Laser diagnostics in Combustion: Temperature Imaging in a two-stroke Engine
Knowledge of the spatial distribution of temperature in the combustion chamber of IC engines prior to ignition is crucial when modeling ignition and flame development. Inhomogeneous mixing, which causes inhomogeneous temperature distributions, occurs particularly in ultra-lean burning modern engines with stratified. 2D temperature distributions between 300 and 1000 K relevant to pre-combustion conditions, can hardly be assessed with laser spectroscopic imaging techniques developed for two-dimensional combustion thermometry so far. A two-line scheme based on laser induced fluorescence of ketones can be used for this purpose.
Figure 1 (left): Temperature dependence of the absorption spectrum of 3-pentanone.
Figure 1 (right): Temperature dependence of the ratio of fluorescence intensities upon excitation at two different wavelengths.
Figure 1 shows the systematic temperature-dependent shift of the absorption spectra of 3-pentanone which is taken advantage of with two-line excitation. The ratio of fluorescence signals upon excitation at 248 nm and 308 nm is strongly temperature dependent (right figure).
Figure 2: Temperature imaging in the unburned endgas of an optically accessible two stroke engine. Black areas represent burned gas areas.
Measurements in the optically accessible combustion chamber of a two-stroke engine fuelled with non-fluorescing iso-octane and 3-pentanone as tracer have shown the great potential of this technique. Two excimer laser / ICCD camera systems were used to excite 3-pentanone at 248 nm and 308 nm respectively and detect the resulting fluorescence. The temperature-dependent ratio of the LIF signals was processed to yield 2D temperature distributions within the unburned endgas region (figure 2). The black areas represent burned gases.
The same signals that are used for temperature measurements also yield information about the local fuel concentration which also has a significant influence on ignition and combustion. Any temperature effect on these measurements can be accounted for using the simultaneously obtained temperature fields. [3,4]
 F. Großmann, P. B. Monkhouse, M. Ridder, V. Sick, J. Wolfrum, Temperature and Pressure Dependences of the Fluorescence of Gas-Phase Acetone and 3-Pentanone, Appl. Phys. B 62, 249-253 (1996).
 S. Einecke, C. Schulz, V. Sick, Dual-wavelength temperature measurements in an IC engine using 3-pentanone laser-induced fluorescence, Laser Application to Chemical and Environmental Analysis VI OSA Technical Digest Series, Volume 3 (Optical Society of America, Washington DC), 84-86 (1998).
 S. Einecke, C. Schulz, V. Sick, R. Schießl, A. Dreizler, U. Maas, Two-dimensional temperature measurements in the compression stroke of a SI engine using two-line tracer LIF, SAE paper No. 982468, 1998 SAE Transactions, Vol. 107, Journal of Fuels & Lubricants, pp. 1060-1068.
 S. Einecke, C. Schulz, C. Sick, Measurement of temperature and equivalence ratio distribution using tracer LIF in IC engine combustion, Appl. Phys. B 71, 717-723 (2000).
More techniques for temperature imaging with tracers:
 M. Luong, W. Koban, and C. Schulz, "Novel strategies for imaging temperature distribution using toluene LIF," in International Conference on Laser Diagnostics, ICOLAD2005 (London, 2005), 155-161.
 W. Koban, J. D. Koch, V. Sick, N. Wermuth, R. K. Hanson, and C. Schulz, "Predicting LIF signal strength for toluene and 3-pentanone under engine-related temperature and pressure conditions," Proc. Combust. Inst. 30, 1545-1553 (2005).
 F. Zimmermann, W. Koban, and C. Schulz, "Temperature diagnostics using laser-induced fluorescence (LIF) of toluene," in Laser Applications to Chemical Security and Environmental Analysis 2006 Technical Digest (2006), TuB4.
 W. Koban and C. Schulz, "Toluene as a tracer for fuel, temperature and oxygen concentrations," SAE technical paper series 2005-01-2091 (2005).
 W. Koban, J. D. Koch, R. K. Hanson, and C. Schulz, "Toluene LIF at elevated temperatures: Implications for fuel/air ratio measurements," Appl. Phys. B 80, 147-150 (2005).
 T. Fuyuto, H. Kronemayer, B. Lewerich, W. Koban, K. Akihama, and C. Schulz, "Laser-based temperature imaging close to surfaces with toluene and NO-LIF," in International Conference on Laser Diagnostics, ICOLAD2005 (London, 2005), 53-61.