Toluene Laser-Induced Fluorescence (LIF) Under Engine-Related Pressures, Temperatures and Oxygen Mole Fractions

Laser-induced fluorescence (LIF) is frequently used for the investigation of mixing processes in internal engine combustion. Toluene is one of the main fluorescing compounds of commercial gasoline. Understanding its fluorescence properties is therefore crucial for the correct interpretation of signal intensities observed under engine (i.e. high temperature and high pressure) conditions. Toluene LIF signal has been investigated as a function of temperature and oxygen concentration in order to enable quantitative fuel tracer imaging. Signal behavior and interpretation for engine-related conditions is demonstrated based on a semi-empirical fluorescence model. Toluene as well as gasoline-LIF is strongly quenched by oxygen. It has therefore been suggested for a direct measurement of fuel/air equivalence ratios. Contrary to this frequently used FARLIF assumption, oxygen quenching is not dominant at elevated temperatures and thus the toluene signal is not proportional to the fuel/air ratio under all the pressure and temperatures conditions in the compression stroke of IC engines. The correct signal interpretation is demonstrated for various practical cases.