Development and demonstration of 2D-LIF for studies of mixture preparation in SI engines

Laser-induced fluorescence (LIF) has been developed for visualization of fuel distribution fields in an operating spark-ignition (SI) engine. Since the standard research fuel iso-octane, does not yield a useful LIF signal a fluorescent additive was used. None of the commonly used seeds were found adequate. A seed not commonly used in this context, 3-pentanone, C₂H₅COC₂H₅, was chosen due to favorable vaporization characteristics and fluorescent properties. Results from preparatory investigations in the actual engine environment are presented and related laboratory data are discussed. The two-dimensional LIF technique was applied to a spark-ignition engine and the fuel distribution at the ignition time was recorded. The resulting images were processed and converted into fuel/air equivalence ratio using an in situ calibration technique. The processed fuel distribution maps presented a noise level of ∼ 10% and a systematic error not exceeding 0.03 fuel/air equivalence units. An increased combustion variability was observed when changing from a homogeneous to an inhomogeneous fuel/air mixture. Correlations of image data to the combustion development indicated that the increased cyclic variability could be largely explained by variations in the mean fuel concentration around the spark gap. The initial flame development therefore seems to be controlled by the average amount of fuel near the spark gap, whereas the actual distribution of the fuel within this volume is of less importance.