Spray and Spray Combustion Characteristics of Diesel-Ethanol Fuel Blends

Licentiatavhandling, 2015

In Diesel engines, the quality of fuel atomization and fuel-air mixing inside the cylinder have important effects on the combustion process, and therefore strongly related to the fuel efficiency and emission formation. To understand how the injected fuel mixes with air and combustion processes in a Diesel engine, fundamental spray experiments can be carried out in a constant chamber under different engine-like conditions. In this thesis, two investigations are included, one to investigate liquid phase spray images and the other to investigate spray characteristics of Diesel-ethanol fuel blends under non-combusting and combusting conditions. Although Diesel spray experiments have been carried out over several decades, some basic questions remain unclear, such as how to define, calculate and evaluate liquid phase spray penetration lengths and cone angles. Hence, in this work, Diesel spray experiments were performed under non-evaporating, evaporating and combusting conditions, and shadowgraph spray images were evaluated. It was found that penetration lengths determined from either instantaneous or averaged images were similar and little influenced by the threshold level. Under combusting conditions, during the early phase of injection, there were little differences in the penetration length when the threshold level was changed, whereas during the later phase of injection, the measured penetration length was sensitive to changes in the threshold level and there were significant differences between the penetration lengths determined from instantaneous and averaged images. The local spray cone angle was calculated along the whole spray profile by measuring the width at different distances from the nozzle tip. It was found that at around 12 mm from the nozzle tip, the measured local spray cone angle was constant during most injections. Further downstream, the local spray cone angle increased up to about 75% of the penetration length under non-evaporating and evaporating conditions, whereas it decreased under combusting conditions. To reduce soot formation, one of the pathways is to increase the content of oxygen atoms in the fuel itself. Since ethanol contains oxygen atoms and has a low carbon-to-hydrogen (C/H) ratio, it can be used as an additive to reduce soot emission. In addition, ethanol can be produced as a sustainable fuel. Therefore, in the present work, 10% and 20% ethanol was blended in Diesel fuel (European EN590 standard Diesel fuel) and the influence of the fuel blends on spray characteristics was investigated under non-combusting and combusting conditions in the Chalmers high-pressure/high-temperature spray chamber. It was found that differences in the fuel composition did not significantly affect the liquid phase spray penetration length or cone angle under non-evaporating or evaporating conditions. Under combusting conditions, reducing the ambient temperature increased the ignition delay and delayed the onset of soot formation for all fuels. Furthermore, E10 and E20 showed longer ignition and soot formation delays than pure Diesel. As the ethanol content of the fuel was increased from 0% to 20%, the lift-off length increased and the detectable soot luminescence decreased.