Fluorescence characteristics of organic tracer molecules for planar laser-induced fluorescence studies in internal combustion engines, part A: non-aromatics

Tracer based planar laser-induced fluorescence (PLIF) has emerged as a powerful in-situ measurement technique with a considerable spatial and temporal resolution for Internal combustion (IC) engines. In PLIF, the emitted fluorescence signals from a tracer molecule are processed to determine distribution of temperature, fuel, residual gases etc. However, it is imperative to have a thorough understanding of the tracer physical properties and its fluorescence intensity dependencies on excitation wavelength, pressure, temperature, and bath gas composition existing inside the combustor for accurate quantitative interpretation. This work consists of a series of two articles providing a detailed review of the existing literature of fluorescence characteristics of various molecules used as tracers in IC engine applications. Due to the overwhelming usage of organic compounds in IC engine environment, the work is restricted to them. Part A of this work is focussed on non-aromatic compounds (acetone, 3-pentanone and biacetyl) whereas part B will focus on aromatics. Due to a small energy gap between the excited singlet and triplet states of ketones, they experience rapid inter-system crossing making them far less sensitive to oxygen quenching effects than aromatic molecules. Addition of tracers to surrogate fuel can lead to difficulties related to co-evaporation, azeotrope formation and stability of tracer molecules in terms of photolysis and pyrolysis effects when subjected to intense laser irradiation and harsh engine environment. In this work, fluorescence signal variation of tracer molecules is divided into variations in absorption cross-section and fluorescence quantum yield (FQY). Absorption cross-section normally increases with temperature but is insensitive to pressure changes. FQY reduces with increase in temperature but increases with pressure for ketones for non-oxygen containing bath gases. The pressure sensitivity increases with the number of atoms in a collider molecule. FQY values decrease with decreasing laser excitation wavelength whereas the temperature and pressure sensitivity of FQY reduce with increasing wavelengths. For simultaneous high pressure and temperature conditions, the pressure sensitivity of FQY is found to reduce due to a reduction in the effective number of collisions with bath gas molecules. Among the three tracers, acetone has been widely used for marking gaseous fuels and 3-pentanone and biacetyl for liquid fuels like iso-octane. Acetone and 3-pentanone have received significant attention for fluorescence studies due to their widespread usage in IC engine applications. Biacetyl on the other hand has recently started to receive attention due to its application in high repetition rates PLIF measurements and requires more fluorescence studies to fully characterise its fluorescence behaviour and construct fluorescence models over the complete pressure and temperature range required in IC engine applications.