Characterization of the Mixing of Fresh Charge with Combustion Residuals Using Laser Raman Scattering with Broadband Detection

Spontaneous Raman scattering with broadband signal collection is used to simultaneously measure the mole fractions of CO₂, H₂O, N₂, O₂, and fuel (C₃H₈) in a spark-ignition engine operating at low load. Both cycle-averaged and single-shot, cycle-resolved measurements of the mixing between residual and fresh charge are made from the beginning of the intake stroke to TDC compression. The measurements are made at twelve locations simultaneously with sub-millimeter spatial precision, which is sufficient to resolve the characteristic scales of inhomogeneity in most cases. Analysis of the spatial covariance functions provides a measure of the noise contribution to the measured mole fractions and, in certain instances, allows the determination of whether the measured composition fluctuations are associated with spatial inhomogeneities or with cyclic variations in overall charge composition.

Results are presented for two different in-cylinder flow fields: a nominally quiescent case and a high-swirl case. The quiescent case is characterized by rapid mixing and low rms fluctuations (∼0.01) in residual fraction at the measurement location at the time of spark, while the swirl case exhibits large amounts of charge stratification and rms fluctuations of approximately 0.05 at the time of spark. Significant skewness in the residual fraction histogram at the time of spark for the swirl case also suggests higher cycle-to-cycle combustion variability and incidence of misfire than would be expected if only the measured rms fluctuations in composition were considered.