Download pdf - Investigating Knock in a commercial Spark-ignition Engine

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





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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


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Table of content





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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


𝒄 𝑨𝑰>𝟎.𝟏+ (𝝎 𝒄


)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





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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





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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

[email protected]

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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


𝒄 𝑨𝑰>𝟎.𝟏+ (𝝎 𝒄


)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



PDF of laminar flame speed



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
