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From the beginning, the development of FSI® direct-injection (DI) gasoline engines at Volkswagen was strictly supported by means of optical diagnostics and CFD-simulations. Basic in-cylinder phenomena, such as the formation of the flow field, the penetration of the spray formed by a hollow-cone swirl-type injector at high fuel pressure, the interaction of spray and flow and the formation of an ignitable mixture were analysed in details. The paper describes the laser-optical techniques - particle-image-velocimetry, laser-Doppler-anemometry, video-stroboscopy, high-speed filming and laser-induced fluorescence - which were used during the development of the DI gasoline combustion process. In addition, CFD-simulations have been carried out simultaneously with the experiments, thus providing the engine designer with complete information about the in-cylinder processes to derive further steps of optimization.

A newly developed measurement technique was employed to investigate flame propagation before and during knocking combustion. Two intensified CCD cameras were used in sequence to record the natural flame light during knocking combustion via a fused silica window, which was fitted to the cylinder head of a one-cylinder four-stroke SI-engine. This arrangement enabled acquisition of two images per cycle. While the first camera was triggered by the rapid rise of the cylinder pressure at the beginning of engine knock, the second camera was activated at a prescribed time of delay. Due to the high sensitivity of the intensified CCD cameras, extremly short exposure times of 100 to 250 ns proved to be sufficient. Consequently, acquisition rates of 200 kHz and more - which are substantially higher than those of conventional natural light photography - could be realized.