Planar Laser-Induced Fluorescence of H₂0 to Study the Influence of Residual Gases on Cycle-to-Cycle Variations in SI Engines

Abstract

A tunable KrF excimer laser operating at 248nm is used for visualization of water vapor (H 2 0 ) at engine-like conditions by two-photon laser-induced fluorescence (LIF). The effects of twophoton absorption in water, laser sheet focusing and two-dimensional fluorescence imaging conditions are discussed, and an estimation of the reliability of the image data is given. The water vapor content measured close to the spark plug prior to ignition in a high-pressure combustion cell is correlated to the ensuing combustion rate, which is determined from the evolving pressure during combustion. The combustion rate is shown to be dependent on the water vapor concentration close to the spark plug prior to ignition, where a high water concentration yields a slow combustion. Using two-photon LIF of water vapor, the influence of residual gases on cycle-to-cycle variations is also investigated in a running spark-ignition (SI) engine. With a low inlet manifold pressure, a correlation coefficient of 0.6 is found between the combustion rate and the residual gas concentration close to the spark plug at the time of ignition. Further, a fast combustion in one cycle is found to yield more residual gases in the subsequent cycle.

CO_2 imaging with saturated planar laser-induced vibrational fluorescence

“…These studies have included imaging of CO at isothermal, room-temperature conditions, 1 imaging of CO and CO 2 in flames, 2 and a computational analysis of linear IR PLIF excitation schemes for CO and CO 2 . 3 Because IR PLIF measurements can generate high signal levels that scale linearly with both laser energy and species concentration, IR PLIF demonstrates advantages over Raman 4 and multiphoton PLIF [5][6][7] techniques to image species with inconvenient or inaccessible electronic spectra. As an example, IR PLIF is unique in that it provides the ability to image nascent CO 2 in combustion systems on a single-shot basis; this can be employed to identify combustion product regions or to measure mixing between postcombustion gases and ambient air or fuel-air charge.…”

Imaging of CO and CO2 using infrared planar laser-induced fluorescence

“…Raman is linear and easily modeled but generates weak signals and can be limited in implementation by a variety of spectral interferences; multiphoton LIF, despite its usefulness for point measurements [5] and imaging [6] of CO in flames, suffers (for most species, such as H 2 O [7]) from strong nonlinearity and low signal due to the predissociative nature of the excited state. In contrast to Raman and multiphoton techniques, IR PLIF is a linear technique that offers higher signal levels than Raman (several orders more signal photons) and is applicable to all IR-active species including CH 4 , CO 2 , and H 2 O.…”