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Investigating Knock in a commercial Spark-ignition Engine
by Large-Eddy Simulation
M. LEGUILLE, O. Colin, C. Angelberger, F. Ravet
2 | ยฉ 2 0 1 8 I F P E N
Limit CO2 footprint of gasoline engines
โข Engine downsizing
Eichler et al. (2016)
โข Thermally more efficient high loads
Severe thermodynamic conditions inside the combustion chamber
Fuel efficiency gain limited by engine Knock
3 | ยฉ 2 0 1 8 I F P E N
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
4 | ยฉ 2 0 1 8 I F P E N
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
5 | ยฉ 2 0 1 8 I F P E N
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
6 | ยฉ 2 0 1 8 I F P E N
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
7 | ยฉ 2 0 1 8 I F P E N
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 ยฐCA
IMEP 23.45 bars
Start of injection -320 ยฐCA
Injection pressure 135 bars
Equiv. ratio 1.05
Inlet / Outlet 0D/1D simulation
Wall temperature distribution
RANS โ CHT simulation with
CONVERGE
ROP : Reference Operating Point
8 | ยฉ 2 0 1 8 I F P E N
Computational meshes
Mesh characteristics
TDC 2.8 million cells
BDC 12.1 million cells
Intake 0.6 mm โ 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
9 | ยฉ 2 0 1 8 I F P E N
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 ยฐCA
Injection pressure 135 bars
Equiv. ratio 1.05
10 | ยฉ 2 0 1 8 I F P E N
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
๐๐๐บ = ๐๐ข๐๐ฟ๐บ ๐
x lf
x 0.15 โ 0.6 mm
lf 0.05 mm
Premixed flames not resolved in
practical LES
ECFM-LES: solving for a filtered flame
11 | ยฉ 2 0 1 8 I F P E N
LES results at ROP
ST
One curve = One cycle
In-cylinder Pressure signals
Good CCVs prediction
30 consecutives cycles with AVBP code โข ~ 2 days per cycle using 256 cores
ROP : Reference Operating Point
12 | ยฉ 2 0 1 8 I F P E N
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
13 | ยฉ 2 0 1 8 I F P E N
3D-CFD based criterion to quantify auto-ignition
Computational Knock Index (Robert et al. & Chevillard et al.):
๐ช๐ฒ๐ฐ (๐) =
๐ ๐๐จ๐ฐ๐ ๐ฝ๐ ๐๐
๐ ๐๐จ๐ฐ๐ ๐ฝ๐ ๐๐ + (๐ ๐
๐บ +๐ ๐๐๐๐
)๐ ๐ฝ๐ ๐๐
Modified expression to get rid of impact of cool flame:
๐ช๐ฒ๐ฐ (๐) = ๐ ๐
๐จ๐ฐ๐ ๐ฝ๐ ๐๐๐ ๐จ๐ฐ>๐.๐
๐ ๐๐จ๐ฐ๐ ๐ฝ๐ ๐๐
๐ ๐จ๐ฐ>๐.๐+ (๐ ๐
๐บ +๐ ๐๐๐๐
)๐ ๐ฝ๐ ๐๐
Give CKI > 0 even in non-knocking cycles
Estimates the proportion of fresh gases burned by AI:
โข ๐ช๐ฒ๐ฐ ๐ = ๐: No AI
โข ๐ช๐ฒ๐ฐ ๐ = ๐: Full combustion by AI
AI : Auto-ignition
cool flame is not knock (Pรถschl et al. 2007)
14 | ยฉ 2 0 1 8 I F P E N
Correlation between CKI and CA50 at ROP
Knock free
Knock
One point = one cycle
Knock limiting ๐ช๐จ๐๐= 15 ยฐCA
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
15 | ยฉ 2 0 1 8 I F P E N
๐๐จ๐ซ๐ซ๐๐ฅ๐๐ญ๐ข๐จ๐ง ๐๐๐ญ๐ฐ๐๐๐ง ๐๐๐๐ ๐ฏ๐ฌ. ๐ช๐จ๐๐
Origins of the combustion variability
๐ช๐จ๐๐ =
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
16 | ยฉ 2 0 1 8 I F P E N
Methodology: Conditionally ensemble averaged cycles
โข Individual cycles do not allow to draw a meaningful conclusion
โข Look for differences between ๐จ๐. ๐ฌ๐๐๐๐ and ๐จ๐. ๐ณ๐๐๐ cycles
20% earliest ๐ช๐จ๐๐ cycles
๐จ๐. ๐ฌ๐๐๐๐ ๐๐๐๐๐
20% latest ๐ช๐จ๐๐ cycles ๐จ๐. ๐ณ๐๐๐ ๐๐๐๐๐
Analysis of conditionally ensemble averaged cycles
17 | ยฉ 2 0 1 8 I F P E N
Comparison of characteristics of early and late cycles
โข Larger velocity towards the spark-plug
in the ๐จ๐. ๐ฌ๐๐๐๐ cycle
โข Internal aerodynamics โข Laminar flame speed
โข More heterogeneous ๐บ๐ณ field around
the spark-plug in the ๐จ๐. ๐ณ๐๐๐ cycle
๐จ๐. ๐ฌ๐๐๐๐
๐จ๐. ๐ณ๐๐๐
At Spark-timing Av. cycles allow to identify characteristic differences between early and late cycles
18 | ยฉ 2 0 1 8 I F P E N
Extending LES database through spark-timing sweep
โข Variations of 4 spark-timings [ -3.3 ยฐCA, -7.3ยฐCA, -9.3 ยฐCA, -11.3ยฐCA]
โข Only combustion phases are re-computed (A.Robert, A. Misdariis)
ST = -5.3 ยฐCA
ST = -3.3 ยฐCA
ST = -7.3 ยฐCA ST = -9.3 ยฐCA
ST = -11.3 ยฐCA
150 LES combustion phases
(ROP)
19 | ยฉ 2 0 1 8 I F P E N
Pertinence of correlation between CKI and CA50
Knock free
Knock
๐ช๐ฒ๐ฐ and ๐ช๐จ๐๐ ๐๐๐ ๐๐๐ ๐๐๐๐๐๐ ๐ช๐ฒ๐ฐ and spark-timing
CKI variability at similar ๐ช๐จ๐๐
โข ๐ช๐ฒ๐ฐ and ๐ช๐จ๐๐ allow to compare cycles independently of spark-timing โข Combustion phasing is the key parameter for engine knock
20 | ยฉ 2 0 1 8 I F P E N
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
21 | ยฉ 2 0 1 8 I F P E N
Cyclic variability of end-gas distribution
๐ช๐๐ ๐ช๐
โข Premixed flame shape for three individual cycles:
๐ช๐๐
3 cycles = 3 different flame shapes = 3 different end-gas distributions
22 | ยฉ 2 0 1 8 I F P E N
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
23 | ยฉ 2 0 1 8 I F P E N
Characterizing end-gas distribution
โข Partitioning of the combustion chamber:
๐ ๐๐=
๐๐ ๐ โ ๐ ๐บ ๐ ๐ฝ๐
๐ฝ
0ยฐ 360ยฐ
90ยฐ
180ยฐ
270ยฐ
๐ฝ
๐ ๐บ = progress varaible for premixed flame
๐๐ = fresh gases density
๐ฝ๐ = volume in section ยซ ๐ ยป
๐ฝ = section angle
๐ฝ = ๐ยฐ 90 sections
Exh
aust
sid
e
Inta
ke s
ide
โข Radial distribution of end-gas:
24 | ยฉ 2 0 1 8 I F P E N
Application to cycles at iso- ๐ด๐น๐ข ๐ก = ๐ก๐ด๐ผ
Cycle Spark-Timing CKI Notation
11 -9.3 ยฐCA 3.91 % ๐ถ113.91%
5 -9.3 ยฐCA 2.84 % ๐ถ52.84%
14 -11.3 ยฐCA 2.44 % ๐ถ142.44%
13 -11.3 ยฐCA 2.41 % ๐ถ132.41%
5 -7.3 ยฐCA 1.81 % ๐ถ51.81%
14 -9.3 ยฐCA 1.64 % ๐ถ141.64%
Partitioning at the onset of AI ๐ = ๐๐จ๐ฐ
= proportion of the initial fuel mass in the combustion
chamber still unburned at the onset of AI
๐ด๐น๐ข ๐ก = ๐ก๐ด๐ผ [%]
25 | ยฉ 2 0 1 8 I F P E N
End-gas distribution & auto-ignition: Strong vs. Weak knocking cycles
๐ช๐๐๐.๐๐% ๐ช๐
๐.๐๐% ๐ช๐๐.๐๐%
๐๐จ๐ฐ,๐
Proportion of end-gas
consumed by auto-ignition
๐๐จ๐ฐ,๐
Auto-ignition delay
Strong knocking cycle Weak knocking cycles
โข Large end gas pocket โข Small ignition delays โข More homogeneous distribution
โข Larger distribution of delays
26 | ยฉ 2 0 1 8 I F P E N
End-gas distribution & auto-ignition: Intermediate knocking cycles
๐ช๐๐.๐๐% ๐ช๐๐
๐.๐๐% ๐ช๐๐๐.๐๐%
๐๐จ๐ฐ,๐
๐๐จ๐ฐ,๐
โข Quite similar to weak knocking cycles โข โฆbut smaller fraction of auto-ignition
27 | ยฉ 2 0 1 8 I F P E N
Statistical analysis over 6 cycles at constant ๐ด๐น๐ข ๐ก = ๐ก๐ด๐ผ
Largest proportion of end-gas consumed by auto-ignition on
the exhaust side
Smallest auto-ignition delays statistically on
the exhaust side
โข Auto-ignition delays ๐๐ด๐ผ โข Fraction of end-gas actually consumed by
auto-ignition ๐๐ด๐ผ
28 | ยฉ 2 0 1 8 I F P E N
Statistical analysis over 6 cycles at constant ๐ด๐น๐ข ๐ก = ๐ก๐ด๐ผ
Larger auto-ignition delays but substantial proportion of end-
gas consumed by AI
29 | ยฉ 2 0 1 8 I F P E N
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
30 | ยฉ 2 0 1 8 I F P E N
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
31 | ยฉ 2 0 1 8 I F P E N
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
32 | ยฉ 2 0 1 8 I F P E N
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 etcโฆ) leading to bifurcations in aerodynamics
Thank you
34 | ยฉ 2 0 1 8 I F P E N
Radial end-gas distribution: Strong vs. Weak knocking cycles
โข Large pocket of end-gas on the exhaust side
โข More homogeneous radial end-gas distribution
๐ช๐๐๐.๐๐% ๐ช๐
๐.๐๐% ๐ช๐๐.๐๐%
Strong knocking cycle Weak knocking cycles
35 | ยฉ 2 0 1 8 I F P E N
Radial end-gas distribution: Intermediate knocking cycles
โข Similar CKI values
โฆ but completely different radial distribution of end-gas
๐ช๐๐.๐๐% ๐ช๐๐
๐.๐๐% ๐ช๐๐๐.๐๐%
Intermediate knocking cycles
36 | ยฉ 2 0 1 8 I F P E N
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
๐๐๐บ = ๐๐ข๐๐ฟ๐บ ๐
Av. early cycles present larger resolved+sgs strain rates leading to faster combustion
=>Aerodynamics field around spark-plug main contributor of CCVs
37 | ยฉ 2 0 1 8 I F P E N
Variability of the radial end-gas distribution
โข Averaged mass of end-gas per angle degree in section ๐
๐ ๐๐ข =
1
๐๐ ๐ ๐
๐ข(๐)
๐๐
๐=1
๐๐= number of cycles
โข Standard deviation:
๐ ๐ ๐๐ข =
๐ ๐๐ข ๐ โ ๐ ๐
๐ข 2๐๐๐=1
๐๐
Regions with statistically a large concentration of end-gas
Regions with statistically a small concentration of end-gas
38 | ยฉ 2 0 1 8 I F P E N
IN / OUT Boundary Conditions
Relax Coefficient number for: Inlet Outlet
Pressure 3000 5000
Temperature 3000 1000
Species 3000 5000
39 | ยฉ 2 0 1 8 I F P E N
Statistical radial end-gas distribution
โข Averaged mass of end-gas per angle degree in section ๐
๐ ๐๐ข =
1
๐๐ ๐ ๐
๐ข(๐)
๐๐
๐=1
๐๐= number of cycles
Regions with statistically a large concentration of end-gas
Regions with statistically a small concentration of end-gas
โข Statistical analysis with the 6 iso- ๐จ๐ญ๐ ๐ = ๐๐จ๐ฐ cycles
40 | ยฉ 2 0 1 8 I F P E N
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 [%
]
โข Large proportion of the end-gas in a single pocket
โข All end-gas pockets in state close to auto-ignition
โข More homogeneous radial distribution of end-gas.
โข Auto-ignition restrained to small end-gas pockets.
โข Different end-gas distribution โข Different scenarii of auto-ignition
๐ถ113.91% ๐ถ5
1.81% ๐ถ41.64% ๐ถ5
2.81% ๐ถ142.44% ๐ถ13
2.41% โข Provides a global characterization of knock in a cycle โข Does not distinguish the multiple AI scenarii in the cycles
๐ช๐ฒ๐ฐ :
41 | ยฉ 2 0 1 8 I F P E N
LES modelling for SIE simulation
Modelling
Turbulence Sigma
Spray Lagrangian particles โ Rosin-Rammler distribution
Spark Ignition ISSIM-LES
Combustion ECFM-LES
Auto-ignition TKI
Wall treatment Wall law Free Slip
42 | ยฉ 2 0 1 8 I F P E N
Computational domain for the LES
โข LES of a 4 cylinder engine possible but extremely expensive
โข Computational domain limited to the 1st cylinder
โข Inlet / Outlet conditions from 0D-1D simulation
โข Wall temperature distribution from CHT simulation
43 | ยฉ 2 0 1 8 I F P E N
Wall temperature estimation by CHT
Valves bottom
Valves
Cylinder dome
Cylinder head boundary
โข Need for accurate wall temperature distribution (A.Misdariis 2015)
โข RANS โ CHT simulation
44 | ยฉ 2 0 1 8 I F P E N
Two peaks in temporal evolution of ๐๐๐จ๐ฐ ๐ ๐ฝ
Auto-ignition is not always a single stage process
Cool flame & main auto-ignition
Cool Flame
โข Results from the low-temperature chemistry of hydrocarbons
โข Weakly exothermic
Main AI
โข Results from the high-temperature chemistry of hydrocarbons
โข Highly exothermic
45 | ยฉ 2 0 1 8 I F P E N
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
46 | ยฉ 2 0 1 8 I F P E N
Temporal evolution of ๐ ๐จ๐ฐ: โข Smooth increase of during cool flame period
โข Sharp increase at main AI onset
Removing cool flame impact in CKI expression (1/2)
Modified CKI formulation:
๐ช๐ฒ๐ฐ (๐) = ๐ ๐
๐จ๐ฐ๐ ๐ฝ๐ ๐๐๐ ๐จ๐ฐ>๐.๐
๐ ๐๐จ๐ฐ๐ ๐ฝ๐ ๐๐
๐ ๐จ๐ฐ>๐.๐+ (๐ ๐
๐บ +๐ ๐๐๐๐
)๐ ๐ฝ๐ ๐๐ โ ๐๐๐
๐๐๐จ๐ฐ ๐ ๐ฝ conditioned to
large ๐ ๐ด๐ผ values
47 | ยฉ 2 0 1 8 I F P E N
Removing cool flame impact in the CKI (1/2)
๐๐๐จ๐ฐ ๐ ๐ฝ = ๐ ๐๐ฎ๐ซ๐ข๐ง๐ ๐๐จ๐จ๐ฅ ๐๐ฅ๐๐ฆ๐ ๐ฉ๐๐ซ๐ข๐จ๐
๐ช๐ฒ๐ฐ(๐) = ๐. ๐ % ๐ช๐ฒ๐ฐ(๐) = ๐. ๐ %
๐๐๐จ๐ฐ ๐ ๐ฝ ๐ซ๐๐ฆ๐๐ข๐ง๐ฌ ๐
๐ช๐ฒ๐ฐ(๐๐) = ๐. ๐๐ % ๐ช๐ฒ๐ฐ(๐๐) = ๐ %
48 | ยฉ 2 0 1 8 I F P E N
Cyclic variability of CKI
โข Initial CKI formulation: All cycles have CKI > 0
โข Modified CKI formulation: CKI = 0 cycles CKI > 0 cycles
49 | ยฉ 2 0 1 8 I F P E N
ST
Fast cycle
Slow cycle
Cyclic Combustion Variability (CCV) at ROP
CCV are variations of the fuel consumption
Slow cycle
Fast cycle
50 | ยฉ 2 0 1 8 I F P E N
Cyclic variability of global operating characteristics
โข No significant cycle to cycle variations of global operating characteristics
โข No correlation with ๐ช๐จ๐๐
Look for CCV sources in the local flow variations
51 | ยฉ 2 0 1 8 I F P E N
PDF of velocity magnitude
๐จ๐. ๐ฌ๐๐๐๐
๐จ๐. ๐ณ๐๐๐
PDF of laminar flame speed
๐จ๐. ๐ฌ๐๐๐๐
๐จ๐. ๐ณ๐๐๐
Comparison of flow conditions seen by the flame
Spatial probability density functions (PDF)
โข 3 ยฐCA after spark-timing
โข Conditionned to 10โ3 < ๐ ฮฃ < 10โ2 Right ahead of flame front
Mean ๐๐ฟ: 1.14 m.s-1 / 1.16 m.s-1 Mean ๐ข : 4.2 m.s-1 / 6.1 m.s-1
52 | ยฉ 2 0 1 8 I F P E N
Impact of retarding the spark-timing on CCV
Retarding the spark-timing overally postpones ๐ช๐จ๐๐
โฆ but it also increases CCV !
53 | ยฉ 2 0 1 8 I F P E N
CKI and available fuel mass fraction at auto-ignition onset
๐จ๐ญ๐ ๐ = ๐๐จ๐ฐ
=
๐ โ ๐๐ญ๐ ๐ = ๐บ๐ป โ ๐๐ญ
๐ ๐ = ๐๐จ๐ฐ๐๐ญ๐ ๐ = ๐บ๐ป
โ ๐๐๐
=
proportion of the initial fuel mass in the combustion chamber still unburned at the onset
of main auto-ignition
๐๐จ๐ฐ = time at which auto-ignition occurs
54 | ยฉ 2 0 1 8 I F P E N
90 sections seems a good compromise
Choice of the number of sections
The section width should be: โข Small enough to capture the resolved wrinkling of the flame
โข Large enough with respect to the cell characteristic length
48 sections 90 sections
120 sections
55 | ยฉ 2 0 1 8 I F P E N
End-gas & auto-ignition distribution
Remaininng time till main auto-ignition in a mesh cell:
๐๐ด๐ผ = ๐ก ๐ ๐ด๐ผ = 0.1 โ ๐ก ๐ ๐ด๐ผ
Remaining time till main auto-ignition in section ยซ ๐ ยป:
๐๐ด๐ผ,๐ = ๐๐๐[๐๐ด๐ผ,๐ 1 ,โฆ , ๐๐ด๐ผ,๐ ๐ , โฆ , ๐๐ด๐ผ,๐ ๐ ]
Mass fraction of end-gas actually consumed by main auto-ignition:
๐๐ด๐ผ,๐ = ๐๐
๐ด๐ผ
๐๐๐ข
56 | ยฉ 2 0 1 8 I F P E N
Engineering criterion to quantify combustion variability at ROP
๐ช๐จ๐๐ =
CA at which 50% of the fuel is consumed
Small ๐ช๐จ๐๐ = Early cycle Large ๐ช๐จ๐๐ = Late cycle
50 %
Variations of ๐ช๐จ๐๐
One curve = One cycle
Evolution of Fuel mass fraction
ROP : Reference Operating Point