Planar laser-induced fluorescence of H₂O to study the influence of residual gases on cycle-to-cycle variations in SI engines

A tunable KrF excimer laser operating at 248 nm is used for visualization of water vapor (H₂O) at engine-like conditions by two-photon laser-induced fluorescence (LIF). The effects of two-photon 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.