Extension of Lean Ignition Limit Using an Elongated Spark Discharge with a Pin-to-Disk Electrode

Lean-burn combustion technology is generally conceived to improve the thermal efficiency of spark ignition (SI) engines. Such an improved efficiency mitigates CO2 emission, and thus, the lean-burn technology helps transition toward a carbon-neutral society. However, it is required to limit the NOx emission at the same time by extending a lean limit as much as possible to lower the flame temperature. Recently, stretched sparks caused by a high-intensity tumble flow have been found to significantly extend a lean-burn limit and shorten an overall combustion duration. Here, we presented an experimental study to better understand the underlying mechanism of the stretched spark for the ignition of lean mixtures. We conceived a spark-to-piston concept and configured a pin-to-disk electrode inside a small combustion chamber introducing an adjustable interelectrode gap. As the interelectrode gap increased, a lean limit for a complete burning regime was extended, and the combustion duration was shortened. A negative applied voltage to the pin-electrode facilitated the extension of the lean limit and a faster combustion for a larger gap compared to a positive one. The effect of a tumble flow mimicked by a high-speed jet on the lean limit and the combustion duration were also investigated. A stretched spark, due to its gliding root on the disk-electrode, extended the lean limit, and the high-speed jet shortened the combustion duration. The restrike phenomenon could not be observed in the present configuration, indicating a minimized cyclic variation due to misfire.