Complex sentence: General Motors, seemed to have concluded that the Wankel engine was slightly more expensive to build than an equivalent reciprocating engine, although claiming having solved the fuel economy issue, but failed in obtaining acceptable exhaust emissions.
Split into simpler sentences: General Motors, seemed to have concluded that the Wankel engine was slightly more expensive to build than an equivalent reciprocating engine. They claimed to have solved the fuel economy issue, but failed in obtaining acceptable exhaust emissions.