<div class="htmlview paragraph">In order to confirm quantitatively the performance and characteristics of the plasma jet ignition in turbulent lean mixtures, combustion tests were carried out in a disk-shaped combustion chamber with lean turbulent methane-air mixtures. In the tests, the governing parameters of the plasma jet ignition such as the plasma cavity size, the orifice diameter, and the discharge energy were varied.</div> <div class="htmlview paragraph">A characteristic lifetime and a characteristic length of the plasma jet and an entrainment volume of the mixture into the plasma jet are defined theoretically and expressed by the parameters of the plasma jet ignition. A series of combustion tests revealed that lean flammability limits, a comparing parameter, and a combustion index are correlated with these characteristic values, where the comparing parameter shows the degree of combustion enhancement by the plasma jet ignition in its initial stage and the combustion index represents the total combustion performance of the plasma jet ignition.</div>