Parametric Tests Demonstrate Stable Ignition with PC Plug: An open-chamber natural gas engine is relatively simple in terms of its cylinder head structure compared to a pre-chamber natural gas engine. Thus, production costs and maintenance costs are lower. Generally, a spark plug on the cylinder head ignites mixture to initiate combustion unless the engine operates on dual fuel with pilot injection of diesel fuel. When the excess air ratio of the mixture is increased (or leaned) to improve thermal efficiency, a conventional spark ignition system does not produce sufficient energy to produce stable combustion. Thus, the highest excess air ratio is limited to a lower level than that of the pre-chamber engine, and the cylinder bore of the open chamber engine is limited to a relatively small size. Engineers at Denso Corporation and Osaka Gas Corporation developed a pre-chamber plug (PC plug) for a natural gas engine with a relatively large bore. A small cavity attached to this spark plug created an environment where a fraction of fuel burned inside the pre-chamber, and the flames ejected from the pre-chamber ignited lean mixture in the main chamber. Because flame energy is greater than spark ignition energy, it can produce stable combustion of lean mixture in the main chamber. The chapter consists of 10 pages including 12 figures and 1 table.