    Senterice :1::What parts -of *a: -conventional  reciprocating, steam. enginé could’ replaced by:  a pistonless ‘rotary engine? .         Sentence 2: The major problem. is the ‘difficulty  of, sealing the: rotors. to. make them steam-tight  “in: the face ‘of wear and. thermal expansion; the  ‘resulting leakage made. them very inefficient.