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A combination of optical imaging diagnostics has been applied to the fuel jet of a direct-injection diesel engine to study the ignition and early soot formation processes. Measurements were made in an optically accessible direct-injection diesel engine of the “heavy-duty” size class at a representative medium speed (1200 rpm) operating condition. Two fuels were used, a 42.5 cetane number mixture of the diesel reference fuels and a new low-sooting fuel (needed to reduce optical attenuation at later crank angles) that closely matches both the cetane number and boiling point of the reference fuel mixture. The combustion and soot formation processes are found to be almost identical for both fuels. Ignition and early combustion were studied by imaging the natural chemiluminescence using a calibrated intensified video camera. The early soot development was investigated via luminosity imaging and simultaneous planar imaging of laser-induced incandescence (LII) and elastic scattering.

Laser-induced incandescence (LII) has been explored as a diagnostic for qualitative two-dimensional imaging of the in-cylinder soot distribution in a diesel engine. Advantages of LII over elastic-scatter soot imaging techniques include no interfering signals from liquid fuel droplets, easy rejection of laser light scattered by in-cylinder surfaces, and the signal intensity being proportional to the soot volume fraction. LII images were obtained in a 2.3-liter, single cylinder, direct-injection diesel engine, modified for optical access. To minimize laser sheet and signal attenuation (which can affect almost any planar imaging technique applied to diesel engine combustion), a low-sooting fuel was used whose vaporization and combustion characteristics are typical of standard diesel fuels. Temporal and spatial sequences of LII images were made which show the extent of the soot distribution within the optically accessible portion the combusting spray plume.