Mixture-Formation Analysis by PLIF in an HSDI Diesel Engine Using C_8-Oxygenates as the Fuel

摘要

With increasing interest in new biofuel candidates, 1-octanol and di-n-butylether (DNBE) were presented in recent studies. Although these molecular species are isomers, their properties are substantially different. In contrast to DNBE, 1-octanol is almost a gasoline-type fuel in terms of its auto-ignition quality. Thus, there are problems associated with engine start-up for neat 1-octanol. In order to find a suitable glow-plug position, mixture formation is studied in the cylinder under almost idle operating conditions in the present work. This is conducted by planar laser-induced fluorescence in a high-speed direct-injection optical diesel engine. The investigated C_8-oxygenates are also significantly different in terms of their evaporation characteristics. Thus, in-cylinder mixture formation of these two species is compared in this work, allowing conclusions on combustion behavior and exhaust emissions. Fuel injection strategy (single and split injection) and in-cylinder air swirl are varied. Results show that the fuel vapor remains in the peripheral region of the bowl volume near top dead center for 1-octanol, i.e., the region close to the cylinder axis is nearly vapor free. Thus, the peripheral region would provide a suitable glow-plug position. This vapor distribution is basically caused by impingement of liquid fuel on the piston wall. Presumably, wall wetting is also a major reason for increased unburnt hydrocarbon (HC) and CO emissions. Furthermore, the influence of wall wetting can be considerably reduced by using DNBE due to its evaporation and ignition properties. This leads to significantly different precombustion fuel/air mixtures and reduced HC and CO emissions compared to 1-octanol.