Residual Gas Mixing in Engines

Abstract

The mixing of fresh charge with residual gases was studied in a spark-ignition engine using planar laser-induced fluorescence (PLIF) of a homogenous air/fuel/tracer mixture. An adjustable, dual-overhead cam cylinder head and throttled operation provided a range of elevated residual gas fractions. The bulk residual fraction was measured with a sampling valve and exhaust emissions were recorded for 15 experimental conditions covering two engine speeds and five valve overlap strategies. Residual gas fractions ranged from 24% to 40% at 600 RPM and 21% to 45% at 1200 RPM. Indicated mean effective pressure ranged from 146 kPa to 271 kPa across all conditions, with variability levels consistently below 6%. Calculated heat release confirmed the high dilution levels with universally slow burning rates. A non-intensified CCD camera was used to capture the PLIF signal and operated with a peak signal-to-noise ratio of 21:1. The negative-PLIF imaging technique was verified with a quantitative measure of intake charge homogeneity, and a fuel-cutoff experiment that isolated unwanted fluorescence signal from residuals. Data images were analyzed with first and second statistical moments of pixel intensity, as well as an ensemble PDF curve. All fired conditions showed a clear increase in spatial variation from the homogeneous condition, a trend that was qualitatively verified visually in the corrected data images. Inhomogeneity in the compressed charge increased rapidly above 35% residual gas fraction, independent of engine speed or overlap strategy. The intake cam advance valve overlap strategy was found to provide reduced spatial variation over equivalent symmetric valve overlaps and exhaust cam retard overlaps.