Exciplex Fluorescence Imaging Technique

An understanding of fuel-air mixing within the combustion chamber is important in the development of efficient combustion in engines and reduced exhaust emissions. The exciplex (Excited State Complex) technique is being refined to allow concentration distributions of liquid and vapor phases of fuel to be imaged during the intake and compression strokes. This serves as a non-intrusive diagnostic tool for fuel mixing/evaporation phenomena prior to ignition. Chemicals, designed with properties similar to automotive fuels, are selected which fluoresce at different wavelengths in each phase (liquid or vapor) when excited by ultra-violet laser light. The fuel is doped with these chemicals and the measured fluorescence intensity of these molecules can then be related to the concentration field of each phase. Extensive development and calibration of this technique is necessary to allow accurate concentration measurements to be made in fuel/air gas mixtures.

THEORY BEHIND EXCIPLEX

The equations describing the mechanism of exciplex (or Monomer) emission fluorescence given in this diagram relate: M, M*, UV, E*, and G. These correspond to ; M - monomer species, M* - excited monomer state, UV - ultra violet laser excitation energy, E* - excited state complex (exciplex) relating to the mixture, and G - ground state reactant of mixture. Upon receiving UV-laser excitation, two different color fluorescence come out. One fluorescence, which is due to the emission from the monomer (M), can track evaporating vapor phase fuel. On the other hand, some portions of the excited monomers react with the ground state reactant (G), and then produce exciplex emission. Emission fluorescence from the exciplex, which has a longer emission band than that of monomer fluorescence, can tag the liquid phase fuel faithfully.

CONCEPT

1,4 : Laser excitation of monomer in liquid or vapor

2 : Exciplex (excited state complex) formation, liquid phase only

3 : Exciplex emission fluorescence occurs in green

5 : Monomer emission fluorescence occurs in blue

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