Tailor-made multi-component fuels are currently being developed for advanced Diesel engines. Accordingly, there is renewed interest in the detailed evaporation characteristics of such multi-component fuels, in particular because soot formation in reacting Diesel sprays generally depends on the mixture formation upstream of the lift-off location. It is also well established that fuel components with different volatility are generally not coevaporative due to fractional distillation in the mixture formation process of spark-ignition engines, but it is not clear if this holds for Diesel-like sprays, in which evaporation and mixing are expected to be more rapid. Unfortunately, little work has been done in this field, and some of the previous results appear to be contradictory.
This paper presents a new laser diagnostic approach, which yields the vapor-phase concentrations of two fuel components simultaneously in Diesel-like sprays. Thus, preferential evaporation of different fuel components can be studied for two-component fuels, namely mixtures of an alkane and an alcohol (without any additives). This technique is first applied to a mixture of ethanol and isooctane, because the evaporation of these two components was found to be significantly different in Diesel-like single-component fuel sprays previously. Thus, preferential evaporation might be expected particularly for the isooctane-ethanol blend. However, the results show that these components are essentially coevaporative in the investigated Diesel-like sprays.
The new diagnostic approach is based on 1-d spontaneous Raman scattering. It is described in detail in the paper. This technique also yields the total stoichiometric ratio and the temperature in the vapor phase, so that the air-fuel mixture can be characterized more completely. The measurements are conducted in a high-temperature, high-pressure vessel under approximately engine-relevant conditions. The investigated Diesel-like sprays are emanating from a state-of-the-art piezo injector. The fuel-rail pressure and injection duration are varied. The results also lead to conclusions on soot formation in Diesel-like sprays.