Single-shot Raman measurements in diesel spray systems as a tool to differentiate two-stage ignition from single stage i

1. Single-shot Raman measurements in diesel spray systems as a tool to differentiate two-stage ignition from single stage ignition Terry Parker, Chris Dryer, Manfred Geier, Jennifer Labs Engineering Division Colorado School of Mines Presented at American Chemical Society 233rd National Meeting &Exposition March 26, 2007

2. Acknowledgements • NSF, Army ARO, NASA • Graduate student support, GANN award, Dept. of Ed. • Custom drilling of injector nozzle, Raycon Corporation • New injection system, Sturman Industries • CSM contributors to the project • Dr. Tom Grover • Dr. Tony Dean

3. Diesel engines are a critical part of the transportation sector of our economy • Heavy duty hauling relies almost exclusively on the diesel • The diesel provides long engine life and superior fuel economy • Fuel tolerance for the cycle is typically greater than for spark ignition engines • Maintenance for diesels is typically lower than for spark ignition engines • The diesel’s advantages are due to: • Higher compression ratios • The basic heterogeneity of the cycle which provides variable power with no throttling penalty • As implemented today, the diesel is a much dirtier cycle than the spark ignition engine • The Diesel, as implemented, is more efficient (less carbon emissions)

4. Overall emission levels for the diesel are THE driving factor in diesel engine research • The 2010 emissions targets are exceptionally aggressive • Emissions for diesels are harder to control than those for the spark ignition engine • Exhaust after treatment is difficult • Soot is difficult to burn out • Catalytic treatment of exhaust is ineffective • Equivalence ratio range is inappropriate for NO reduction • Catalysts are susceptible to plugging and deactivation • The diesel cycle is plagued by the soot-NOx tradeoff

5. Significant improvement in emissions requires a detailed understanding of diesel combustion • CSM Research focus • Experimental investigations of the fundamentals of diesel combustion • Diesel spray measurements • Post combustion exhaust measurements • Line-of-sight temperature (time resolved) • All have led to a unique view of NO destruction • Critical question is ignition behavior in experiments • Today’s discussion • Experimental capability: apparatus and ignition dilema • Sprays: measurement results • Nitric oxide measurement techniques and results • Temperature measurement results • Raman measurements: resolution to dilema • Conclusions

6. A unique facility is used to mimic the diesel combustion environment • Simulator is a cold-wall pressure vessel with a heated air core • Hard wall interaction simulated with a plate on top of packed bed • 1000 K and 50 atm capability • System includes central air flow and side arm nitrogen flows • limited oxidant used to control pressure rise in system • central air flow used to control temperature gradients (~ 15oC)

7. The simulator provides a controlled environment for diesel engine research • Temperature uniformity was ensured using constant temperature walls and the packed-bed heater • Stainless steel plate acts as a “hard wall” for vapor jet to interact with • Standard operation was at 873 K and 12.5 atm • Maximum: 1173 K, 16.5 atm • Air and nitrogen flows used to maintain temperature and avoid window fouling • Significant differences between an engine and the simulator • Simulator is isobaric • Total pressure for simulator is low • 12.5 atm versus 30 to 100 atm.

8. Injection Start (0.0 ms) Ignition (~2.2 ms) Ignition Occurs During Injection Event: Combusting Spray • Injection End (3.05 ms) Why the Delay

9. Lasers Nd:YAG (1.06mm) and CO2 (9.27mm) lasers are co-aligned and focused to a 150 mm waist Waist size is experimentally verified Scattering measurements 1.06mm at 90° 9.27mm at 10° Extinction measurements at both wavelengths Beam power monitored to compensate for power fluctuations Droplet Size and Volume Fraction Measurements Use a Pair of Infrared Lasers

10. The spray event is exceptionally repeatable Direct comparison of events is possible Data Acquired: Axially every 5 mm from 10mm to 50 mm Radially every 0.3 mm from centerline to spray edge All measurements acquired at 500 kHz for an event Cold spray and reacting spray data Scattering Measurements Provide Spatial Resolution Within the Spray Center Line Axial Position Radial Position

11. Spray Measurements show a jet with a high velocity core and low momentum exterior • Liquid Penetration Length • Predicted* ~32 mm • Measured ~35 mm • Spray Half Angle • Predicted** 2.8-4.3° • Measured ~5.0° *Higgins, B.S., C.J. Mueller, and D.L. Siebers, SAE Paper No. 1999-01-0519. **Wu, K.-J, C.-C. Su, R. L. Steinberger, D. A. Santavicca, and F. V. Bracco, Journal of Fluids Engineering 105:406-413 (1983).

12. Time averaged over 0.1 ms per frame  43 frames Look for: Spray development High volume fraction area visible during steady state Loss of any structure after injection shut-off Combusting Volume Fraction Movie (t = 0  4.3 ms)

13. 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.

14. Post Combustion Measurements: Expected NO/CO2 Trend • 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

15. 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)

16. Line averaged temperatures can be used to monitor the temperature evolution of a system • Temperature measurements rely on emission/extinction of soot

17. Temperature measurements indicate a very similar temperature trend for very different NO levels • Temperatures are very similar • Crudely, this would imply similar NO levels

18. Results indicate that the production of NO in combusting diesel plumes is more complex than simply Zeldovich NO Production Does Not Scale with Temperature

19. Global equilibrium indicates that Diesels should make high levels of NO • Overall diesel combustion is fuel lean but majority of burn is at equivalence ratio 1 • Simple equilibrium indicates that current emission levels should be hard to meet

20. Injection and Combustion Time Scales Overlap: IF we understand ignition • 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

21. The ignition controversy: two stage ignition and the NTC region R-H +O2 => R* +HO2 R* + O2 <=> ROO* • Classic shock tube and RCM work has shown a first and second stage of ignition for lower pressures in the 700 to 900 K range • Two stage ignition:Initial reactions produce radicals and then heat release as intermediate products are formed. The increase in temperature acts to shift the reaction pathways so that radical consumption is enhanced which slows the overall reaction down. Slower reactions rebuild the radical pool and after a time, vigorous reaction occurs. • Second reaction, net rate falls • as T increases, shuts off overall reaction • Observed ignition behavior: • Two stage ignition • Ignition of all “premixed” fuel, mixing delay, diffusion burn

22. Ignition results as a function of pressure are inconclusive • At low pressures, ignition results indicate two-stage ignition • At 12.5 atm., results are inconclusive

23. RAMAN signals are wavelength shifted from the pump beam and are weak • System setup: 100 mJ @ 355 nm, imaging spectrometer with gated intensified camera Wavelength shifts

24. Smoothed Data Raw Signal Corrected for vignetting Smoothed Data Nitrogen Oxygen CO2 Nitrogen (presumed) Nitrogen Oxygen Signals from pressurized cold airand CO2 provide the basis for interpretation • Vignetting correction is applied to spatial dimension • Parabolic smoothing is applied across 39 pixels in in space and 5 pixels in wavelength • Results are as expected and show good quality in a simple well understood field Distance along Beam

25. Cold Hot Nitrogen Carbon Dioxide Oxygen Continued investigation illustrates expected trends at elevated temperatures • At high temperatures (873 K) signal falls as number density scales with inverse T • Nitrogen normalized signals: • subtle spectral changes • identifying temperature via spectral shape probably not viable

26. Probe Region Spray To investigate Ignition, signals must be acquired at appropriate time and spatial position • Times are pre, mid, and post burn • Spatial location is 5 mm from tip and at spray edge • Avoid plasma formation due to laser interaction with spray post-burn Mid-burn Pre-burn

27. Preburn results show a broadband fluoresence that complicates signal processing • Nitrogen and C-H signals clearly identifiable • Broadband fluorescence in regions that we expect to see fuel

28. Smoothed Data Center Edge CO2 O2 CO2 Center edge Elastic scatter CO2 O2 N2 CH stretch Fluorescence? Post Burn Results indicate spatial variation, C-H, and CO2 • Signals at edge are dominated by nitrogen and oxygen • Central regions also include CO2 and CH (unburned HC) • Carbon Dioxide signals are weak C-H Nitrogen

29. CO2 CO C-H CO CO2 CO CO2 Mid burn results indicate “full” combustion • Signals are elastic, CO2, O2, CO, N2, C-H • Central region shows evidence of burning • CO signal reasonably strong • CO and CO2 indicate combustion with final products • First pressure pulse is “true” ignition center edge center edge

30. Ignition results indicate simulator provides “true” ignition during the spray • NO/CO2 expected to be constant at steady state • NO/CO2 levels begin to fall for both simulator and test engine experiments • There is no kinetic pathway to destroy NO in air • 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

31. Conclusions • Raman signals confirm: the observed ignition behavior is not two stage ignition • NO in the simulator does not follow expected trends