A Study of Volumetric Ignition Using High-Speed Plasma for Improving Lean Combustion Performance in Internal Combustion Engines

It is well known that ultra-lean combustion can result in higher thermal efficiency, better fuel economy, and greatly reduced NOx emissions. Accomplishing ultra-lean combustion is very difficult with a conventional spark plug, and ignition instability can be cited as one of the factors. Therefore, it is thought that ignition system innovation is important for the achievement of ultra-lean combustion in gasoline engines. This study investigated high-speed plasma ignition as a new ignition system for internal combustion engines. High-speed plasma refers to the transient (non-equilibrated) phase of plasma before formation of an arc discharge; it is obtained by applying high voltage with an ultra-short pulse between coaxial cylindrical electrodes. High-speed plasma can inherently form a multi-channel discharge, with the electrical discharge spreading over a much larger volume than a spark discharge does. Accordingly, it is expected to achieve volumetric ignition different from the point ignition of a conventional spark plug. The results of verification experiments conducted with a single-cylinder engine showed that volumetric ignition was achieved with low energy consumption. It was also confirmed that ignitability was improved in comparison with a conventional spark plug. It was observed that the electric field strength in the air gap between the coaxial cylindrical electrodes during high-speed plasma formation was increased more by applying a shorter pulse. It was also clarified experimentally that the shorter pulse improved ignitability even further.