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Diesel combustion and SI knock modeling

Diesel combustion and SI knock modeling 19
Part 7: Diesel combustion and SI knock modeling Reciprocating Internal Combustion Engines Prof. Rolf D. Reitz Engine Research Center University of WisconsinMadison 2014 PrincetonCEFRC Summer School on Combustion Course Length: 15 hrs (Mon. Fri., June 23 – 27, 2014) Copyright ©2014 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 CEFRC47, 2014 Part 7: Diesel combustion and SI knock modeling Short course outine: Engine fundamentals and performance metrics, computer modeling supported by indepth understanding of fundamental engine processes and detailed experiments in engine design optimization. Day 1 (Engine fundamentals) Part 1: IC Engine Review, 0, 1 and 3D modeling Part 2: Turbochargers, Engine Performance Metrics Day 2 (Combustion Modeling) Part 3: Chemical Kinetics, HCCI SI Combustion Part 4: Heat transfer, NOx and Soot Emissions Day 3 (Spray Modeling) Part 5: Atomization, Drop Breakup/Coalescence Part 6: Drop Drag/Wall Impinge/Vaporization/Sprays Day 4 (Engine Optimization) Part 7: Diesel combustion and SI knock modeling Part 8: Optimization and Low Temperature Combustion Day 5 (Applications and the Future) Part 9: Fuels, Aftertreatment and Controls Part 10: Vehicle Applications, Future of IC Engines 2 CEFRC47, 2014 Part 7: Diesel combustion and SI knock modeling Diesel engine applications OnHighway Vehicles Marine – Propulsion or auxiliary Power Generation – Prime power for remote locations, or standby power for edifices Locomotive – Switchyard engines, passenger engines, European light freight OffHighway Vehicles – Mine trucks (haulers, loaders, etc) OffHighway Stationary – Petroleum industry (drill rigs, pumps, etc.) 3 CEFRC47, 2014 Part 7: Diesel combustion and SI knock modeling Diesel engine applications http://maniacworld.com/Worlds MostPowerfulDieselEngine.html 4 CEFRC47, 2014 Part 7: Diesel combustion and SI knock modeling Diesel combustion Air alone is drawn in and compressed Fuel injected into high temperature, high pressure air to initiate combustion Parameter Quiescent Medium Swirl High Swirl Cylinder Size Largest Medium Smallest Load controlled by quantity of fuel injected Maximum Speed 100–1800 1800–3000 3000–5000 (rpm) Range of Bore 900–150 mm 150–100 mm 130–80 mm Highly heterogeneous (mm, inches) Compression 12–15 15–16 16–18 mixture in cylinder Ratio Combustion Shallow Bowl Moderate Bowl Deep Bowl wide range of mixture Chamber concentration over which Injection Pressure Highest High Moderate combustion occurs Number of nozzle Multiple Multiple Multiple wide range of operating holes InCylinder Air Quiescent Medium Swirl High Swirl airfuel ratios Flow 5 CEFRC47, 2014 Part 7: Diesel combustion and SI knock modeling Diesel combustion General Rules Center fuel injector in bowl, and if possible, center bowl in piston Trade off fuel injection pressure versus air motion to provide required mixing while not overmixing at light load Injection timing Injection pressure and Number, diameter, and angle of rate shape spray holes Reentrancy Intake port geometry Rim width Air supply pressure and temperature Bowl depth and geometry Compression ratio Exhaust Gas Recirculation Supply sufficient air to meet peak torque smoke limits Select injection timing and compression ratio for best fuel economy within emission constraints Other constraints: peak cylinder pressure, fuel injection pressure 6 CEFRC47, 2014 Part 7: Diesel combustion and SI knock modeling Dec, 1997 Diesel combustion (Conceptual model of Dec, 1997) Time to mix fuel/air to “combustible” ratios Time at given T and P to initiate combustion 7 CEFRC47, 2014 Part 7: Diesel combustion and SI knock modeling Park, 2007 Diesel combustion regimes Kamimoto plot 2.0 1.5 Soot 1.0 High Efficiency Clean Regime 0.5 NOx HCCI CO 0.0 1400 1600 1800 2000 2200 2400 Cylinder Temperature K HCCI Requires precise charge preparation and combustion control mechanisms (for autoignition and combustion timing) 8 CEFRC47, 2014 Equivalence Ratio Part 7: Diesel combustion and SI knock modeling Musculus 2006 Dec, 1997 Diesel combustion regimes Conventional Diesel LTC diesel Combustion Dec used optical diagnostics to Musculus developed a similar develop a conceptual model of conceptual model for diesel LTC conventional diesel combustion OH exists only at periphery of the jet Low temperature reactions fill the  thin diffusion flame surrounding a head of the jet with intermediates soot filled jet (e.g., CH O) 2 OH is observed across the entire jet crosssection Dec’s conceptual model LTC conceptual model 9 CEFRC47, 2014 Part 7: Diesel combustion and SI knock modeling Diesel combustion modeling KIVACHEMKING code CHEMKIN II based chemistry solver used for volumetric heat release Each cell considered a wellstirred reactor (WSR) Diesel fuel chemistry is modeled with ERC reduced nheptane mechanism Flame propagation considered through level set based model  G equation Spray modeled with ERC spray models injector nozzle tip firedeck Gasjet theory used to reduce grid size dependency of droplet drag squish region calculations Collision model considers bounce, coalescence, and fragmenting and nonfragmenting separations cylindrical piston bowl KHRT breakup model piston bowl wall crevice region Tetradecane used for fuel physical properties 10 CEFRC47, 2014 Part 7: Diesel combustion and SI knock modeling Singh, 2009 Engine setup – Sandia Cummins Base engine type Cummins N14 DI diesel Number of cylinders 1 Bore x stroke 13.97 x 15.24 cm Connecting rod length 30.48 cm Displacement 2.34 L Geometric compression ratio 10.75:1 Simulated compression ratio 16:1 Bowl width 9.78 cm Bowl depth 1.55 cm Fuel injector type Commonrail Cup (tip) type minisac Number of holes 8, equally spaced Spray included angle 152° Nozzle orifice diameter 0.196 mm Nozzle orifice L/D 5 11 CEFRC47, 2014 Part 7: Diesel combustion and SI knock modeling Singh, 2009 Operating conditions Three cases ranging from Conventional to LTC diesel combustion HTC HTC LTC short ID med. ID Long ID O Conc. (Vol ) 21 21 12.7 2 Speed (RPM) 1200 1200 1200 IMEP (bar) 4.5 4.5 3.9 Intake Temp (C) 111 47 90 Intake Pressure (kPa) 233 192 214 TDC Motored Temperature (K) 905 800 870 TDC Motored 24 22.3 22.9 Density (kg/m3) Peak Adiabatic 2760 2700 2256 Flame Temp. K Rail Pressure (bar) 1200 1200 1600 Start of Injection (ATDC) 7 5 22 Duration of Injection (CA) 10 10 7 Injection Quantity (mg) 61 61 56 Ignition Delay (CA) 4 8.75 11 Ignition Dwell (CA) 6 1.25 +4 12 CEFRC47, 2014 Part 7: Diesel combustion and SI knock modeling Kokjohn, 2011 Combustion characteristics • Simulations capture combustion characteristics accurately over a range of combustion regimes • Results with and without consideration of flame propagation show nearly identical results HTCShort HTCMed LTCLong 13 CEFRC47, 2014 Part 7: Diesel combustion and SI knock modeling Kokjohn, 2011 Conventional diesel combustion 10 500 Simulations and experiments show HTCShort Experiment diffusion flame is 1 to 2 mm thick Simulation 8 400 6 300 4 200 2 100 injection profile Dec’s conceptual model 0 0 SAE 970873 20 15 10 5 0 5 10 15 20 Crank ATDC O2 21 SOI1 7 °ATDC P 1200 bar inj Intake P 2.33 bar Fuel Diesel Optical engine experiments Singh, CNF 2009 14 CEFRC47, 2014 Pressure MPa Heat Release Rate J/Part 7: Diesel combustion and SI knock modeling Kokjohn, 2011 Conventional diesel combustion Role of flame propagation: Flame propagation model predicts edge or triple flame structure near liftoff location Nose like structure with stoichiometric region closest to nozzle Stoichiometric region shows the only nonnegligible flame speed 30 Metghalchi et al. Present study 25 20 15 S S T L 10 G  5 0 u (vv )G SGD kG f vertex T T 0 t 0.0 0.5 1.0 1.5 2.0 2.5 Equivalence Ratio,  15 CEFRC47, 2014 0 S (cm/sec) L,refPart 7: Diesel combustion and SI knock modeling Kokjohn, 2011 LTC diesel combustion Increased mixing time allows OH to fill jet 10 2000 LTCEarly Experiment Simulation 8 1600 6 1200 4 800 injection 2 profile 400 LTC conceptual model Musculus et al. 0 0 SAE 2006010079 25 20 15 10 5 0 5 10 15 20 Crank ATDC O2 12.7 SOI1 22 °ATDC P 1600 bar inj Intake P 1.92 bar Fuel Diesel Optical engine experiments Singh, CNF 2009 16 CEFRC47, 2014 Pressure MPa Heat Release Rate J/Part 7: Diesel combustion and SI knock modeling Kokjohn, 2011 Dependence of flame structure on mixing time HTC – Short ID Simulations Experiments Diffusion flame is 12 mm thick Short 0.004 HTC – Medium ID Mixing Diffusion flame is 4 – 6 mm Time (exp. show 5 – 6 mm) LTC – Early (Long ID) 7 OH fills jet 0.003 Increase in reaction zone thickness with mixing time Flame structure is captured without 11 considering subgrid scale turbulent/chemistry interactions 0.001 Ignition occurs in premixed region Combustion is controlled by diffusion and large scale Long 14 (resolved) mixing processes Mixing Time Reaction rate dependence on injectiongenerated mixing decreases with increasing ignition delay. If ID extends past endofinjection, combustion generatedmixing dominates injectiongeneratedmixing 17 CEFRC47, 2014 LTCLong HTCMedium HTCShort ID Part 7: Diesel combustion and SI knock modeling Kokjohn, 2011 Singh, 2009 Flame thickness dependence on equivalence ratio Singh et al. (2009) studied dependence of equivalence ratio on OH LIF using OH Mass Fraction homogenous reactor and 1D opposed flow diffusion flame simulations HTCShort ID (21 O2) OH detectable when 0.2 Φ 1.6 1.65 LTCLong ID (12.7 O2) OH detectable when 0.5 Φ 1.2 Φ = 0.35 Kokjohn Reitz (2011) CFD simulations Φ = 1.2 show similar dependence on equivalence ratio For a given intake oxygen concentration, Φ = 0.55 OH LIF can be used to estimate local equivalence ratios OH Mass Fraction Reaction zone thickness and equivalence ratio are correlated. 18 CEFRC47, 2014 LTCLong HTCShort Part 7: Diesel combustion and SI knock modeling Kokjohn, 2011 Relative importance of turbulence and chemistry HTCShort ID Initial reactions occur in a well mixed zone on the periphery of the jet (Φ1) Kinetically controlled premixed spike followed by a mixing controlled energy release Turbulent mixing due to the injection event remains elevated during energy release Reaction rate is controlled by the rate of transport of reactive material to the reaction zone 19 CEFRC47, 2014 ε/tke 1/s FCR 1/s HRR 1/s Part 7: Diesel combustion and SI knock modeling Kokjohn, 2011 Relative importance of turbulence and chemistry HTCMedium ID Sharp increase in turbulent mixing rate due to injection event Injection event is nearly complete by SOC Turbulent mixing rate has started to decrease prior to autoignition Second spike in the mixing rate due to expansion of hot products Occurs after a significant quantity of the fuel has already been consumed Spray induced mixing less important as ignition dwell approaches zero 20 CEFRC47, 2014 ε/tke 1/s ε/tke 1/s FCR 1/s FCR 1/s HRR 1/s HRR 1/s Part 7: Diesel combustion and SI knock modeling Kokjohn, 2011 Relative importance of turbulence and chemistry LTCLong ID Sharp increase in turbulent mixing rate due to injection event Injection event is completed prior to secondstage combustion Mixing rate has dissipated nearly completely prior to autoignition Fuel is consumed in two distinct stages HR occurs rapidly and has nearly completed by the second spike in the mixing rate Turbulent mixing appears to play a secondary role to kinetics 21 CEFRC47, 2014 ε/tke 1/s ε/tke 1/s ε/tke 1/s FCR 1/s FCR 1/s FCR 1/s HRR 1/s HRR 1/s HRR 1/s Part 7: Diesel combustion and SI knock modeling Liang, 2007 Subramaniam, 2003 Sparkignition gasoline engine knock Bore × Stroke 89 mm × 79.5 mm Compression Ratio 12 : 1 Engine Speed 1500 rev/min PFI Mode Spark timings (ATDC) 44, 40, 36, 32 MAP (kPa) 65 DI Mode (Spark timing sweeps) Spark timings (ATDC) 32, 28, 24, 20 MAP (kPa) 75 End of Injection (ATDC) 72 Air/Fuel DI Mode (ManifoldAbsolutePressure sweeps) Ratio MAP (kPa) 75, 80, 90, 100 Spark timing (ATDC) 33 End of Injection (ATDC) 68 DI Mode (EndOfInjection sweeps) End of Injection (ATDC) 76, 72, 68, 64 MAP (kPa) 75 Spark timing (ATDC) 32 22 CEFRC47, 2014 Part 7: Diesel combustion and SI knock modeling Liang, 2007 Subramaniam, 2003 Direct injection flame propagation ERC spray models DPIK ignition model KIVAChemkinG with ERC PRF mechanism ERC reduced NOx mechanism Spark Timing = 32 ATDC 23 CEFRC47, 2014 Part 7: Diesel combustion and SI knock modeling Liang, 2007 Validation spark timing sweep 3.5 25 3.5 25 o o Spk = 20 ATDC EXPT EXPT Spk = 24 ATDC 3.0 3.0 SIMU SIMU 20 20 2.5 2.5 2.0 2.0 15 15 1.5 1.5 10 10 1.0 1.0 0.5 0.5 5 5 0.0 0.0 0 0 0.5 0.5 80 60 40 20 0 20 40 60 80 100 80 60 40 20 0 20 40 60 80 100 o o Crank Angle ( ATDC) Crank Angle ( ATDC) 3.5 25 3.5 25 o o Spk = 28 ATDC EXPT EXPT Spk = 32 ATDC 3.0 3.0 SIMU SIMU 20 20 2.5 2.5 2.0 2.0 15 15 1.5 1.5 10 10 1.0 1.0 0.5 0.5 5 5 0.0 0.0 0 0 0.5 0.5 80 60 40 20 0 20 40 60 80 100 80 60 40 20 0 20 40 60 80 100 o o Crank Angle ( ATDC) Crank Angle ( ATDC) 24 CEFRC47, 2014 Pressure (MPa) Pressure (MPa) Heat Release Rate (J/Deg) Heat Release Rate (J/Deg) Pressure (MPa) Pressure (MPa) Heat Release Rate (J/Deg) Heat Release Rate (J/Deg)Part 7: Diesel combustion and SI knock modeling Liang, 2007 Validation Spark Timing Sweep NOx Unburnt HC 1.2 1.2 Spark Timing Sweep Spark Timing Sweep EXPT 1.0 1.0 SIMU (cm =3) 3 SIMU (cm =0) 3 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 EXPT SIMU 0.0 0.0 32 30 28 26 24 22 20 32 30 28 26 24 22 20 o o Spark Timing ( ATDC) Spark Timing ( ATDC) Local flame quench due to mixture stratification is modeled: l C l Cl K m33 m F 25 CEFRC47, 2014 Normalized NO x Normalized UHC Part 7: Diesel combustion and SI knock modeling Liang, 2007 Role of turbulence in SI combustion KineticsControlled Formulation for Turbulent Flame Propagation: After ignition kernel stage, each cell modeled as WSR, detailed chemistry applied “Flame propagation” is controlled by heat conduction and autoignition instead of G equation model. 2.5 3.5 EXPT EXPT 3.0 Gequation Gequation 2.0 Kinetics only Kinetics only 2.5 DI mode PFI mode 1.5 2.0 Spark timing 1.5 1.0 Spark timing 32 ATDC 44 ATDC 1.0 0.5 Transition from kernel to Geqn 0.5 Transition from kernel 20 ATDC to Geqn at 20 ATDC 0.0 0.0 80 60 40 20 0 20 40 60 80 80 60 40 20 0 20 40 60 80 o o Crank Angle ( ATDC) Crank Angle ( ATDC) PFI (Spark timing = 44 ATDC) DI (Spark timing = 32 ATDC) 26 CEFRC47, 2014 Pressure (MPa) Pressure (MPa)Part 7: Diesel combustion and SI knock modeling Liang, 2007 Assessment of kineticscontrolled combustion models Kinetics Controlled Gequation PFI case Spark timing = 44 ATDC Conclusion: Autoignition chemistry alone is NOT sufficient to properly model flame propagation. Turbulence enhancing effect on flame propagation speed in premixed charge SI engines CANNOT be neglected. However, in direct injection cases combustion is controlled by mixing rates (diffusion combustion). 27 CEFRC47, 2014 Part 7: Diesel combustion and SI knock modeling Liang, 2007 Knocking combustion in SI engines Simulated local pressures are filtered by a Butterworth bandpass filter Passband frequencies used: 525 kHz Resonant frequencies based on classical wave equation (C. Draper 1938) : c s f m,, n m n B Numerical Transducers 16 0.8 Resonant f f f f f f 10 20 01 30 40 11 Position 1 14 0.6 frequency 12 Analytical 6.7 11.2 14.0 15.4 19.4 19.5 0.4 value (kHz) 10 0.2 8 0.0 N 6 1 PPmax KI PP 0.2  max,n Knock Index: 4 N n1 0.4 2 V 1 2 0 0.6 20 10 0 10 20 30 40 Power Index: PI pdV o  V Crank Angle ( ATDC) 1 V disp 28 CEFRC47, 2014 Pressure (MPa) Bandpass filtered pressure (MPa) Part 7: Diesel combustion and SI knock modeling Liang, 2007 Knocking combustion in SI engines Baseline conditions Operating mode Direct Injection Engine Speed 1500 rev/min 1.2 2.0 Boost pressure 160 kPa Baseline (Singleinjeciton, 0 EGR) 1.0 1.8 EGR 0 0.8 1.6 Equivalence ratio Stoichiometric 0.6 1.4 Injection timing 270 ATDC 0.4 1.2 Spark timing 25,20,15,10 ATDC 0.2 1.0 0.0 0.8 25 20 15 10 5 Spark Timing (CA ATDC) 29 CEFRC47, 2014 Knock intensity index, KI (MPa) Power Index, PI (MPa)Part 7: Diesel combustion and SI knock modeling Liang, 2007 Knocking combustion in SI engines 16 0.8 16 0.8 Spark Timing = 25 ATDC Spark Timing = 20 ATDC Severe Knock 14 14 Medium Knock 0.6 0.6 12 12 0.4 0.4 10 10 0.2 0.2 8 8 0.0 0.0 6 6 0.2 0.2 4 4 0.4 0.4 2 2 0 0.6 0 0.6 20 10 0 10 20 30 40 20 10 0 10 20 30 40 o o Crank Angle ( ATDC) Crank Angle ( ATDC) 16 0.8 16 0.8 Spark Timing = 15 ATDC Spark Timing = 10 ATDC 14 14 Light Knock 0.6 0.6 Light Knock 12 12 0.4 0.4 10 10 0.2 0.2 8 8 0.0 0.0 6 6 0.2 0.2 4 4 0.4 0.4 2 2 0 0.6 0 0.6 20 10 0 10 20 30 40 20 10 0 10 20 30 40 o o Crank Angle ( ATDC) Crank Angle ( ATDC) 30 CEFRC47, 2014 Pressure (MPa) Pressure (MPa) Bandpass filtered pressure (MPa) Bandpass filtered pressure (MPa) Pressure (MPa) Pressure (MPa) Bandpass filtered pressure (MPa) Bandpass filtered pressure (MPa) Part 7: Diesel combustion and SI knock modeling Liang, 2007 Knocking combustion in SI engines Spark timing=25 ATDC Severe knock Spark timing=10 ATDC Light knock Factors affecting knock intensity: Endgas autoignition tendency; Piston movement. 31 CEFRC47, 2014 Part 7: Diesel combustion and SI knock modeling Wang, 2013 Pressure oscillations during knock Frequency 5000 10000 15000 20000 25000 30000 8 pressures at wall 60000 1e7 80000 1e6 100000 1e5 0.20 6 0.15 0.10 4 0.05 2 0.00 5000 10000 15000 20000 25000 30000 0 5 10 15 20 Crank (Deg) Frequency Power spectrum shows high rd energy in 3 circumferential mode 32 CEFRC47, 2014 Press (MPa) Phase AmplitudePart 7: Diesel combustion and SI knock modeling Wang, 2013 Pressure oscillations during knock 33 CEFRC47, 2014 Part 7: Diesel combustion and SI knock modeling Wang, 2013 Acoustic characteristics of pressure oscillations 0.20 + 0.15 34 0.10 + + 0.05 + 0.00 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 Frequency ign. offset 1mm Knocking oscillation is mainly from 1st resonant mode oscillating energy is focused at 7.2 kHz. Although resonances also occur at 12.0, 15.0 and 16.4 kHz, their amplitudes are much smaller than 1st resonant mode. Draper resonance modes from equation 34 CEFRC47, 2014 AmplitudePart 7: Diesel combustion and SI knock modeling Wang, 2013 Heat transfer during knock Oscillating flow during knocking at 10.0 CA and 10.2 CA 1000 1000 800 800 600 600 new WHT model orig. WHT model 400 400 200 200 nonknocking  1 dp  c u T ln(T /T ) (2.1y 33.4) (Q ) p w c u1 dt 0 0 q w  10 0 10 20 30 40 2.1ln(y ) 2.5 Crank Angle (deg) Compared to nonknocking case, engine knock significantly enhances heat transfer to walls. Energy loss via heat transfer during combustion period is nearly 40 of total fuel energy under heavy knocking conditions, and is nearly 4 times heat transfer of nonknocking condition. 35 CEFRC47, 2014 Energy (J) wall heat transfer / JPart 7: Diesel combustion and SI knock modeling Wang, 2014 “Superknock” Superknock is severe engine knock triggered by preignition randomly, sometimes after many thousands of engine cycles 36 CEFRC47, 2014 Part 7: Diesel combustion and SI knock modeling Wang, 2014 “Superknock” Can lead to catastrophic engine damage: (a) spark electrode breakup, (b) exhaust valve melt, and (c) piston ring land broken 37 CEFRC47, 2014 Part 7: Diesel combustion and SI knock modeling Wang, 2014 “Superknock” Simulations of Deflagrationto Detonation Transitions (DDT) Thought to be due to preignition of hotspots from “particles/deposits” 38 CEFRC47, 2014 Part 7: Diesel combustion and SI knock modeling Wang, 2014 “Superknock” exacerbated in highly boosted SI engines Ring pack design has been shown to play an important role in reducing frequency of “Superknock” events Possible process of preignition to superknock: (a) hotspot autoignition, (b) flame and spark ignition, and (c) hotspot induced endgas detonation. 39 CEFRC47, 2014 Part 7: Diesel combustion and SI knock modeling Tan, 2004 Reitz, 2009 Summary CFD modeling is capable of describing both diesel and sparkignition combustion characteristics over a wide range of conditions. Diesel (mixingcontrolled) and premixed combustion is adequately represented without requiring subgridscale turbulencechemistry interactions to be modeled. The effect of turbulence on combustion is modeled satisfactorily using an integrated Gequationbased combustion model with detailed chemical kinetics (CHEMKIN). Very similar results are achieved with and without consideration of flame propagation for diesel combustion: Gequation flame propagation model does reveal edge flame at liftoff location (not observed with the kineticsonly calculation) Flamepropagationdominated premixed charge sparkignition engine combustion requires specification of turbulent flame speed in the model. The GequationCHEMKIN model allows all combustion regimes to be modeled (GAMUT: Gequation for All Mixtures – a Universal Turbulent combustion model – Tan Reitz, 2004, Reitz Sun, 2009). Integration with chemistry model allows model to predict both diesel ignition and SI engine knock processes. 40 CEFRC47, 2014