Optical evaluation of orifice orientation and number effects on active pre-chamber spark ignition combustion

Abstract

Pre-chamber spark ignition combustion has shown several advantages for its implementation in engines, mainly related to its capability to achieve stable operation at lean or diluted mixtures and increased combustion speed. However, such systems have two challenges compared to traditional spark-ignition combustion: on the one hand, the stratification of the combustion process implies the usage of advanced tools for the analysis and optimization of the combustion system; on the other hand, the ignition process and overall combustion development is very sensitive to variations in the pre-chamber geometry. Previous works have explored particularly the effects of the pre-chamber volume compared to the engine displacement, the relationship between this volume and the total flow area of the nozzle connecting it to the rest of the cylinder, as well as the number and distribution of the nozzle orifices. In this work, the effect of the orientation of such orifices is explored on an optically-accessible rapid compression-expansion machine. Two main configurations are used: straight nozzle, where the axes of all the orifices intersect in a single point approximately in the center of the pre-chamber volume; and tilted nozzle, where the axes of two opposed orifices are displaced 1.5 mm, inducing a swirling motion both in the pre-chamber during the compression process and in the main chamber once ejection starts. Additionally, a sensitivity to the number of orifices is performed for the tilted nozzle layout. The results show that the induced swirling motion has little effect on the pre-chamber combustion development and initial ejection velocity, but it has a positive impact on the subsequent flame propagation and combustion efficiency, especially for ultra-lean conditions.