed
Small 𝑪𝑨𝟓𝟎 = Early cycle Large 𝑪𝑨𝟓𝟎 = Late cycle
50 %
Variations of 𝑪𝑨𝟓𝟎
One curve = One cycle
Evolution of Fuel mass fraction
ROP : Reference Operating Point