Abstract: An engine for converting temperature differentials to power or for converting power to heating or cooling. In the more obvious use, heat is applied to designated segments of metal belts to cause the designated segments to expand for the conversion of heat to power. If desired, cooling fluid is also applied to other segments of said belts to cause these other segments to contract and thus cooperate with the above stated application of heat in the conversion of temperature differences to power. In the preferred embodiment, two pairs of pulleys are provided. The two pulleys of each pair are mounted on and keyed to a common shaft. The two shafts are spaced apart but are parallel to each other. One of the pair of belts extends around one pulley of each pair and the other belt extends around the other two pulleys. The belts are positioned in planes parallel to each other. One pulley of each pair is larger than the other pulley of the pair to which it is connected by the associated shaft.