Perspectives on NO Formation in Combusting Diesel Spray Systems Jennifer Labs and Terry Parker

1. Perspectives on NO Formation in Combusting Diesel Spray Systems Jennifer Labs and Terry Parker Engineering Division Colorado School of Mines Golden, CO Presented at: The 3rd Joint Meeting of the US Sections of the Combustion Institute Chicago, IL March 18, 2003

2. Understanding of Diesel Spray Combustion Still Evolving • Complicated by: • Transient Mixture Formation • Variable Ignition Delay • Soot-Nox Tradeoff • Soot Product of Rich Combustion • NO formed at Diffusion Flame Front and in Post-Combustion Hot Gases • NO a product of high temperature, oxygen bearing environment • The 2007 Emissions targets for diesels are exceptionally stringent

3. Current View of Diesel Combustion • Put forth by Flynn, et. al.* • Two Phases: • Premixed • Diffusion • Soot considered a product of fuel-rich central spray zones • NO is formed in the: • Diffusion flame front • Post-combustion hot gases ** **Flynn, P.F, R.P. Durrett, G.L. Hunter, A.O. zur Loye, O.C. Akinyemi, J.E. Dec, C.K. Westbrook, SAE Paper No. 199-01-0509.

4. A Unique Facility Is Used to Simulate Diesel Combustion • System Capable of 50 atm and 1000 K • System includes central air flow and side arm nitrogen flows • Temperature gradients (~ 15oC) • Cooling BaF2 windows • 3-D Translation Capabilities • Data Acquisition and Timing Control • LabVIEW software controls system timing • 8 channels, 500 kHz

5. The Facility Captures the Physics of Diesel Combustion • Hard wall interaction simulated with a plate on top of a packed bed • Injection System • Single shot pressure amplifier produces peak pressures of 150 MPa • Custom Lucas CAV nozzle is drilled with a single hole in the center • (0.16 mm, L/D~4)

6. Combusting Spray Experiments Include Extractive Gas Analysis • Experiment Measurements • Injection Line Pressure • Vessel Pressure • FTIR Calibrated for Exhaust NO, CO2, CO and H2O • Constant Cell Pressure and Temperature • Operating conditions • 873K • 12.5 atm absolute • Central air flow, 165 slpm Heated Extraction Line FTIR 10 m gas cell

7. Diesel Combustion is a Two-Stage Process • Ignition apparent by small, sharp pressure rise • Low level heat release between premixed burn and diffusion burn • Supported by engine data • non-zero AHRR values between premixed and diffusion

8. Ignition Delay for Dodecane Is Similar to Previously Published Results ** • Ignition Delay • Device and Measurement Method Dependent • Current Experimental Data for Dodecane • Agrees with Previous Published Data • Repeatable Within Fuel Type New Data Points ** Parker, T.E., MD. Forsha, H.E. Stewart, K. Hom, R.F. Sawyer, and A.K. Oppenheim, SAE Paper No. 850087.

9. Experimental Results are for Controlled Mass Injection Events • Nitric oxide (NO) and carbon monoxide (CO) emissions normalized by carbon dioxide (CO2) • No shaft work in simulator • Assume small fraction of fuel carbon results in soot or carbon monoxide (CO) • CO2 representative of fuel input • Experiments measured total NO produced from a single injection event • Injection duration controlled • Mass injected controlled (3-28 mg)  representing light to heavy load conditions • Also controlling fraction of fuel consumed in premixed or diffusion combustion

10. Carbon Monoxide Results Support an Early Stage Rich Burn • High CO/CO2 levels at small injection masses • Large fraction of fuel consumed by fuel-rich premixed burn • Low CO/CO2 levels at large injection masses • Large fraction of fuel consumed by lean diffusion burn

11. Expected NO/CO2 Trend is Simple • NO formation rate hypothesized to be constant for steady state diffusion combustion • Low NO/CO2 levels for small injection events • Larger fraction of fuel consumed in fuel-rich premixed combustion • NO/CO2 level rising asymptotically to a constant NO production rate • Majority of fuel burned in the diffusion phase

12. NO/CO2 for Simulator and Test Engine is Maximized at an Intermediate Load **McCormick, R.L., M.S. Graboski, T.L. Alleman, A.M. Herring, and K.S. Tyson, Environmental Science & Technology 35(9):1742 (2001)

13. Injection and Combustion Time Scales Overlap • Ignition delay and start of secondary burn relatively constant • Negligible NO/CO2 for injection events shorter than ignition delay • Peak NO/CO2 production occurs when injection event begins to interact with major heat release event • Steady state reached at long injection events

14. NO is Destroyed Via Post-Combustion Product Re-entrainment • NO/CO2 expected to be constant at steady state • NO/CO2 levels begin to fall for both simulator and test engine experiments • From examining the data, it is postulated that: • Products of combustion re-entrained into hydrocarbon rich plume • Nitric oxide destroyed via NO re-burn • Reactions between NO and unburned hydrocarbon radicals • Radicals present in the interior of jet • Initially, formation/destruction is transient

15. Formation of NO during the non-steady combustion event Steady formation and transient destruction of NO during steady state combustion Steady formation and destruction during steady state combustion Three Regimes of Nitric Oxide Formation/Destruction

16. Three Regimes of Nitric Oxide Formation/Destruction

17. Conclusions • A system capable of simulating diesel combustion has been demonstrated • Gas extraction and analysis • Variable mass injection • Two-stage burning process is exhibited by the simulator, similar to those found in engine data • NO formation as a function of CO2 is peaked with the highest value coinciding with the spray interacting with the major heat release

18. Conclusions • Three regimes of formation for overall NO production were hypothesized • Non-constant formation rate associated with the unsteady combustion process associated with light loads • Constant formation rates and transient destruction of NO associated with the steady state combustion of the fuel • Steady production and destruction associated with sufficiently long injection events which allow the combustion event and the NO re-burn to come to steady state

19. Acknowledgements • Biodiesel directed studies supported by NREL, Shaine Tyson, Contract Monitor • Facility development supported by a National Science Foundation Career Award, Dr. Farley Fisher, Contract Monitor • Ongoing research support by NSF, Dr. Farley Fisher, Contract Monitor • Graduate student support, GANN award, DOE • Custom drilling of injector nozzle, Raycon Corporation • CSM contributors to the project • Dr. Tom Grover • Eric Jepsen • Dr. Heather McCann • Jon Filley

20. Extractive Measurements Rely on Capturing the Entire Burned Charge • “Cup” used to capture exhaust gases • Valve timing is used to control the gas quantity extracted • Extract into secondary tank • Mix for 2 minutes • Then fill cell • Monitoring CO2 mass as a function of valve open time shows when entire charge is captured • 4.0 second extraction time used

21. Verification: Carbon Dioxide Mass in Exhaust and Fuel • Fuel mass per injection • Capture fuel and weigh • Knowing fuel composition allows conversion to mass CO2 • Assumes all fuel carbon produces CO2 • CO2 in exhaust reported by FTIR • Similar results for diesel and methyl oleate

22. Characteristic Engine Simulator Injection Pressure 20-80 MPa [6] 20-170 MPa [3] 55-150 MPa Nozzle orifice diameter 0.15-0.35mm [7]0.184mm[6] 0.160 mm Nozzle L/D 2-8 [3], 4[8] 4 Number of nozzle orifices 3-8 holes 1 hole Types of nozzles used Pintle and Hole-type nozzles Hole-type nozzle Mass injection 2-24 mm3 per hole[9] 4 mm3 min 37 mm3 max Chamber Temperature 1000-1200 K [7] 700-1300 K [12] 873 K Chamber Pressure 40-120 atm [7] 50-100 atm [3] 12.5 atm Hard wall interaction length 50-60 mm [10] 100 mm Liquid Length 18 mm[1] 30 mm Chamber Pressure Change 6 MPa [3] 25 kPa Compression Ratio 16-22 [3] None Overall Stoichiometry Fuel Lean [3] Fuel Lean Comparison of Typical Diesel Engine Characteristics with the Simulator

23. Example of AHRR Plot • Flynn, P.F., Hunter, G.L., Durrett, R.P., Farrell, L.A., Akinyemi, W.C., SAE Paper No. 2000-01-1177

24. 2 Stage Ignition • Curran, H.J., P. Gaffuri, W.J. Pitz, and C.K. Westbrook,.Combustion and Flame, 1998, vol. 114(1): p. 149-177. • Griffiths, J.F., P.A. Halford-Maw, and D.J. Rose, Combustion and Flame, 1993, 95(3): p. 291-306. • Chevalier, C., W.J. Pitz, J. Warnatz, and C.K. Westbrook, Twenty-Fourth Symposium (International) on Combustion (1992).