Development of laser-induced fluorescence to quantify in-cylinder fuel wall films

Abstract

Laser-induced fluorescence of a fuel tracer is a very sensitive technique to image in-cylinder liquid fuel films, but quantification of the measured film thickness has proven difficult so far. This article describes improvements in the quantification procedure and presents an example application in a motored, optically accessible spark-ignition engine with direct injection. We designed a calibration tool that could be pressurized and heated, allowing investigation of the laser-induced fluorescence intensities at temperatures exceeding the liquid's standard-pressure boiling point. The fluorescence intensity of liquid toluene and 3-Pentanone dissolved in isooctane upon excitation with a pulsed laser at 266 nm was investigated as a function of temperature and pressure. Consistent with the literature results on gas-phase laser-induced fluorescence, the signal from toluene was much stronger than from 3-Pentanone, about two orders of magnitude for films thinner than 50 mm. Laser-induced fluorescence from both tracers decreased with increasing temperature but that of toluene significantly more. The response to pressure was less pronounced. For imaging across a large field of view, the spatial non-uniformity of laser excitation and detection efficiency was taken into account using a solid fluorescing substrate, an inexpensive Schott-glass WG280 filter. Isooctane with 0.5 vol.% toluene was used for application in the motored engine, imaging the liquid film on the piston-top window after direct injection from a central multi-hole injector. Air as a bulk gas was found to be advantageous over nitrogen in that gas-phase fluorescence was quenched by oxygen. The imaged film distributions and thicknesses and the derived total fuel film mass were physically plausible. Consistent with the recent literature results from a constant pressure vessel, increasing injection pressure from 50 to 100 bar did not decrease wall wetting but further increase to 200 bar did.

Large Eddy Simulations and Tracer-LIF Diagnostics of Wall Film Dynamics in an Optically Accessible GDI Research Engine

“…f and thus a are a function of environmental conditions. In particular, f decreases with increasing temperature [37,38,39]. In our experiments, we estimate this to cause a systematic error of up to 35% in magnitude, such that the measurement always tends to underestimate the real film thickness.…”

“…The following section gives a brief overview of the fluorescence technique employed to image liquid-film thickness. Further details on the experimental technique and some results can be found in [37,38,39].…”

“…LIF has been previously applied in engines, e.g. to detect liquid fuel inside the intake manifold of engines with port fuel injection [41,42] or within the combustion chamber of engines with GDI [37,43,44,45,38]. Fuelfilm thickness measurements by LIF were also done in constant volume chambers at engine-like conditions [46,47,48,49].…”

“…Since many factors influence the fluorescence signal, in particular, temperature, the tracer has to be well-characterized with respect to its photo-physical properties. For the case of toluene, details can be found in [37,38,39].…”

Imaging of Fuel-Film Evaporation and Combustion in a Direct-Injection Model Experiment

“…They found that the film temperature follows the piston temperature quite well during the cycle. Quantitative LIF fuel-film imaging on the piston window of an optical DISI engine was recently performed by Geiler et al [20]. An important outcome was that, when performing LIF fuel-film imaging, the interfering LIF signal from the gas phase is suppressed by quenching by oxygen.…”