Two-photon nitric oxide laser-induced fluorescence measurements in a diesel engine

A two-photon nitric oxide (NO) laser-induced fluorescence (LIF) technique was developed and applied to study in-cylinder diesel combustion. The technique prevents many problems associated with in-cylinder, single-photon NO planar-laser-induced fluorescence measurements, including fluorescence interference from the Schumann-Runge bands of hot O₂, absorption of a UV excitation beam by in-cylinder gases, and difficulty in rejecting scattered laser light while simultaneously attempting to maximize fluorescence signal collection. Verification that the signal resulted from NO was provided by tuning of the laser to a vibrational off-resonance wavelength that showed near-zero signal levels, which resulted from either fluorescence or interference at in-cylinder pressures of as much as 20 bar. The two-photon NO LIF signal showed good qualitative agreement with NO exhaust-gas measurements obtained over a wide range of engine loads.