Wankel engine Your text is sure not documented I think you have never seen a Wankel engine from inside In other respects would you know that common piston ring material like Goetze cast iron IKA 3 and IKA 33 is used for the apex seals Trochoiden surface chrome chrome molybdenum molybdenum combined with construction steel apex seals Nikasil etc The side surface is reagular cast iron or over eutectic alu KSxx etc The cooling surface is smaller as for a comparable reciprocating piston engine and then should you compare the same displacement volume Chamber volume is two times for the displacement A 1 3ltr Wankel has a displacment of a 2 6ltr and only a surface like a 1 3ltr 4 T reciprocating piston engine That why water cooler is smaller per kW