Quantitative measurements of liquid fuel film on the piston top of an optical-direct-injection engine by laser -induced fluorescence.

Abstract

This dissertation presents the development and application of a tracer-based laser induced fluorescence (LIF) imaging technique for quantitative measurements of piston-top fuel film thickness in an optical spark-ignition direct-injection (SIDI) engine. The effort was mainly focused on selecting a suitable tracer for use with iso-octane and the design of a LIF diagnostic arrangement in an optical SIDI engine. Two potential tracers, 3-pentanone and toluene, were evaluated extensively for their spectral characteristics and evaporation behavior in iso-octane. A dilution technique was adopted to establish complete absorption and emission curves of pure tracers in a calibration flow cell. Appropriate tracer concentrations were then determined based on these curves to ensure the linear dependence of LIF signals on film thickness. The evaporation behavior of selected tracers in iso-octane including air was assessed with simulations that used predictive Soave-Redlich-Kwong (PSRK) equation of state calculations. Quantitative measurements of the liquid fuel film thickness evolution were achieved in an optical SIDI engine with a single-laser, two-camera configuration. Initial investigations have excluded 3-pentanone as a suitable fluorescence tracer for fuel film measurements due to preferential evaporation and background interference. Measurements with low-concentration toluene (3% by volume) have provided good results in characterizing the film evolution. However, the high susceptibility of toluene fluorescence to oxygen quenching has required working in a nitrogen atmosphere. A lens blocking technique was employed to improve the signal-to-noise ratio and interfering signal from the vapor-phase mixture was estimated from measurements using homogeneous fuel/air mixtures. Crank angle resolved evolutions of the fuel film thickness and extension on the piston surface were successfully obtained using toluene as fluorescence tracer. Quantification was obtained by relating the measurements to an in situ calibration at room temperature. The results indicate that the evaporation of the fuel film is strongly dependent on the engine temperature and to less extend the intake air swirl level. Evidence of charge cooling and influence of swirl level on cylinder pressure are discussed.