Fluorescence characteristics of organic tracer molecules for planar laser-induced fluorescence in internal combustion engines, Part B: 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 whereas part B will focus on aromatics (toluene, anisole, naphthalene, 1-methylnaphthalene and fluoranthene). Due to a large energy gap between the excited singlet and triplet states of aromatics, they are highly sensitive to oxygen quenching effects than ketones. Absorption cross-section might increase or decrease with temperature but is insensitive to pressure changes. Fluorescence quantum yield of aromatics show a very strong reduction with increase in temperature but might either increase or decrease with increasing pressure. The pressure sensitivity is found to increase with the number of atoms in a bath gas molecule. Fluorescence spectra are found to undergo redshift with temperature which can be used to measure temperature using 2 colour thermometry. The large fluorescence quenching by oxygen can also be used to directly measure fuel–air ratio using FARLIF methodology. Towards the end several IC engine studies are reviewed to discuss various aspects of mixture formation and temperature distribution.