Toluene LIF at elevated temperatures: implications for fuel-air ratio measurements

Toluene laser-induced fluorescence (LIF) was investigated for 266- and 248-nm excitation in the temperature range of 300 - 650 K in a nitrogen/oxygen bath gas of 1 bar total pressure with oxygen partial pressure in the range 0 - 400 mbar. Contrary to a popular assumption, the toluene LIF signal is not directly proportional to the fuel - air ratio ( termed the FAR-LIF assumption) for many conditions relevant to reciprocating IC engines. With increasing temperature, a higher oxygen partial pressure is required to justify the FAR-LIF assumption. The required oxygen pressure becomes unrealistic (> 5 bar) for T> 670 K at 266-nm excitation and for T> 625 K at 248-nm excitation.