Characterization of in-cylinder fuel distribution from an air-assisted fuel injection system using advanced laser diagnostics

摘要

During the injection and mixture formation processes of a GDI engine fuel may exist in either the liquid or vapour phase and 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 whilst normal fluorescing occurs from the vapour 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-dimethlylaniline (DMA) (7%). The excitation source used was an XeCl eximer laser with a wavelength of 308nm. 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 centre of the cylinder, normal to the crankshaft and from 2 horizontal planes, 5mm and 25mm below the cylinder head gasket face. Two fuel injection strategies were used for each observation plane, one representing an 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 though 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 vapour. The fuel present in the liquid phase during injection vaporises 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 vapour. The complete vaporisation of liquid fuel produces a well-homogenised mixture around the time of ignition.