Small Orifices - Diesel Combustion and Spray Investigations
Doctoral thesis, 2003
Future demands on diesel engines, such as the need to meet stricter emission legislation, improve performance and lower fuel consumption, for lowering transportation cost and carbon dioxide emissions, forces the development of engines to take a major step in the near time. During the years of engine development, all components have been optimised in small steps at a time, but larger steps must now be taken to meet requirements of the future. One of the large steps that can be made is to change the orifice size of the diesel injector nozzle. In this thesis, the orifice size and the effects of downsizing the orifice diameter have been investigated. The investigations were done using a constant pressure chamber in fundamental studies and a single cylinder engine to study the effects on exhaust emissions and fuel consumption.
In the constant pressure chamber the liquid and gas phases of the fuel were investigated by two different laser-based methods: Mie scattering and laser-induced exciplex fluorescence. It was found that a reduction in orifice diameter leads to reduced fuel penetration for both the liquid and gas phases. However, a change in the orifice size did not affect the spray angle. The spray volume, which is formed, becomes smaller as the reduction of the orifice size progresses. But the liquid spray volume fraction decreases at the same time, making the gas spray volume fraction larger. Increased mixing and increased relative air entrainment are the reasons for the leaner and less fuel-rich conditions inside the spray. In addition to the relatively larger flame lift-off length for a reduced orifice diameter, this would lead to lower emissions of soot, which have been found in the engine experiments. Besides the soot reduction, lower fuel consumption and lower levels of hydrocarbon and carbon monoxide were found at the same time as the nitrogen oxide emissions increased. The ignition delay was also reduced as the orifice diameter was decreased. Furthermore, a strong emission dependency on the relative air to fuel ratio for small orifices was found which originated from the nozzle configurations larger number of orifices in the nozzles used.
small orifice diameter