Characterization of the Ignition and Early Flame Propagation of Pre-Chamber Ignition System in a High Pressure Combustion Cell

The Pre-Chamber spark plug, already in its most simple configuration, allows a cycle fuel consumption reduction of 2-3% (WLTC) by enabling a significant compression ratio increase due to its huge knock mitigation effect. This benefit can be strongly extended in the homogeneous lean burn operation mode with very low nitrogen oxide emissions by a novel approach of injecting a well prepared fuel-air-mixture inside the Pre-Chamber. An increase of the engine compression ratio allows further the development of a new combustion process referred as the Pre-Chamber supported self-ignition process, which enable an increased thermodynamic efficiency at part load operating points of a gasoline engine. The development of a suitable Pre-Chamber ignition system requires the technical understanding of the Pre-Chamber geometry parameters on the combustion process. The impact of the overflow channel design on the flame propagation and ignition of the fuel-air mixture inside the main chamber must be understood in greater detail. This in turn requires a high-fidelity combustion model which is capable of predicting the impact of the overflow channel geometry on e.g. flame extinction, radical recombination on the walls of the Pre-Chamber orifice and finally the behavior of main chamber inflammation regardless of the Pre-Chamber ignition regime. Focus of this work is to discuss the impact of the Pre-Chamber geometry onto the inflammation and early flame propagation inside the main combustion chamber by means of experiments in an optical high pressure vessel under simplified boundary conditions. As a basis, the simultaneous high speed measurement of Schlieren and OH* chemiluminescence serve as a fundamental means to analyze ignition performance and early flame propagation in order to develop and validate an accurate combustion model. Initially, the general impact of the chamber pressure will be discussed emphasizing specifically on the differences between certain Pre-Chamber layouts and the conventional ignition system onto flame speed and ignition probability. Furthermore, variations of e.g. the number of overflow holes, the orifice diameter and the volume of the Pre- Chamber aids to identify the most relevant parameters responsible for flame extinction and combustion performance inside the main chamber.