Williams, Paul Andrew; (1994) Characterization of fuel sprays in spark ignition engines. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis describes techniques to quantify spark ignition engine fuel spray quality, a study of the effect of fuel properties and other influences upon spray formation, and engine testing to assess the effect of fuel droplet sizes upon combustion. The literature survey reveals little published work on the effect of fuel properties on the sprays, and identifies the need for suitable parameters to describe the spray. 'Equivalent diameters' are defined as an alternative to Sauter mean diameter. Use of the diffraction particle sizer identified a problem caused by the evaporation of the volatile fuels. A solution is discussed. A parametric test was carried out, investigating the effect of variation of the liquid surface tension, viscosity and density on the droplet sizes produced by an injector. Injection pressure, air velocity, manifold pressure and injector type were also considered. A 'pencil-jet' phenomenon is described, where, at low values of Reynolds and Ohnesorge numbers, the injector produces a disintegrating column of fuel instead of a conical spray. Risk of occurrence increased at low air densities. A hypothesis is presented to explain the phenomenon. Pencil-jet formation occurred at low manifold pressure when using a gasoline, but not when using n-heptane. It is suggested that alterations to a gasoline could prevent pencil-jet formation. A correlation between measured droplet sizes and a set of non-dimensional equations was performed, allowing prediction of mean droplet sizes from knowledge of the fuel properties and manifold conditions. The most influential liquid property was viscosity, followed by surface tension, although the changes likely to be achieved are modest. Droplet sizes were almost insensitive to liquid density. Engine testing revealed no clear relationship between droplet size and emissions. Trends were noted but large changes in droplet sizes were required to produce a significant effect. Avoidance of pencil-jet formation could give significant benefits.

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