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, C,H,COC,H,, 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 w 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.

Investigations concerning the potential for the visualization of water vapor in combustion processes have been made. The water molecules were excited through a two-photon excitation process at 248 nm with a tunable excimer laser; this was followed by fluorescence detection between ~ 400 and 500 nm. In the experimental work special care was taken to map the possible spectral interferences from hot O(2), which also absorbs in the same spectral region and which produces fluorescence emission that interferes with the water fluorescence. Experimental investigations of high-pressure applications are also presented.

Finally, two-dimensional (2-D) measurements made at room temperature, taken in an atmosphericpressure flame, and taken in an engine simulator at elevated pressure are presented. These results indicate that the detection limit for 2-D single-shot registrations under optimized experimental conditions was estimated to 0.2% at atmospheric pressure and at room temperature. Extrapolations to flame conditions are also presented.

To study the effect of different injection timings on the charge inhomogeneity, planar laser-induced fluorescence (PLIF) was applied to an operating engine. Quantitative images of the fuel distribution within the engine were obtained. Since the fuel used, iso-octane, does not fluoresce, a dopant was required. Three-pentanone was found to have vapour pressure characteristics similar to those of iso-octane as well as low absorption and suitable spectral properties.A worst case estimation of the total accuracy from the PLIF images gives a maximum error of 0.03 in equivalence ratio. The results show that an early injection timing gives a higher degree of charge inhomogeneity close to the spark plug. It is also shown that charge inhomogeneity gives a more unstable engine operation. A correlation was noted between the combustion on a cycle to cycle basis and the average fuel concentration within a circular area close to the spark plug center. The correlation coefficient when a second order polynomial was fitted reached a value of -0.8 when the engine operated with a high degree of inhomogeneity. The highest correlation coefficient between the duration of 0-0.5% heat released and the average fuel concentration was obtained within a radius of approximately 5-10 mm from the spark plug. When the standard disc-shaped combustion chamber was replaced with a turbulence generating geometry, the inhomogeneity of the charge became very low, independent of injection timing. No cycle to cycle correlations between fuel/air equivalence ratio and combustion were then noted.

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Optical characteristics of dimethyl ether (DME) are presented, with emphasis on laser-based combustion diagnostics. OME is a well-known substance which has excellent properties as fuel for compression ignition (CI) engines. It is also believed to have suitable properties for laser diagnostics in CI engines, but reports of its optical properties are sparse in the literature. OM E has therefore been investigated by flame-emission, optical absorption, laser-induced fluorescence (UF), Raman spectroscopy, and rotational CARS. A preliminary evaluation of the potential for measuring NO in a OME flame is also presented. The Raman cross section of OME is more than twice as large as that of methane. DME absorbs in the VUV, but one absorption band extends into the UV where many tunable lasers radiate. This tail is displaced towards longer wavelengths with increasing temperature. Excitation at 193 nm yields a structured fluorescence between 350-550nm. The DME rotational CARS signal is -10 times weaker than that of nitrogen, and the non-resonant susceptibility is 9 times that of nitrogen.