Reciprocating Internal Combustion Engines

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Reciprocating Internal Combustion Engines Prof. Rolf D. Reitz, Engine Research Center, University of Wisconsin-Madison 212 Princeton-CEFRC Summer Program on Combustion Course Length: 9 hrs (Wed.

1 Reciprocating Internal Combustion Engines Prof. Rolf D. Reitz, Engine Research Center, University of Wisconsin-Madison 212 Princeton-CEFRC Summer Program on Combustion Course Length: 9 hrs (Wed., Thur., Fri., June 27-29) Hour 3 Copyright 212 by Rolf D. Reitz. This material is not to be sold, reproduced or distributed without prior written permission of the owner, Rolf D. Reitz. 1 CEFRC3 June 27, 212

2 Hour 3: Chemical Kinetics, HCCI & SI Combustion Short course outine: Engine fundamentals and performance metrics, computer modeling supported by in-depth understanding of fundamental engine processes and detailed experiments in engine design optimization. Day 1 (Engine fundamentals) Hour 1: IC Engine Review,, 1 and 3-D modeling Hour 2: Turbochargers, Engine Performance Metrics Hour 3: Chemical Kinetics, HCCI & SI Combustion Day 2 (Spray combustion modeling) Hour 4: Atomization, Drop Breakup/Coalescence Hour 5: Drop Drag/Wall Impinge/Vaporization Hour 6: Heat transfer, NOx and Soot Emissions Day 3 (Applications) Hour 7: Diesel combustion and SI knock modeling Hour 8: Optimization and Low Temperature Combustion Hour 9: Automotive applications and the Future 2 CEFRC3 June 27, 212

Hour 3: Chemical Kinetics, HCCI & SI Combustion Modes of Engine Combustion http://www.erc.wisc.edu/combustion.php HCCI uses a hybrid combustion strategy.

3 Hour 3: Chemical Kinetics, HCCI & SI Combustion Modes of Engine Combustion HCCI uses a hybrid combustion strategy. Premixed fuel and air is inducted, but instead of igniting with a spark as in a SI engine, the high temperature from compression causes the mixture to spontaneously react, like in a diesel engine. Ignition occurs at slightly different times at different locations in the chamber. One feature of HCCI combustion is how quickly the fuel is consumed. 3 CEFRC3 June 27, 212

4 Basic Combustion Concepts Spark Ignition (SI) How can SI engines operate with engine speeds from 1 to 2, rev/min? x Hour 3: Chemical Kinetics, HCCI & SI Combustion T burned fuel/air S T Turbulence! Because turbulent flame speed, S T, scales with rpm! Characteristic Time Combustion (CTC) model Kinetic energy, k~v piston 2 Integral length scale l I ~ L piston Kinetic energy dissipation rate, V piston3 /L piston Diffusivity, D ~ k 2 / V piston L piston Reitz & Bracco, 1983; Abraham, 1985 Species conversion rate (Y i, species mass fraction, * local equilibrium solution) ; c ~ k/ L piston / V piston Mallard-Le Chatelier propagating wave speed: S T dy D dt i ~ V piston Glassman, CEFRC3 June 27, 212

5 Hour 3: Chemical Kinetics, HCCI & SI Combustion Basic Combustion Concepts Diesel (CI) Halstead, 1977 Ignition Delay Q R* B Shell Ignition Model RH +O2 2R* R* R* + P + Heat R* R* + B R* R* + Q R* + Q R* + B B 2R* R* termination 2R* termination Af4 Switch to Characteristic Time Combustion model Turbulence generated by fuel injection c ~ k/ L nozzle / V nozzle Kong, CEFRC3 June 27, 212

6 Turbulent Mixing Hour 3: Chemical Kinetics, HCCI & SI Combustion Spark-ignition Hot products with Cold reactants unburned burned ~ k/ ~ L piston / V piston S T /S L S T High turbulence - faster combustion Injected fuel with entrained air ~ k/ ~ L nozzle / V nozzle air fuel Diesel Delayed ignition (PCCI) - better mixing S T = air Matalon, CEFRC CEFRC3 June 27, 212

7 Hour 3: Chemical Kinetics, HCCI & SI Combustion Summary of combustion regimes Gasoline engine spark-ignition with flame propagation: High turbulence for high flame speed heat losses. Issues: NOx and UHC/CO, knock (CR, fuels), throttling losses low thermal efficiency TE ~25% Diesel engine with spray (diffusion) combustion: Rich mixtures (soot) & high temperatures (NOx) higher TE ~45% H/Premixed Charge Compression Ignition LTC, chemistry controlled: Sensitive to fuel, poor combustion/load control, low NOx-soot TE ~5% spark-ignition diesel H/PCCI 7 CEFRC3 June 27, 212

$Well-Stirred-Reactor (WSR) ; Williams, 1988 ; specific internal energy chemical label n r reactions n s species reactant/product stoichiometric coefficients mass fraction molecular weight species$

8 Hour 3: Chemical Kinetics, HCCI & SI Combustion Premixed Volumetric Combustion & Chemical Kinetics Species and energy conservation equations i i c s ( i u) [ D ( )] i i t Constant volume combustion Well-Stirred-Reactor (WSR) ; Williams, 1988 ; specific internal energy chemical label n r reactions n s species reactant/product stoichiometric coefficients mass fraction molecular weight species energy 8 CEFRC3 June 27, 212 I

9 Hour 3: Chemical Kinetics, HCCI & SI Combustion Chemical Kinetic Mechanisms for Engine Simulations Requirements for mechanisms for practical engine simulations: Size can not be too large due to CPU time limitation ~ 1 species Capable of predicting auto-ignition delay time accurately Contain proper reactions for pollutant formation precursors Biodiesel surrogates - Significant mechanism reduction is required. Soy biodiesel - Methyl: - palmitate (C16:) - stearate (C18:) - oleate (C18:1) - linoleate (C18:2) - linolenate (C18:3) C 2 C 4 C 9 9 CEFRC3 June 27, 212

Hour 3: Chemical Kinetics, HCCI & SI Combustion Hydrocarbon kinetics - NTC O 2 Second Stage Ignition H 2 O 2 = OH + OH Induction Period (s) 14 12 1 8 6 4 2

10 Hour 3: Chemical Kinetics, HCCI & SI Combustion Hydrocarbon kinetics - NTC O 2 Second Stage Ignition H 2 O 2 = OH + OH Induction Period (s) Acceleration by Q OOH branching Warnatz, Initial Temperature (K) First Stage Ignition Isomerization steps 1 CEFRC3 June 27, 212

11 R - RH H Hour 3: Chemical Kinetics, HCCI & SI Combustion HCCI combustion kinetics Fast High Temperature Combustion Typical HCCI Combustion Temperature and Heat Release Rate profiles + O 2 OO H 2 O 2 T, P + HO 2 OOH O + + OH + O 2 OOH OO - OH HOO O O Degenerate Branching Path O + OH + O + OH Ethers/ olefins HRR Aldehydes/ketones TDC Mehl, CEFRC3 June 27, 212 CAD

12 Hour 3: Chemical Kinetics, HCCI & SI Combustion Mechanism reduction identify key reaction steps ERC n-heptane mechanism Temperature 6 1. n-c 7 H 16 + OH= C 7 H H 2 O 2. C 7 ket 12 = C 5 H 11 CO + CH 2 O + OH 3. H 2 O 2 + M = OH + OH + M 4. HO 2 + HO 2 = H 2 O 2 + O 2 5. CH 4 + HO 2 = CH 3 + H 2 O 2 6. CO + OH= CO 2 + H, 7 7. C 7 H 15 O 2 + O 2 = C 7 ket 12 + OH Time Patel, SAE CEFRC3 June 27, 212

13 Hour 3: Chemical Kinetics, HCCI & SI Combustion Reduced mechanisms - match shock tube and RCM data First stage (t1), main ignition (t2=tig) delay 1 Expts: Fieweger, 1997 Predicted ignition delay times validated against shock tube tests (data from Fieweger) =1. and P=4 bar n-heptane/air ignition delay [ms] 1.1 Cal, tig Exp, tig Cal, t1 Exp, t /initial temperature [1/K] Ra and Reitz, CEFRC3 June 27, 212

14 Mechanism reduction methodology Reduction of reaction pathways and species number - combination of chemical lumping, graphical reaction flow analysis and elimination methods Reaction rate optimization - ignition delay curve sensitivity analysis Ignition delay sensitivity coefficient S S ig 1 k1 1 k2 ( T ) 1 log t log ( k k ) gr Hour 3: Chemical Kinetics, HCCI & SI Combustion Pre-exponential: (log t log t ) 1 base Ignition delay gradient sensitivity coefficient d log1 tk d log 1 1 tk2 ( T) dt dt 1 log ( k k ) Positive S gr : counter-clockwise rotation Negative S gr : clockwise rotation Ra and Reitz, 211 A = k A base ignition sensitivity [%] ignition delay [ms] tetra-decane: ROO+O =R-keto+OH A = k A base initial temperature [K] initial temperature [K] ignition delay sensitivity gradient sensitivity 14 CEFRC3 June 27, baseline k=2 k= gradient sensitivity coefficient

15 Hour 3: Chemical Kinetics, HCCI & SI Combustion Mechanism reduction group reaction classes No Reaction As Bs Cs Ar Br Cr effect I RH H R H 2O II RH OH R H 2O C τ1, p2, p3, τ4 III RH HO 2 R H 2O2 τ1, P2, P3, τ4 IV RH O 2 R HO 2 P2, P3 V R O 2 ROO VI-a ROO = QOOH C τ 1, P 2 VI-b QOOH + O 2 = OOQOOH P 2, P 3 VI-c OOQOOH = R-keto + OH P 2, P 3 Ignition delay sensitivity coefficient VII R-keto = CH2O + R'CO + OH C τ 1 VIII R'CO = X 1 + X 2 + CO P 2, P 3, τ 4 IX R = S 1 + S 2 + S3 Sensitivity of ignition delay curves of n-heptane oxidation - solid circle, open circle and blank entry denote dominant, mild and not significant influence, respectively. - C indicates counter-clockwise rotation. - Circle only indicates clockwise rotation. Ignition delay gradient sensitivity coefficient Ra and Reitz, CEFRC3 June 27, 212

16 ERC-MultiChem: PRF 41 species, 158 reactions base mechanism Source mechanisms: LLNL n-heptane (56 species; 2,539 reactions), isooctane (857 species; 3,66 reactions), ERC n-heptane (29 species; 52 reactions) ignition delay [ms] 1 1 Hour 3: Chemical Kinetics, HCCI & SI Combustion PRF MultiChem Exp, Fieweger et al. (1997).1 1 PRF nc7h16/air MultiChem 1 Exp, Fieweger et al. (1997) ignition delay [ms] 1/T [1/K] 1.1 ignition delay [ms] =1., ic 8 H 18 /air, 4 bar /T [1/K] =1., 4 bar /initial temperature [1/K] Ra and Reitz, CEFRC3 June 27, 212 ic8 PRF9 PRF8 PRF6 nc7 Exp, ic8 Exp, PRF9 Exp, PRF8 Exp, PRF6 Exp, nc7

mechanism Physical property surrogates Cyclo

mechanism ERC reduced mechanism* cyclohexane

n-octadecane iso-alkanes heptamethyl nonane

tetralin n-pentylbenzene n-heptylbenzene

species 3586 reactions 25 species 51 reactions

17 Hour 3: Chemical Kinetics, HCCI & SI Combustion Chemical class grouping MultiChem skeletal mechanism Physical property surrogates Cyclo alkanes Chemistry surrogates LLNL Detailed mechanism ERC reduced mechanism* cyclohexane decalin cyclohexane n-alkane n-dodecane n-alkane n-octadecane iso-alkanes heptamethyl nonane tetramethyl hexane Aromatics naphthalene mcymene tetralin n-pentylbenzene n-heptylbenzene n-heptane n-tetradecane iso-octane toluene 857 species 3586 reactions 25 species 51 reactions MultiChem Mechanism 1 species, 348 reactions Ra and Reitz, CEFRC3 June 27, 212

18 Hour 3: Chemical Kinetics, HCCI & SI Combustion ignition delay [ms] Ignition delay validations of MultiChem Gauthier CNF 24 Experiment Model /T [1/K] Propane phi=1. Pin= 3 bar ignition delay [ms] Model Fieweger CNF 1997 Exp, Fieweger et al. (1997) nheptane phi=1. Pini=4 bar /T [1/K] 8 Surrogate fuels: n-heptane, iso-octane, tetradecane, cyclohexane, toluene, decalin, ethanol, MB/D ignition delay [ms] Model Experiment /T [1/K] ic8h18 phi=1. Pini=4 bar Fieweger CNF 1997 ignition de lay [micr-sec] EXP (Bounaceur et al.) Bounaceur et al. Andrae et al. ERC-MultiChem Toluene tig n [mic ro-s] MCH Phi=1. E xperiment Model ignition delay [ms] Experiment Model Decalin phi=1. Pini=4 bar /T [1/K] Bounaceur IJCK 25; Andrae CNF /T [1/K ] 1/T [1/K] Shen,Energy & Fuels, CEFRC3 June 27, 212

19 Hour 3: Chemical Kinetics, HCCI & SI Combustion Biofuel models biodiesel and ethanol Soy-based biodiesel: Methyl palmitate (C16:) Methyl stearate (C18:) Methyl oleate (C18:1) Methyl linoleate (C18:2) Methyl linolenate (C18:3) n-heptane + MD.1 LLNL 3299 species long-chain methyl ester surrogate Methyl decanoate (MD), Methyl-9-decenoate (MD9D) ERC reduced: 85 species LLNL MD+MD9D mechanism PRFmulti + MD/MD9D reduced.1 nc 7 H 16 + MD nc 7 H 16 + MD9D LLNL MD+MD9D mechanism PRFmulti + MD/MD9D reduced n-heptane + MD9D Brakora SAE Igniton Delay Time (s) 1E-3 1E-4 1E-5 1E-5 = 1., MD fuel = 1., MD9D fuel / T (K) Igniton Delay Time (s) 1E-3 1E-4 1 / T (K) 19 CEFRC3 June 27, 212

20 Hour 3: Chemical Kinetics, HCCI & SI Combustion HCCI Engine validations pressure [MPa] phi=.12, Cal phi=.2, Cal phi=.28, Cal phi=.12, Exp phi=.2, Exp phi=.28, Exp 2-D grid with 2,734 cells for iso-octane HCCI combustion simulation crank angle [deg atdc] Measured and predicted pressure profiles of iso-octane fueled HCCI combustion for various equivalence ratios. Measured data: Hessel, SAE Ra and Reitz, CEFRC3 June 27, 212

21 Validation - Gasoline HCCI Engine Cases Engine Speed (rev/min) Equivalence Ratio Intake Temperature (K) Start of Injection (BTDC) Hour 3: Chemical Kinetics, HCCI & SI Combustion Caterpillar 34 engine Bore = 137.2mm Stroke = 165.1mm Compression ratio = Tamagna, SAE Pressure (bar) Pressure (bar) Expt Simu 7 rpm.27 CASE 1 Crank Angle ( o ATDC) CASE 3 Expt Simu 1737 rpm HRR (J/Deg) Pressure (bar) Pressure (bar) Expt Simu 7 rpm.35 Crank Angle ( o ATDC) CASE 4 Expt Simu 1737 rpm.28 CASE CEFRC3 June 27, 212 Crank Angle ( o ATDC) HRR (J/Deg) Crank Angle ( o ATDC) HRR (J/Deg) HRR (J/Deg)

Hour 3: Chemical Kinetics, HCCI & SI Combustion Efficient chemistry solvers Adaptive multi-grid chemistry (AMC) model Group thermodynamically-similar cells to reduce the calling frequency to save

22 Hour 3: Chemical Kinetics, HCCI & SI Combustion Efficient chemistry solvers Adaptive multi-grid chemistry (AMC) model Group thermodynamically-similar cells to reduce the calling frequency to save computer time Extended dynamic adaptive chemistry (EDAC) scheme Liang, 29 Dynamically determine the size of fuel chemical mechanism based on the local and instantaneous thermal conditions of the cells Thermodynamically similar cells (similar temperature, equivalence ratio ) Shi, Ge and Reitz, Springer, 212 Remap back to cells Chemkin Solver 22 CEFRC3 June 27, 212

23 Hour 3: Chemical Kinetics, HCCI & SI Combustion HCCI engine validation ERC PRF mech. (39 sp, 141 rxn) Full AMC AMC+EDAC hrs hrs hrs Pressure (MPa) Experiment Simulation-Full Chemistry Simulation-AMC + EDAC model Crank Angle 23 Shi, Ge and Reitz, Springer, CEFRC3 June 27, 212

Hour 3: Chemical Kinetics, HCCI & SI Combustion Flame propagation Models with Detailed Chemistry Platform: KIVA3V-G CFD code + Detailed Chemistry Solver Turbulent Flame Propagation G-equation

24 Hour 3: Chemical Kinetics, HCCI & SI Combustion Flame propagation Models with Detailed Chemistry Platform: KIVA3V-G CFD code + Detailed Chemistry Solver Turbulent Flame Propagation G-equation description of combustion Laminar and turbulent flame speeds Primary heat release calculation Flame quench due to mixture stratification Post-flame Chemistry CO oxidation, H 2 -O 2 reactions Pollutant formation mechanisms Knocking Combustion Spark Ignition Engine Auto-ignition mechanisms Location / intensity Liang and Reitz, SAE CEFRC3 June 27, 212

Gas Auto-ignition (detailed kinetics) Φ>1 CH4, CO, H, H2 Fuel Droplets ST from flame speed correlations End-gas Zone Burned gas:

25 Hour 3: Chemical Kinetics, HCCI & SI Combustion Ignition and Level set (G-equation) Models Partially Premixed Flame (DI Engine) Discrete particle ignition model Φ<1 O2, O, NO G-Equation Flame Φ 1 propagation Diffusion CO2, H2O, CO, NO Burnt Gas Diffusion End Gas Auto-ignition (detailed kinetics) Φ>1 CH4, CO, H, H2 Fuel Droplets ST from flame speed correlations End-gas Zone Burned gas: G> Unburned gas: G< ~ G ~ ~ ~ ~ (v f vvertex ) G u ST G DT k G t 25 Flame Front Post-flame Zone Liang SAE CEFRC3 June 27, 212

26 Hour 3: Chemical Kinetics, HCCI & SI Combustion Turbulent flame structure Combustion regime diagram Kolmogorov/Batchelor length scales: Peters, 2 Quenched broken reaction zones 1/ 4 3 l K Re 3/ 4 l I l K l Flamelets lk lf Ghandhi 212 Laminar flame thickness: lf / c p u S L T If lk Cm 3 l Cm 3.1lF, local ST = ~ 2 m Liang SAE It is not possible to resolve a turbulent flame on a practical engine simulation grid 26 CEFRC3 June 27, 212

27 Hour 3: Chemical Kinetics, HCCI & SI Combustion Turbulent Flame Speed Correlation Cm 2 t tign ST 1 1 exp SL I 1/ 2 Peters, 2 1/ a4b3 l l a4b3 l 2 u a4b3 Sl lf 2b1 lf 2b1 lf Progress Term Liang SAE / 2 Stretch factor: l I 1 F 15 l 3/ 2 u l F u 2 S rk K L Turbulence stretch Characteristic Timescale: Discrete Particle Ignition Kernel (DPIK) model Curvature k k1.5 l I Cm1.16 Transition criterion: rk C m1 ~ ~ ~ G ~ ~ ( v f vvertex ) G u ST G DT k G t 27 rk Fan & Reitz 2 CEFRC3 June 27, 212

28 Hour 3: Chemical Kinetics, HCCI & SI Combustion Laminar Flame Speed Correlations Liang SAE Power Law (Metghalchi & Keck, 1982): 3 T p S L S L, 1 2.1Ydil T p 25 (cm/sec) ( 1) L,ref ( 1) S Reference State: 3K, 1bar S L, Bm B2 ( m ) SL, exp ( ) Liang et al. : For iso-octane, Metghalchi et al. Present Liang et study al Equivalence Ratio, CEFRC3 June 27,

29 Hour 3: Chemical Kinetics, HCCI & SI Combustion Turbulent flame front zone (primary heat release) Species Production Rate within Flame Containing Cells: u Yi b Yi u Ai4 ST d i dt Vi 4 Equilibrium within Vb 1. Unburned Vu and burnt Vb volumes are tracked 2. Yi b is determined by the element potential method 3. ρib is determined by equation of state of burnt mixture b p MWmix b Yi Y i RuT b b i b Vu Vb b i4 4. ρiu is determined by mass conservation of species i Flame surface (G=) i Vi 4 ib Vb Vu u i 5. Unburned species mass fractions Yi u iu iu Liang SAE i 29 CEFRC3 June 27, 212

30 Hour 3: Chemical Kinetics, HCCI & SI Combustion Validation - PFI/DI Gasoline Engine Bore Stroke 89 mm 79.5 mm Compression Ratio 12 : 1 Engine Speed 15 rev/min -44, -4, -36, -32 MAP (kpa) 65 Based on MIT PRF Mechanism (25 species, 51 reactions) Model constants: Cm1=2., Cm2=1. (Fixed in all cases) PFI Mode Spark timings (ATDC) Liang SAE DI Mode (Spark timing sweeps) Spark timings (ATDC) -32, -28, -24, -2 MAP (kpa) 75 End of Injection (ATDC) - 72 DI Mode (Manifold-Absolute-Pressure sweeps) MAP (kpa) 75, 8, 9, 1 Spark timing (ATDC) - 33 End of Injection (ATDC) - 68 DI Mode (End-Of-Injection sweeps) End of Injection (ATDC) -76, -72, -68, -64 MAP (kpa) 75 Spark timing (ATDC) - 32 DI Configuration 3 CEFRC3 June 27, 212

CA = -2 ATDC CA = 1 ATDC CA = -5 ATDC CA = 2 ATDC Evolution

31 Hour 3: Chemical Kinetics, HCCI & SI Combustion Validation - PFI Engine Operation Liang SAE Spark Timing = 4 BTDC CA = -2 ATDC CA = 1 ATDC CA = -5 ATDC CA = 2 ATDC Evolution of the G= surface Evolution of Temperature 31 CEFRC3 June 27, 212

32 Hour 3: Chemical Kinetics, HCCI & SI Combustion Validation - PFI Engine Operation PFI mode O -32 ATDC Pressure (MPa) EXPT SIMU Pressure (MPa) o 5 1 Crank Angle ( ATDC) EXPT SIMU.5-5 o 5 1 Spark Timing -44, -4, -36, -32 ATDC Engine Speed 15 rpm PFI mode O -36 ATDC.5 2. PFI mode O -4 ATDC EXPT SIMU Pressure (MPa) Pressure (MPa) 2. Liang SAE o 5 1 Crank Angle ( ATDC) EXPT SIMU PFI mode O -44 ATDC o Crank Angle ( ATDC) Crank Angle ( ATDC) 32 CEFRC3 June 27, 212

33 Hour 3: Chemical Kinetics, HCCI & SI Combustion Assessment of the role of flame propagation Liang SAE Explore Kinetics-Controlled Formulation for Turbulent Flame Propagation: After ignition kernel stage, each cell is modeled as a WSR, detailed chemistry is applied. Flame propagation is controlled by heat conduction and auto-ignition EXPT G-equation Kinetics only PFI mode 3. Pressure (MPa) Pressure (MPa) Spark timing -44 ATDC o Spark timing -32 ATDC 1..5 Transition from kernel to G-eqn -2 ATDC EXPT G-equation Kinetics only DI mode 8 Crank Angle ( ATDC). -8 Transition from kernel to G-eqn at -2 ATDC o 4 Crank Angle ( ATDC) Mallard-Le Chatelier propagating wave speed: ST D 33 dyi dt CEFRC3 June 27,

alone is NOT sufficient to properly model flame propagation.

34 Hour 3: Chemical Kinetics, HCCI & SI Combustion Role of flame propagation Kinetics Controlled G-equation PFI case Spark timing = -44 ATDC Summary: Auto-ignition chemistry alone is NOT sufficient to properly model flame propagation. Turbulence enhancing effect on flame propagation speed in SI engines CANNOT be neglected. Liang SAE CEFRC3 June 27, 212

CFD Combustion Models for IC Engines. Rolf D. Reitz

CFD Combustion Models for IC Engines Rolf D. Reitz Engine Research Center University of Wisconsin-Madison ERC Symposium, June 7, 27 http://www.cae.wisc.edu/~reitz Combustion and Emission Models at the