In an example, an engine includes a thermal expansion unit comprising expansion material that expands in response to a temperature increase of the expansion material and contracts in response to a temperature decrease of the expansion material. The engine includes a structure comprising a heat receiving region, wherein at least a portion of the thermal expansion unit is disposed within the structure. The heat receiving region is configured to transfer thermal energy from a source of thermal energy to the expansion material through a first thermal energy transference path. The transfer of thermal energy to the thermal expansion unit causes expansion of the expansion material within the thermal expansion unit. The expansion of the expansion material causes an increase in length of the thermal expansion unit. The increase in length of the thermal expansion unit causes establishment of a second thermal energy transference path through which thermal energy is transferred from the expansion material to outside the thermal expansion unit.