Breuninger, T., Schmidt, J., Tschoeke, H., Hese, M. et al., "Optical Investigations of the Ignition-Relevant Spray Characteristics from a Piezo-Injector for Spray-Guided Spark-Ignited Engines," SAE Int. J. Engines 8(1):89-100, 2015, doi:10.4271/2014-01-9053.
The spray-guided combustion process offers a high potential for fuel savings in gasoline engines in the part load range. In this connection, the injector and spark plug are arranged in close proximity to one another, as a result of which mixture formation is primarily shaped by the dynamics of the fuel spray. The mixture formation time is very short, so that at the time of ignition the velocity of flow is high and the fuel is still largely present in liquid form. The quality of mixture formation thus constitutes a key aspect of reliable ignition.In this article, the spray characteristics of an outward-opening piezo injector are examined using optical testing methods under pressure chamber conditions and the results obtained are correlated with ignition behaviour in-engine. The global spray formation is examined using high-speed visualisation methods, particularly with regard to cyclical fluctuations. In order to characterise the recirculation zone of the hollow cone spray that is relevant to the ignition behaviour, laser-optical measuring methods were used. It was possible to achieve a significant increase in knowledge, in particular by combining different methods. Both droplet diameter and velocity were determined with a high degree of temporal and spatial resolution using Phase Doppler Anemometry (PDA). Furthermore, an extinction measurement method was used with which the liquid part of the fuel in the recirculation zone of the outward-opening piezo injector could be quantitatively determined under experimental conditions for the first time. To do this, a new approach was developed for calculating the mean extinction coefficients. In conjunction with the results of the PDA measurements it was thus possible to determine the air/fuel equivalence ratio achieved with the liquid fuel phase. The tests were carried out for a typical partial load range point whilst varying the number of injections, the chamber pressure and the injector control parameters. By comparing the spray sizes with the misfire rates determined in-engine, it was possible to assess the spray status with regard to ignition behaviour.