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The combustion process in high-speed direct-injection diesel engines is characterized by random turbulent mixing between turbulent eddies having different fuel concentrations. Nitric oxide and soot are formed in hot eddies and fuel-rich eddies. In the present study, the authors elucidate the diesel combustion process, from the viewpoint of such heterogeneity and turbulent mixing, by analysis of high-speed flame photographs. Based on this study the following points are suggested: jet-like flames are formed just after ignition but soon disintegrate into random turbulent flamelets as each flame quickly expands. In the middle and later stages of combustion, uniform and isotropic turbulent motions prevail over the entire space, gradually decaying with time. Such turbulent motions favor the destruction of fuel concentration heterogeneity. Gas expansion due to combustion enhances such random motions, and the swirl prevents their early decay in every burning stage.

A laser homodyne technique is applied to measure turbulence intensities and spatial scales during compression and expansion strokes in a non-fired engine. By using this techinique, relative fluid motion in a turbulent flow is detected directly without cyclic variation biases caused by fluctuation in the main flow. Experiments are performed at different engine speeds, compression ratios, and induction swirl ratios. In no-swirl cases the turbulence field near the compression end is almost uniform, whereas in swirled cases both the turbulence intensity and the scale near the cylinder axis are higher than those in the periphery. In addition, based on the measured results, the k-ε two-equation turbulence model under the influence of compression is discussed.