Examination of Iso-octane/Ketone Mixtures for Quantitative LIF Measurements in a DISI Engine

Mixtures of low concentrations of 3-pentanone in iso-octane are used widely in an attempt to obtain quantitative measurements of fuel equivalence ratio in direct-injection, spark-ignition engines. Despite similar boiling temperatures and heats of vaporization, 3-pentanone has been found to evaporate from the mixture more rapidly than the iso-octane. Thus, the signal detected by the planar laser-induced fluorescence (PLIF) diagnostic cannot accurately represent fuel distribution during spray evaporation and air-fuel mixing in an engine. Using an evaporation chamber, we demonstrate the non-steady PLIF signal of the iso-octane/3-pentanone mixture during steady evaporation. Significant improvement in the consistency of the PLIF signal during evaporation is achieved by adding a heavier ketone (3-hexanone) tracer to compensate for the early depletion of the 3-pentanone. Use of the modified fuel-tracer mixture is demonstrated during quantitative equivalence ratio measurements in an optically accessible DISI engine, and the result is compared to that obtained using the iso-octane/3-pentanone mixture.