Spark plug for an engine for a cogeneration system

A spark plug for a cogeneration engine comprising:

• a tubular housing;
• a central bar electrode supported by said tubular housing in said tubular housing with electrical insulation therebetween;
• a first bar discharge member being arranged at one end of said central bar electrode, comprising Ir alloy, protruding from one end of said tubular housing along an axis of said center electrode, and having an outer circumferential surface along said axis;
• a plate ground electrode being arranged at said one end of said tubular housing in a radial direction of said axis and having an end surface confronting said outer circumferential surface of said first bar discharge member;
• and
• a second discharge member being arranged on said end surface and having a surface confronting said outer circumferential surface, spark discharge being generated at a gap between said first and second discharge members, wherein if it is assumed that a distance of said gap is G, a width of said outer circumferential surface in a normal direction of a plane including said radial direction and said axis is D, and a width of said surface in said normal direction is A, G is equal to or greater than 0.2 mm and equal to or lower than 0.6 mm, D is equal to or greater than 1.6 mm, and |A−D| is equal to or lower than a result of adding 0.5 mm to G.