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Characterization of In-Cylinder Fuel Distribution From An Air-Assisted Fuel Injection System Using Advanced Laser Diagnostics
Published November 26, 2002 by Institution of Mechanical Engineers in United Kingdom
Event: Fuel Injection Systems
During the injection and mixture formation processes of a GDI engine, fuel may exist in either the liquid or vapor phase it is therefore advantageous to be able to observe both phases simultaneously. However, traditional LIF techniques are not suitable for this purpose since the large differences in fluorescence intensity from the two phases make the selection of a suitable light collection level virtually impossible.
In an attempt to overcome this problem the Laser-Induced Exiplex Fluorescence (LIEF) technique was applied to a motored, single-cylinder research engine with full optical access. The engine was fitted with a GDI cylinder head featuring an air-assisted injection system with an injection pressure of 6.5 bar.
The LIEF technique relies on the reaction of two fuel dopants to produce a red-shifted fluorescence emission from the liquid phase whist normal fluorescing occurs from the vapor phase. Selection of appropriate filtering then allows the separation and subsequent collection of fluorescence emissions from each phase simultaneously.
The substitute fuel used was iso-octane with dopants Naphthalene (7%) and N,N-dimethylaniline (DMA) (7%). The excitation source used was an XeCl eximer laser with a wavelength of 308 nm. In order to collect both emissions using a single ICCD camera an image doubling and filtering system was designed and constructed.
The fluorescence from the fuel injection was observed in both a vertical plane in the center of the cylinder, normal to the crankshaft and from 2 horizontal planes, 5 mm and 25 mm below the cylinder head gasket face. Two fuel injection strategies were used for each observation plane, one representing a late injection, part load, "stratified' condition with injection beginning during the second half of the compression stroke and the other representing an early injection, full-load, "homogenous' condition with injection beginning midway through the intake stroke.
Initial results show clear and well-defined images of the fuel spray and mixture development. The late injection condition shows a narrow spray of liquid and vapor. The fuel present in the liquid phase during injection vaporizes in the late stage of the compression stroke to leave a well-stratified mixture around the spark plug around the time of ignition. The homogeneous condition shows a wider spray of both liquid and vapor. The complete vaporization of liquid fuel produces a well-homogenized mixture around the time of ignition.