The effects of fuel composition on soot processes in diesel fuel jets were studied in an optically-accessible constant-volume combustion vessel at experimental conditions typical of a DI diesel. Four fuel blends used in recent engine studies were investigated, including three oxygenates and one diesel reference fuel: (1) T70, a fuel blend containing the oxygenate tetraethoxy-propane; (2) BM88, a fuel blend containing the oxygenate dibutyl-maleate; (3) GE80, a fuel blend containing the oxygenate tri-propylene-glycol-methyl-ether and (4) CN80, a diesel reference fuel composed of an n-hexadecane and heptamethyl-nonane mixture. Measurements of the soot distribution along the axis of quasi-steady fuel jets were performed using laser extinction and planar laser-induced incandescence (PLII) and were compared to previous results using a #2 diesel fuel (D2). In addition to the soot measurements, lift-off length and ignition delay measurements were performed for an extensive range of ambient gas temperatures and densities. Flame lift-off lengths were used in the interpretation and analysis of the soot measurements.
Lift-off lengths, ignition delays and soot levels for these fuel blends follow similar trends with respect to ambient temperature or density established using D2 fuel. With increasing ambient temperature or density, lift-off length and ignition delay decrease and peak soot levels in a fuel jet increase. The increase in peak soot level is linear with respect to temperature and non-linear with respect to ambient density. Although following established trends with temperature or density, at a given experimental condition there is significant variation in lift-off length, ignition delay, and soot level for each fuel blend. The soot level in decreasing order with respect to fuel composition is: D2 > CN80 > BM88 > T70 > GE80. The distance from the injector to the region of first soot formation has an inverse relationship to the sooting propensity given above. That is, the first-soot distance is longest for GE80 and shortest for D2. The order in sooting tendency is found at either fixed ambient and injector operating conditions or at equivalent fuel-oxygen mixtures at the jet lift-off length, confirming that fuel molecular structure effects are important to the soot processes at diesel conditions. Differences in soot level with respect to fuel composition are quantified at many experimental conditions and axial positions of the fuel jet.