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A photographic study was conducted using an optical-access compression-expansion machine in order to reveal the mechanism of ignition and flame propagation initiated by plasma igniters. The tests included a jet igniter with inert cavity liner (quartz), a jet igniter with reactive cavity liner (paraffin), and a J-gap spark plug. Schlieren cinematographic records were obtained for each condition along with concomitant pressure traces. Basic features of ignition and combustion at lean limit were determined for each igniter. The spark plug permited lean operation down to an equivalence ratio of 0.7, after which mis-ignition occurred. Jet igniters provided an extension of lean limit to 0.5 in equivalence ratio. For jet igniters, these limits were imposed by either extinction of the flame or too slow burning rate, rather than by misfire.