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The exciplex fluorescence technique has been used to separately visualize liquid and vapor phase fuel in engines since its development by Melton. However, as a fluorescence technique it has the potential to be quantitative and the underlying assumptions have been outlined by Melton. An initial quantitative application of the TMPD/naphthalene system, based on these assumptions, applied to a hollow-cone spray in a two-stroke engine, indicated that it substantially over-estimates the concentration of fuel vapor about TDC. The reasons for the discrepancy were investigated and it was concluded that a major factor is the effect of temperature on the photophysics of the species involved. Thus the absorption spectra of the exciplex dopants were determined at temperatures up to 700 K. These experiments showed that the increase in absorption with temperature above 500 K is responsible for the failure of the earlier calibration.

Results of experiments performed on a direct-injection two-stroke engine using an air-assisted injector are presented. Pressure measurements in both the engine cylinder and injector body coupled with backlit photographs of the spray provide a qualitative understanding of the spray dynamics from the oscillating poppet system. The temporal evolution of the spatial distribution of both liquid and vapor fuel were measured within the cylinder using the Exciplex technique with a new dopant which is suitable for tracing gasoline. However, a temperature dependence of the vapor phase fluorescence was found that limits the direct quantitative interpretation of the images. Investigation of a number of realizations of the vapor field at a time typical of ignition for a stratified-charge engine shows a high degree of cycle to cycle variability with some cycles exhibiting a high level of charge stratification.