Measurement of temperature, fuel concentration and equivalence ratio fields using tracer LIF in IC engine combustion

A technique based on planar laser-induced fluorescence of 3-pentanone, for measurements of absolute concentration, temperature and fuel/air equivalence ratios in turbulent, high-pressure combustion systems such as an internal combustion engine is presented. Quasi-simultaneous excitation with 248 nm and 308 nm of 3-pentanone that is used as a fluorescence tracer doped to iso-octane, yields pairs of strongly temperature-dependent fluorescence images. Previous investigations have resulted in information on temperature and pressure dependence of absorption cross-sections and fluorescence quantum yields. Using these data the ratio of corresponding fluorescence images can be converted to temperature images. Instantaneous temperature distribution fields in the compression stroke and in the unburned end-gas of an SI engine were measured. The temperature fields obtained from the two-line technique are used to correct the original tracer-LIF images in order to evaluate quantitative fuel distributions in terms of number densities and fuel/air equivalence ratio. PACS: 07.20.Dt; 42.62.Fi; 33.50.Dq Knowledge of the spatial distribution of temperature and local fuel/air equivalence ratios in the combustion chamber in IC engines prior to ignition is of major interest when modeling engine combustion and modifying combustion chamber geometries. Especially in modern engines with stratified load and systems with exhaust gas recirculation that both strongly influence ignition and flame development, temperature and fuel concentration inhomogeneities are present. For quantitative measurements of fuel vapor concentrations ketones such as 3-pentanone are frequently used because their evaporation properties are similar to those of common model fuels such as iso-octane [1, 2]. Furthermore, the influence of collisional quenching mainly by molecular oxygen is much reDedicated to Professor Jürgen Wolfrum on the occasion of his 60th birthday duced compared to aromatic compounds since the lifetime of excited states is controlled by rapid intersystem crossing. 3-pentanone possesses an absorption feature between 220 and 340 nm with the peak near 280 nm at room temperature [3]. The absorption spectrum exhibits a temperature-induced shift towards longer wavelengths of about 10 nm per increase of 100 K (Fig. 1). Upon excitation in this region, fluorescence is emitted between 330 and 550 nm, with a spectral distribution almost independent of the absorbed wavelength. This spectral shift of the absorption, albeit undesired for concentration measurements can be used for measuring temperature, for example when 3-pentanone is seeded to non-fluorescing model fuels, as the fluorescence intensity is a function of the absorption coefficient for a given excitation wavelength, and thus, of temperature. After excitation at two different wavelengths the ratio of the fluorescence signal intensities reflects the local temperature. This was first described by Großmann et al. [3] and later applied to temperature measurements using acetone Fig. 1. Temperature shift of the absorption band of 3-pentanone. Wavelengths accessible with excimer laser systems are marked (λ1: KrF, λ2: Raman-shifted KrF (first Stokes line in hydrogen), λ3: XeCl) ([18])