Vapor/Liquid Visualization with Laser-Induced Exciplex Fluorescence in an SI-Engine for Different Fuel Injection Timings

Laser-induced exciplex fluorescence has been applied to the mixture formation process in the combustion chamber of an optically-accessible four-cylinder in-line spark-ignition engine in order to distinguish between liquid and vapor fuel distribution during the intake and compression stroke for different injection timings. The naphthalene/N,N,N′N′-tetramethyl p-phenylene diamine (TMPD) exciplex system excited at 308nm with a broadband XeCl excimer laser is used to obtain spectrally-separated, single-shot fluorescence images of the liquid or vapor phase of the fuel. For different timings of the fuel injector this technique is applied to obtain crank-angle-resolved images of the resulting mixture in the combustion chamber. The fluorescence light is detected with an intensified slow-scan CCD-camera equipped with appropriate filters. For fuel injection starting with the intake valve lift at its maximum droplets are detected in the combustion chamber during intake stroke and in the air-fuel mixture at the time of ignition. In the case of the fuel injector opening late in the compression stroke, i.e. with maximum time for liquid fuel evaporation in the intake manifold, no droplets are detected in the measuring volume at any time. For an intermediate injection timing during the late exhaust or the early intake stroke, droplets are detected during early portion of the intake event in the probe volume, but evaporate during mixture preparation in the subsequent compression stroke. In addition, simultaneous acquisition of pressure traces and analysis of engine-out HC-, NO-, and CO-emissions allow correlations between injection timing and engine performance.