Experiments and Analyses Examine Relationship between Ignition Delay and Gas Density: Lean-burn combustion has been an attractive approach to improve the fuel economy of gasoline engines. Extending the lean-limit is the key to improving fuel economy over the entire engine operating range. Thus, both ignition and initial combustion must be reliable to complete combustion in the limited time. A plasma ignition system has been considered as one technology that has the potential to increase ignition reliability and extend the lean limit. Nissan engineers previously developed a high-speed, low-temperature plasma system (HSP) to produce plasma energy quickly, on the order of nanoseconds. According to the experimental results, streamer discharge produced by the HSP was confirmed as effective in reducing the initial combustion duration. The investigation continues for better understanding of the mechanism of ignition and initial combustion, and this chapter reports the experimental results obtained in a single-cylinder engine equipped with a barrier discharge igniter and those obtained through chemical kinetic analyses performed for characterization of ignition delay.