An In-Cylinder Imaging Study of Pre-chamber Spark-Plug Flame Development in a Single-Cylinder Direct-Injection Spark-Ignition Engine

Prior work in the literature have shown that pre-chamber spark plug technologies can provide remarkable improvements in engine performance. In this work, three passively fueled pre-chamber spark plugs with different pre-chamber geometries were investigated using in-cylinder high-speed imaging of spectral emission in the visible wavelength region in a single-cylinder direct-injection spark-ignition gasoline engine. The effects of the pre-chamber spark plugs on flame development were analyzed by comparing the flame progress between the pre-chamber spark plugs and with the results from a conventional spark plug. The engine was operated at fixed conditions (relevant to federal test procedures) with a constant speed of 1500 revolutions per minute with a coolant temperature of 90 oC and stoichiometric fuel-to-air ratio. The in-cylinder images were captured with a color high-speed camera through an optical insert in the piston crown. The images showed plumes of reacting gases originating from the pre-chamber orifices and the subsequent flame development in the main combustion chamber of the engine. Flame characteristics were quantified from the images. Quantitative analysis of the images showed all the pre-chamber spark plugs consistently yielded faster flame development (approximately 4.7 CAD) compared with flames created by the conventional spark plug. The flame fronts from the pre-chamber spark plugs were 1.54-2 times larger than those from the conventional spark plug. The imaging data also showed significant cycle-to-cycle variability during the initial stage of the flame development from the pre-chambers with smaller/later jets for some cycles. However, the flame progress recovered rapidly to more uniform propagation later in the cycles. The different pre-chamber geometries did not lead to significant differences in the combustion characteristics at the studied conditions, although the pre-chamber with asymmetric orifice sizes yielded slightly larger variability and delayed flame development compare with the other pre-chamber designs. The relatively modest effects of the different pre-chamber designs are attributed to potentially offsetting changes in the dominant physical mechanisms affecting the ignition process at the conditions studied.