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

Abstract

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.

Laser diagnostics and minor species detection in combustion using resonant four-wave mixing

“…A range of stable species and radicals have been detected in this way including NO [118,182], NO 2 [181,187,198,206,207], OH [175,208,209] [197], CH 3 OH [176] and C 3 H 8 [212,213] have also been detected but in combustion situations these are usually present in relatively large concentrations. Nonetheless it may be useful to consider LIGS for detection of fuel compounds as they may be present as unburned hydrocarbons in exhaust gases.…”

“…By additionally applying Raman spectroscopy Weikl et al [286] simultaneously determined mixture composition and gas temperature in gaseous and liquid propane injection systems. In the same propane gas injection process Seeger et al [212,213] used laser-induced gratings to measure the local equivalence ratio as a function of time after the start of injection. At high fuel concentration the grating oscillation period was utilized while at low propane concentration the ratio between electrostrictive and thermal gratings yielded results with good accuracy.…”

Advanced Laser-Based Techniques for Gas-Phase Diagnostics in Combustion and Aerospace Engineering

“…The LIGS signal is very rich in information and can provide measurements of the temperature, 168 Compared to CARS, laser-induced grating methods have been applied to combustion-related systems to a very limited extent. Seeger et al 170,177 measured the local equivalence ratio as a function of time after the start of injection in a propane gas injection process. At high fuel concentration, the grating oscillation period was utilized to derive composition information, while at low propane concentration the ratio between electrostrictive and thermal signal contributions was the measure of choice.…”

Recent Advances in the Characterization of Gaseous and Liquid Fuels by Vibrational Spectroscopy

“…If this is done for all species with respect to the same reference species, all the relative concentrations add up to 1. Hence, absolute concentration values can be derived [35]. The ratio method can even be used when there are overlapping peaks.…”