In addition to the removal of internal reciprocating stresses by the complete removal of reciprocating internal parts typically found in a piston engine, the Wankel engine is constructed with an iron rotor within a housing made of aluminium, which has a greater coefficient of thermal expansion. This ensures that even a severely overheated Wankel engine cannot seize, as is likely to occur in an overheated piston engine. This is a substantial safety benefit when used in aircraft. In addition, the absence of valves and valve trains increases safety. GM tested an iron rotor and iron housing in their prototype Wankel engines, that worked at higher temperatures with lower specific fuel consumption.