System Simulation for Aftertreatment LES for Engines Christopher Rutland Engine Research Center University of Wisconsin-Madison Acknowledgements General Motors Research & Development Caterpillar, Inc. National Science Foundation Department of Energy: FreedomCAR and Vehicle Technologies Program
ERC Work: Integrated System Level Model Engine and Emissions Simulink Aftertreatment Devices Engine Flow Models Combustion Models Emission Models DOC Model DPF Model LNT, etc. Models Exhaust flow Numerical parameters Control connection 2
Regeneration During Mode 5-2 Transient Runaway regeneration occurs when DPF is very hot and: Exhaust flow rate is suddenly lowered in mode transitions Excess O2 is available in exhaust Consistent with results from Koltsakis, et al. (SAE 2007-01-1127) Prevent runaway regeneration by intake throttling to reduce available O2 in exhaust DPF Wall Temperatures [ C] 1200 1000 800 600 400 200 Regeneration: Runaway after Mode Switch DPF Wall Temperature Regeneration Mode 5 Mode 2 Inlet Midpoint Outlet 0 0 200 400 600 800 1000 Time [s] DPF Wall Temperature [ C] 1200 1000 800 600 400 200 Prevent Runaway with Intake Throttling DPF Wall Temperature Mode 5 Inlet Midpoint Outlet 0 0 200 400 600 800 1000 Time [s] Mode 2 3
Large Eddy Simulation Purpose Enable better representation of local mixing and flow unsteadiness Application of LES for engine studies and LTC-D Ability to study mixture formation in more detail Engine cyclic variability amenable to analysis Improved sensitivity to operating conditions (ex: VVA) LES implementation Relatively coarse mesh used - often same as RANS No change in KIVA numerical scheme Next generation of turbulence models 4
Global Quantities Pressure LTC: Early Injection, High EGR HRR Operating Conditions LTC Case (Cummins N-14 Singh, et al. 2006) Speed IMEP Intake temperature Intake pressure O 2 concentration Start of injection Duration of injection Injection pressure 1200 RPM 3.9 363 K 214 kpa 12.7 % (volume) -22 o CA atdc 7 crank angles 160 MPa 5
Experiment Local Quantities Temperature Distribution LES - CHEMKIN RANS - CHEMKIN CA = -14.5 o atdc CA = -12 o atdc Same oscillatory structure Temp (K) Ignition chemiluminscence compared with temperature predictions LES develops more flow structures of appropriate length scale (Experimental images courtesy Singh et al., 2006) 6
Fuel Vaporization - RANS RANS simulations have very similar fuel vaporization rates Leads to similar temperature and equivalence ratio profiles prior to combustion Temperature prior to SOI RANS RNG k- In-cylinder Liquid Fuel Cycle 3 Temperature (K) Cycle 4 Cycle 5 7
Fuel Vaporization - LES LES simulations show variability in rate of fuel vaporization Leads to differences in combustion characteristics Temperature prior to SOI In-cylinder Liquid Fuel LES Dynamic Structure Cycle 3 Temperature (K) Cycle 4 Cycle 5 8
Variable Valve Actuation HCCI control by changing thermodynamic conditions Late IVC to control effective compression ratio Investigate influence of VVA on local flow and mixing phenomena Intake valve closing Stock IVC Case 217 o atdc Late IVC Case 275 o atdc IVO Stock IVC Late IVC Speed 1737 RPM 1737 RPM Intake valve lift profiles Fuel flow (Kg/hr) 3.0 (25% Load) 3.0 (25% Load) EGR 40 % 40 % Start of injection Intake pressure 305 o atdc 184.09 kpa 305 o atdc 184.09 kpa Operating conditions (Nevin, 2006) Intake flow rate 2.52 Kg/min 2.03 Kg/min Intake temperature 305 K 305 K 9
LES Responds to V V A Pressure Pressure Stock IVC Case Late IVC Case Temperature Cases simulated using RANS and LES Ignition and combustion calculations using CHEMKIN Mass at IVC: LES values closer to experimental calculations ~ 25 K difference 217 o IVC 275 o LIVC 10
Velocity Vectors RANS Stock IVC Case Late IVC Case Magnified valve profile CA = 235 o atdc Comparison of in-cylinder flow after stock IVC CA = 259 o atdc Similar Flow Fields Flow appears similar in both cases RANS predicts limited sensitivity to change in boundary conditions 11
Velocity Vectors LES Stock IVC Case Late IVC Case Magnified valve profile CA = 235 o atdc Flow in stock IVC case responds to valve closure Stagnation and reversal CA = 259 o atdc Continues to move up LES sensitive to change in boundary conditions Improved ability to predict mixing related phenomena later in cycle 12
DNS Spray Studies Evaporating liquid sprays N-heptane fuel Detailed chemical kinetics Movie: Temperature contours Temperature contours from evaporating spray 13
Temperature and Heat Release Rate: Movies 14
Developed Spray Combustion Lifted Flames Ignition sites Flamelet Regions flame fuel Conventional View* * From SAE 970873, J. Dec flame 15
Thank You 16
Objectives and Results Explore steady state and transient scenarios Examine and help explain device interactions Suggest guidelines of operation Based on model results and analysis Recent results LTC combustion under load transients Effect of DPF loading and regeneration on engine operation Compare different DPF regeneration techniques Model prevention and control of runaway DPF regenerations 17
Diesel-PCCI Mode Change: Cooled EGR and IVC Model transition: conventional PCCI conventional -120 Intake valve closes early with increase in cooled EGR Conventional SOI: 12 deg ATDC PCCI mode SOI: IVC + 5CAD IVC response for PCCI under different EGR conditions IVC actuation during the mode transition decreases with increase in EGR IVC [CAD] -125-130 -135-140 -145-150 0 1 2 3 4 Time [s] EGR 10% EGR 27% EGR 40% CA50 set to - 5.7 ATDC IMEP set to 5.3 bar 18
DPF Regeneration Fuel injected in exhaust before DOC Increases reactions in DOC and exhaust T Controlled to achieve a desired DPF inlet temperature of 600 C BSFC [g/kw-h] 300 290 280 270 260 Fuel Consumption DPF Regeneration 250 0 10 20 30 40 Time [min] Total BSFC Engine BSFC Exhaust Temperature [ C] 650 600 550 500 450 400 350 300 Exhaust Temperatures DPF Regeneration Engine Out DPF Out DOC Out 250 0 10 20 30 40 Time [min] 19
Throttle Assisted Regeneration Engine out NOx increased significantly during DPF regeneration Intake throttling required reducing EGR to maintain sufficient O2 for given load Reduced EGR resulted in higher engine out NOx 100 Engine-Out NOx NOx [ppm] 90 80 70 60 50 40 DPF Regeneration 30 0 10 20 30 40 Time [min] 20
Runaway Regeneration BSFC [g/kw-h] 350 325 300 275 Regeneration by fuel injection before DOC Try to reduce regeneration time by injecting more fuel Results in runaway regeneration Fuel Consumption Total BSFC Engine BSFC 250 0 1 2 3 4 Time [min] Exhaust Temperature [ C] 1200 1000 800 600 400 200 Exhaust Temperatures Engine Out DPF Out DOC Out 0 0 1 2 3 4 Time [min] DOC fuel injection stop Temperature continues to rise after fuel shutoff 21
Regeneration Phase Diagrams Runaway regeneration occurs when DPF is very hot and: Exhaust flow rate is low Excess O2 is available By using the phase diagram unsafe operating conditions can be identified and avoided by: Intake throttling Air injection into DPF inlet Air Injection Mode mode 2 Mode 8 mode 8 Mode Change Regen Intake Throttling 22
Ongoing Activities LES Improved combustion model for LES Modify and adapt spray models for LES Improve scalar mixing models with detailed experimental results Continue validation of LES approach for engines Test LES models in other engine codes System Simulations Develop additional emissions models: CO and HC Incorporate SCR device model Improve ease of use and run times Continue to study device interactions Suggest guidelines for operation 23