| The temporal and spatial evolution of fuel distribution in an internal combustion engine has considerable influence on combustion characteristics such as ignition, flame stability and emissions. Accompanying the development of modern engine concepts, such as spark-ignition direct-injection (SIDI) engines, laser-based optical diagnostic methods like tracer-based laser-induced fluorescence (LIF) have become effective techniques to investigate in-cylinder processes. In general, the interpretation of LIF images and the accuracy of the results depend heavily on how accurately the tracer can track the parent fuel and how well the LIF signals can be corrected for temperature, pressure and quenching effects. This requires a thorough understanding of how fluorescence tracers behave and how they can be more broadly and effectively used in engine research and development.; This dissertation includes a series of studies focusing on the investigation and development of tracer-based LIF techniques. First, the impact of fluorescence tracers on engine combustion performance and the stability of common tracer/fuel mixtures are addressed. It is found that the addition of 3-pentanone or toluene at a suitable level for LIF measurements to iso-octane does not have a significant impact on engine performance at homogeneous and near stoichiometric conditions. The stability of 3-pentanone/iso-octane and biacetyl/iso-octane mixtures is examined via gas chromatography. The analysis shows that these mixtures are chemically stable at test conditions and preferential evaporation occurs when they are open to atmosphere. Second, this work addresses the application of LIF to SIDI engine research and development. A single-laser two-tracer LIF imaging technique is designed and developed to study the significance of sequential evaporation of a multicomponent fuel in an optical SIDI engine. Acetone and p-xylene are selected to represent the low and high boiling point fractions of gasoline, respectively. Fluorescence signals from the two tracers are simultaneously recorded with one camera. The results show negligible difference between acetone and p-xylene distributions under stratified charge mode, putting droplet evaporation under liquid-phase mass diffusion controlled limit. The same LIF imaging technique with toluene as the single tracer is applied to measure in-cylinder temperature distributions in a motored engine. The measured temperatures compare well with temperatures calculated through thermodynamic analysis but now provide a means for spatially resolved temperature measurements. |
