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Detailed Chemical Kinetics
Modeling of Knock in GT-POWER
Apoorv Agarwal
Eric Curtis
Ford Motor Company
GT-SUITE Conference November 5, 2012
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Acknowledgment
• Gamma Technologies – Ryan Dudgeon – Syed Wahiduzzaman
• Ford Motor Company
– Brad VanDerWege – Shiyou Yang – Jim Leiby
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Agenda • Introduction • GT-POWER v7.2 Knock Model • Chemical Kinetic Mechanism Selection • Parametric Studies
– Octane Rating, EGR, Intake Temperature and Ethanol Ratio
• Comparison with Engine Knock Data • GT-POWER v7.3 Knock Model Results • Conclusions
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Introduction Autoignition theory is the most common explanation for the origin of knock
– Spontaneous combustion occurs in the end-gas region when fuel-air mixture is compressed to sufficiently high pressures and temperatures
– Chemistry of fuel-air oxidation plays a vital role in describing pre-flame reactions and rapid energy release at the time of autoignition
Fig 9-63 (Heywood, 1988)
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Current Models for Borderline Knock
1. Peak Unburned Gas Temperature
2. Single-Reaction Induction Time Models
3. Multi-Step Chemistry
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Objectives
• To develop a chemistry based spark knock model that simulates end-gas autoignition by solving multi-step chemistry of gasoline oxidation within GT-POWER
• Goal is better prediction of spark knock within the limitations of a 1-D engine model
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GT-POWER 7.2 Knock Model Schematic
Single Cylinder Engine
Knock Chamber
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Knock Chamber Pressure & Temperature Matching
Knock Chamber
Main Cylinder
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Chemical Kinetic Mechanism • MC9 mechanism developed by Ra & Reitz at
ERC, University of Wisconsin – Developed specifically for PRF & Ethanol oxidation
• 46 species and 142 reactions – Extracted from MultiChem mechanism for
multicomponent fuels described in Combustion and Flame 158 (2011) 69–90
– Validated with ignition delay measurements in shock tubes and engines at T < 1600 K and P < 55 bar
– Comparison with three other kinetic mechanisms performed at Ford showed MC9 to be the best in predicting ignition delay under knock like conditions
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Parametric Studies 1000 rpm, Pin = 1 bar
• Fuel Octane Rating – 80 RON, 90 RON, 100 RON
• EGR – 0%, 30%, 45%
• Intake Temperature – 300 K, 330 K
• Ethanol Blend – E0, E20, E50, E100
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Effect of Octane Rating at 1000 rpm, 0 EGR, CA50 = 10 ATDC
80 RON
90 RON
100 RON
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Effect of Octane Rating at 1000 rpm, 0 EGR, CA50 = 20 ATDC
80 RON
90 RON
100 RON
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Effect of Octane Rating at 1000 rpm, 0 EGR, CA50 = 30 ATDC
80 RON
90 RON
100 RON
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Effect of EGR and Intake Temperature on Knock
• Swept CA50 (surrogate for spark timing) for 90 RON fuel at 1000 rpm to determine the impact of EGR ratio and intake temperature on autoignition
• Conducted DOE at 0%, 30%, 45% EGR and 300 K, 330 K Tin
• Defined “Knock Limited CA50” as the latest CA50 at which autoignition occurs – no autoignition at KLCA50 + 1° CA
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Main Effects Plot M
ean
of C
A50
0.450.300.00
25
20
15
10
330300
EGR Intake Temp
Main Effects Plot (data means) for CA50
~4° CA50 advance / 10%
EGR ~1° CA50 retard /
15°C increase in Tin
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Effect of Ethanol Blend in Fuel 2000 RPM, 13 bar BMEP
E0 = 80 RON
CA50 = 17° ATDC
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Comparison with High BMEP SCE Knock Data
2000 RPM, 18 bar NMEP
EGR Sweep 0 - 25%
Data provided by Brad VanDerWege, Ford Motor Company
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Knock Assessment • Cycles evaluated
– Mean • Burn rate specified by average CA50 and B1090 of
300 engine cycles – 5th percentile of fastest cycles
• CA50 = CA50avg – 1.64σCA50
• B1090 = B1090avg – 1.64σB1090 – 2nd percentile of fastest cycles
• CA50 = CA50avg – 2σCA50
• B1090 = B1090avg – 2σB1090 -20 -10 0 10 20
5th Percentile
2nd Percentile
Mean
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Modeling Assumptions • Temperature and concentration of the
end gas assumed to be uniform – Autoignition chemistry solved at bulk
unburned zone temperature – Model predicted knock onset will be later
than reality
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Modeling Procedure 1. Model Calibration (no EGR) - matched in-cylinder
pressure and air flow rate by performing a DOE of intake and exhaust runner lengths at – measured MAP, N, Tin, CA50 (mean), B10-90 (mean), cylinder
geometry, valve lift profiles/phasing, RON, cylinder head temperatures
– used default values of wall & piston temperatures and heat transfer coefficients
2. Knock Chamber - Matched pressure, temperature and mass in the knock chamber with the unburned zone
3. Simulated Mean, 5th and 2nd percentile cycles with kinetics • Criteria for autoignition – alignment between autoignition time in
the knock chamber and cylinder mass burn profile
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Comparison to Measurements Pressure Matching – No EGR
Intake runner = 900 mm
Exhaust runner = 950 mm
TWALL = 400 K
MAP (bar)
Air Flow (kg/hr)
Measured 1.59 50.45
Simulated 1.59 50.16
% Diff 0 -0.58
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Simulations – No EGR, Mean Cycle
Autoignition at 61° ATDC
CA50 = 22.1° ATDC
B1090 = 24.7°
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Simulations – No EGR, Mean Cycle
Autoignition at 61° ATDC
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Simulations – No EGR, Mean Cycle
Mean cycle will not knock
Unburned zone mass already consumed
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Simulations – No EGR, 5th Percentile Cycle
CA50 = CA50 – 1.64σCA50
B10-90 = B10-90 – 1.64σB1090
Autoignition at 45° ATDC
(-16° from mean)
CA50 = 19.4° ATDC (-2.7° from mean)
B1090 = 21.6° (-3.1° from mean)
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Simulations – No EGR, 5th Percentile Cycle
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Simulations – No EGR, 2nd Percentile Cycle
CA50 = CA50 – 2σCA50
B10-90 = B10-90 – 2σB1090
Autoignition at 41° ATDC
(-4° from 5th percentile)
CA50 = 18.8° ATDC (-0.6° from 5th percentile )
B1090 = 20.9° (-0.7° from 5th percentile)
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Simulations – No EGR, 2nd Percentile Cycle
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Summary of No EGR Case • Autoignition occurs in the mean cycle much after
the unburned zone mass has been completely consumed – Mean cycle is not likely to knock
• For the 2nd and 5th percentile cycles, autoignition occurs close to the end of combustion in the main chamber – These cycles will likely knock in a real engine
• Occurrence of autoignition in the knock chamber close to the end of combustion will be used as the criteria to indicate knock at other operating conditions
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Comparison with Measurements EGR: 5% – 25%
• Kept all geometric and thermal variables the same – Intake and exhaust runner lengths – Wall temperature – Heat transfer coefficient
• CA50 and B1090 specified from measurements • Intake temperature varied with EGR
– 35.7°C (no EGR) to 42°C (25% EGR) • Varied MAP to match air flow rate within 2.5% of
measurements
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Calibration of MAP and Air Flow for the Different EGR Cases
MAP (bar)
Air Flow (kg/hr)
MAP (bar)
Air Flow (kg/hr)
MAP (bar)
Air Flow (kg/hr)
MAP (bar)
Air Flow (kg/hr)
MAP (bar)
Air Flow (kg/hr)
Measured 1.65 49.77 1.72 49.55 1.76 48.28 1.84 47.54 1.91 47.06Simulated 1.62 48.82 1.69 48.45 1.73 47.16 1.81 46.68 1.89 45.98
% Diff -1.8 -1.91 -1.94 -2.22 -1.7 -2.32 -1.63 -1.81 -1.05 -2.29
25% EGR5% EGR 10% EGR 15% EGR 20% EGR
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Comparison to Measurements Pressure Matching – 5% EGR
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Simulations – 5% EGR, Mean Cycle
No Auto Ignition
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Simulations – 5% EGR, 5th Percentile Cycle
Autoignition at 49° ATDC
CA50 = CA50 – 1.64 σ
B10-90 = B10-90 – 1.64σ
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Simulations – 5% EGR, 5th Percentile Cycle
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Simulations – 5% EGR, 2nd Percentile Cycle
Results for 10 – 25% EGR are similar
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• User defined kinetics mechanism for predicting autoignition
• Kinetics reactions are active in the unburned zone during the combustion period
• Can be used with all SI combustion models (SIWiebe, SITurb, profile combustion, etc.)
• Autoignition is detected based on rate of pressure rise in the unburned zone
• Additional criterion can be used to simulate the onset of detectable knock, e.g. unburned mass fraction, knock index, etc.
Detailed Kinetics in Unburned Zone (V7.3)
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KLSA & CA 50 advance vs. EGR
Model : SI Turb, Speed : 1000 rpm, BMEP 15, EGR 0-25%, Intake temperature : 300K Maintained constant BMEP with EGR using pressure boost.
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1000 rpm, 15 bar BMEP
Unburned Mass Fraction at Knock Onset vs. Spark Timing
Spark Advance
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Hoepke, B., Jannsen, S., Kasseris, E., Cheng, W. K., EGR Effects on Boosted SI Engine Operation and Knock Integral Correlation, SAE 2012-01-0707
KLSA vs. EGR Comparison with Experimental Data from Literature
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Conclusions • The detailed chemical kinetic mechanism used in this study was
able to describe the effect of fuel chemistry and EGR ratio well on autoignition – Validation with single cylinder engine dyno data shows autoignition
occurring towards the end of combustion with combustion phased 3 to 5° crank angle advanced of the mean cycle over the range of loads and EGR investigated
– Mean cycle did not autoignite in any case when there was unburned mass left in the main chamber
• The chemical kinetic model provides a good trade-off between computational speed and model fidelity
• GT-POWER v7.3 incorporates knock kinetics in the main chamber, thereby eliminating the approximations made in the indirect approach used in this study
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BACKUP
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Unburned Mass Fraction at Knock Onset vs. Spark Timing KLSA vs. Intake Temperature
Speed : 1000 rpm BMEP : 15 bar
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Pressure Traces (v 7.3)
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