Local fuel concentration measurements for mixture formation diagnostics using diffraction by laser-induced gratings in comparison to spontaneous Raman scattering

The potential of the laser-induced gratings (LIGs) technique for the determination of fuel concentrations is investigated in comparison to the well-established spontaneous Raman scattering (SRS) technique. At pulse-repetitive excitation, the temporal shape of the LIG signals, containing non-resonant electrostrictive and resonant thermal contributions, as well as the Raman spectra were recorded in single-shot and accumulation modes. The measurements were performed in a laboratory system: in mixtures of propane as a fuel with nitrogen and air under stationary conditions in order to provide calibration data and to investigate the single-shot precision of measurements; and in the course of pulsed injection of gaseous propane into air at elevated pressures and temperatures. In the latter case, parameters of the experiment such as temperature, amount of injected gas and pressure were adjusted to be close to conditions of practical interest existing in real engines with direct injection (DI). The temporal variation of local propane concentration was tracked after the start of the injection pulse. Various approaches to the fast evaluation of the temporal shape of the measured LIG signals were tested, and their applicability in different ranges of propane concentrations, as well as single-shot measurement precision, were investigated. The results of employing the LIG technique were compared to those provided by the evaluation of the ratio of the integrated line intensities in the recorded SRS spectra. Copyright (c) 2008 John Wiley & Sons, Ltd.