Plasma Ignition Improves Combustion in Lean Mixture in Rig and Engine Tests: Combustion of highly diluted mixture for lean-burn operation and EGR decreases the combustion rate and generally requires higher ignition energy to produce stable combustion and then reduce fuel consumption and NOx. To improve ignition of highly diluted mixture, several ignition systems including, for example, plasma jet ignition, laser ignition, and railplug ignition have been studied at various sites. A group of researchers from Ooita University, Chiba University, Tokyo Institute of Technology, and Nihon Gaishi has been investigating repetitively pulsed plasma discharge for the ignition system of a spark-ignition engine. The preliminary results obtained in the mixture of propane and air indicated favorable results in improving lean limit and indicated thermal efficiency. They reported the results obtained for the mixture of iso-octane and air in both a test rig and a 0.265-liter, single-cylinder engine. The chapter consists of 12 pages including 14 figures.