For the purpose of comparison, hypothetical designs for reciprocating and a rotary vane type expanders are considered in relation to a prototype trochoidal machine built and tested by the author. Geometrical and thermodynamic features of the three machines are discussed in addition to some of the practical problems associated with their configurations. Parameters such as chamber surface area to volume ratio and swept to bulk volume ratio have been evaluated for the designs considered and estimates are made for mechanical and diagram efficiencies likely to be achieved in each case. A new parameter is proposed whereby the instant turning moment at a given shaft angle may be determined as a function of chamber pressure and it is suggested that this parameter provides a convenient means for determining the optimum expansion ratio for various expander configurations.

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