Quantitative Measurements of Residual and Fresh Charge Mixing in a Modern SI Engine Using Spontaneous Raman Scattering 1999-01-1106
Line-imaging of Raman scattered light is used to simultaneously measure the mole fractions of CO2, H2O, N2, O2, and fuel (premixed C3H8) in a modern 4-valve spark-ignition engine operating at idle. The measurement volume consists of 16 adjacent sub-volumes, each 0.27 mm in diameter × 0.91 mm long, giving a total measurement length of 14.56 mm. Measurements are made 3 mm under the centrally-located spark plug, offset 3 mm from the spark plug center towards the exhaust valves. Data are taken in 15 crank angle degree increments starting from top center before the intake stroke (-360 CAD) through top center of the compression stroke (0 CAD). Ensemble-averaged measurements obtained over 200 cycles provide data on the mean level of mixing between fresh charge and combustion residuals, and sets of 500 single-shot, cycle-resolved measurements provide statistics of charge composition fluctuations in the vicinity of the spark plug and permit resolution of mixing scales through analysis of the spatial covariance.
Single-shot data are analyzed over the middle 12 locations (∼11 mm length), where signal levels are high and noise interference is minimal. The statistical cycle-to-cycle variance in the residual mole fraction is observed to be highest at points where the mean spatial and temporal gradients in residual are large as determined from the cycle-averaged data. At time of spark (-15 CAD), however, rms fluctuations in the residual are less than 1%. Analysis of the spatial covariance functions permits assessment of the noise contribution to the measured mole fractions. These spatial correlations also indicate that there are large mixing length scales (∼ a centimeter or more) early in the intake stroke and at bottom center (-180 CAD). The mixing scales decay as the piston compresses the charge, to 2-4 millimeters at time of spark. Thus, the data presented indicate that, at idle, mixing between fresh charge and residuals is not complete at time of spark, though the level of fluctuation in gas composition is small.