Real-time calibration of planar laser-induced fluorescence air-fuel ratio measurements in combustion environments using in situ Raman scattering

A laser-based technique for recording absolute air-fuel ratio maps in a single-laser pulse in combustion devices, for instance, prior to ignition in internal combustion engines, is demonstrated. The method is based on an in situ pulse-to-pulse calibration of two-dimensional laser-induced fluorescence (LIF) images of relative fuel distributions by a point Raman measurement. A single excimer laser pulse is used to quasisimultaneously detect planar LIF from a fuel tracer as well as spectrally resolved Raman scattering from oxygen and fuel in one point inside the LIF image plane. By determining the air-fuel ratio in this particular point from the measured Raman signals, the whole LIF image can be scaled, and quantitative two-dimensional air-fuel ratio data can be obtained. In the method, it is crucial to achieve a Raman signal that can be spectrally separated from the much stronger LIF signal. Therefore, the behavior of the Raman signals from air and fuels, like iso-octane and methane, was studied in a cell as well as in a four-stroke spark ignition engine. Especially, spectral interferences from commonly used fluorescent additives (3-pentanone and acetone) with the Raman signals were investigated. Moreover, possibilities for suppressing the background originating from fluorescent tracers by taking advantage of the polarization characteristics of Raman scattering is briefly addressed. Finally, demonstration measurements of the calibration technique are presented, and the precision and accuracy of the method are shortly discussed.