Detailed Analysis of Mixture and Combustion of Diesel Jets by Laser Induced Fluorescence Techniques * G. Bruneaux Engine System Analysis Department IFP, France The evolution of Diesel engines in the last decades has been driven by the stringent pollution legislation and increas- ing environmental concern. In this context the Diesel engine appears like a very good candidate due to its naturally high efficiency. However, in order to further reduce engine out pollutant and green house gases to the required limits, a detailed study of the fuel-air mixing and combustion processes is necessary. An investigation of those processes by Laser Induced Fluorescence (LIF) techniques is presented. The mixing process is studied by tracer LIF, enabling a statistical analysis of the mixture structure and of the effect of injection parameters. The combustion process is stu- died by simultaneous formaldehyde, Poly-Aromatic-Hydrocarbons (PAH) and OH LIF, giving access to detailed in- formation on the combustion process. A strong coupling is observed between the two processes. Introduction mass concentration field is illustrated in Figure 1 High Pressure Direct Injection (HPDI) has es- for a set of injection conditions. tablished itself as a proven technology which al- lows the efficient reduction of engine-out emissions in DI Diesel engines. But although HPDI is becom- ing a standard, the consequences of this new technology on mixing and combustion processes inside the combustion chamber have yet to be further understood because the physical pheno- mena involved are extremely complex, in particular since the two processes occur on overlapping time scales and therefore a strong coupling is happen- ing. For a detailed investigation of the physical processes occurring in the combustion chamber, optical diagnostics are the best candidates since they are the only way to obtain two dimensional quantitative measurements inside the combustion chamber. Various techniques are available for the F study of mixing[1,2] and combustion processes[3], [kg/m3] each one having its advantages and limitations. Figure 1 : Schematics of tracer LIF images configura- Among them, Laser Induced Fluorescence (LIF) is tion and palette connecting image colors and fuel mass a complete and powerful tool giving access to de- concentrations. tailed qualitative and quantitative information on 2.5 the mixing and combustion processes. Examples of application of these techniques are presented 2 here. Volume histogram, [] A A 1.5 B Mixing process Tracer LIF is appropriate for the study of the 1 mixing process since it can provide two- dimensional quantitative information appropriate 0.5 for statistical analysis. The first step of the applica- tion of tracer LIF to the measurement of fuel distri- B bution in the jet is to develop a method to obtain 0 0 1 2 3 4 5 normalized fuel mass concentration fields. In ref.[4] Vapor fuel mass concentration, [kg/m3] such a methodology was developed based on the knowledge of the injected mass at a given instant Figure 2 : Comparison of statistical fuel distribution and taking into account laser sheet profile shape, (volume histograms) obtained in the mixing zone (A) and beam steering effect corrections, and the effect of the stagnation zone (B) (left), and illustration of the zone temperature on the LIF signal. The resulting fuel locations (right). * Corresponding author: gilles.bruneaux@ifp.fr Towards Clean Diesel Engines, TCDE2009 A statistical analysis of the mixture fields ob- The results were then synthesized by a concep- tained by tracer LIF was carried out and is illu- tual model of Diesel jet combustion illustrated in strated in Figure 2. It showed that the Diesel jet Figure 5. mixture can be separated in two distinctive zones: One important result that came out of those in- the mixing zone on the upstream sides, where air vestigations is the clear connection between the entrainment due to shear turbulence dominates the mixture and combustion structures, as well as for jet dynamic, and the stagnation zone at the tip the transition from auto-ignition to diffusion limited where the jet pushes away the dense surrounding combustion than for the structure of the diffusion gases. The latter is characterized by a lower mix- flame itself. ing rate since small scales turbulence are missing. Such analysis was then carried out to investi- gate the effect on the mixing process of different injection parameters such as injection pressure (illustrated in Figure 3), nozzle hole diameter, injec- tion duration. Figure 4: Simultaneous OH and 355 LIF images dur- ing different stages of the diffusion-limited combustion phase. OH LIF in gray, formaldehyde and PAH LIF in white. Pinj=1000bar Pinj=1500bar Pinj=2000bar Figure 3: Comparison of individual vapor fuel mass concentration fields obtained by LIF for different injection pressures. Combustion process The combustion of Diesel jets is a complex process involving different steps from auto-ignition to the stabilization of a diffusion flame, and differ- ent regimes of combustion (premixed, diffusion, soot formation...). In order to carry out a detailed analysis of the different combustion regions, a simultaneous for- Figure 5:Schematic for conceptual model of Diesel maldehyde, PAH and OH LIF technique was de- jet combustion. veloped and applied to the Diesel jet configuration [5]. The technique allows to obtain simultaneous Perspectives information on the different regions of the combus- The study of mixture and combustion processes tion zone: the low temperature zones with formal- of Diesel jets by LIF clearly brought up the advan- dehyde, the fuel-rich high temperature zones with tage of using such techniques for this kind of com- PAH and the fuel-lean high temperature zones with plex investigation. Furthermore, this kind of re- OH. search can clearly benefit in the future from the Figure 4 shows a typical combined image ob- improvement of LIF technique. Indeed in recent tained with this technique during the diffusion li- years, a lot of efforts has been made to obtain mited combustion. A detailed analysis of the struc- more quantitative information with LIF[6], in par- ture of those images was carried out in order to ticular for measurements of temperature. identify the different regions of combustion and Also, it will be interesting in the future to use the understand the evolution of the process, in particu- LIF techniques presented here to study more com- lar the transition from premixed auto-ignition to plex configurations. For instance multiple injection diffusion controlled combustion. strategies are a promising technology to further reduce engine out emission, and a more detailed analysis of the physical phenomena involved dur- ing those strategies will certainly help the optimiza- tion process of Diesel combustion systems towards cleaner engines. References 1 Espey, C., Dec, J. E., Litzinger, T. A., Santavic- ca, D. A., Quantitative 2-D Fuel Vapor Concen- tration Imaging in a Firing D.I. Diesel Engine Using Planar Laser-Induced Rayleigh Scatter- ing, SAE 940682, 1994. 2 Bruneaux, G., Liquid and Vapor Spray Structure in High Pressure Common Rail Diesel Injection, Atomization and Sprays, vol 11, issue 5, pp533- 556, 2001. 3 Dec, J., A conceptual Model of DI Diesel Com- bustion Based on Laser-Sheet Imaging, SAE paper 970873, 1997. 4 Bruneaux, G., Mixing Process in High Pressure Diesel Jets by Normalized Laser Induced Excip- lex Fluorescence - Part I: Free Jet, SAE 2005- 01-2100, 2005. 5 Bruneaux G., Combustion Structure of Free and Wall Impinging Diesel Jets by Simultaneous Laser Induced Fluorescence of Formaldehyde, PAH and OH, International Journal of Engine Research, volume 9, issue 3, 2008. 6 Schulz, C., Sick, V., Tracer-LIF diagnostics: Quantitative measurement of fuel concentra- tion, temperature and air/fuel ratio in practical combustion situations, Prog. Energy Combust Sci. 31, 75-121 (2005).