Editing Spark plug (section)

===Surface-discharge spark plug===
A piston engine has a part of the combustion chamber that is always out of reach of the piston; and this zone is where the conventional spark plug is located.  A [[Wankel engine]] has a permanently varying combustion area; and the spark plug is inevitably swept by the rotor's apex seals. If a  spark plug were to protrude into the Wankel's combustion chamber it would be hit by the passing apex seal, but if the plug were recessed to avoid this, mixture access to the spark would be reduced, leading to misfire or incomplete combustion. So a new type of "surface discharge" plug was developed, presenting an almost flat face to the combustion chamber.  A stubby centre electrode projects only very slightly, and the entire earthed body of the plug acts as the side electrode. The electrodes thus sit just beyond the reach of the passing apex seal, while the spark is accessible to the fuel/air mixture. The arc gap remains constant throughout the entire service life of a surface-gap spark plug, {{citation needed span|and the spark path will continually vary (instead of darting from the centre to the side electrode as in a conventional plug).|date=March 2021}} A further advantage of the surface-gap design is that the side electrode cannot break off and potentially cause engine damage, though this also doesn't often happen with conventional spark plugs.{{CN|date=March 2021}}