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Verification and Application of Fuel-Air-Ratio-LIF

These investigations aim to validate the applicability of a concept for fuel-air-ratio measurements by laser-induced fluorescence (FARLIF) at elevated temperatures. For the commonly used model fuel isooctane with the fluorescence tracer toluene the FARLIF applicability was confirmed with an excitation wavelength of 266 nm for fuel-air mixtures with λ ≥ 0.2 at pressures above 1.5 bar and temperatures at least up to 433 K. The FARLIF-concept with 266 nm excitation was validated as well for a multi component fuel (nearstandard gasoline) at λ ≥ 0.1, pressures above 1 bar and temperatures at least up to 500 K. To the authors’ knowledge this is the first time that FARLIF could be validated for an auto-fluorescent multi-component fuel without any additional tracer. For both fuels, the mixture temperature affects the FARLIF-intensities. Therefore, isothermal conditions are best suited for FARLIF-investigations; otherwise the temperature has to be known for signal correction. The application of planar FARLIF-imaging is successfully demonstrated for both considered fuels. In both cases signal strengths were sufficient for quantitative single-shot fuel-air-ratio measurements in transient mixing flows under engine relevant conditions. For long excitation light paths through high fuel densities, absorption shows to affect the measurement and FARLIF-images need to be corrected. Finally, first results of double-pulse FARLIF-imaging are presented, giving insight into dynamic mixture processes. In general, the FARLIF-concept shows to be a powerful tool with multifunctional applications if used in its valid constraints and – if necessary – with appropriate corrections.