Abstract: A fluid engine comprising a coiled conduit having a fluid inlet and a fluid outlet and supported to rotate about its axis in a partially immersed condition within a first fluid which interfaces with a second fluid having a different fluid density. Rotation of the conduit about its axis causes the fluid inlet to alternately be immersed in the first and second fluids whereby a quantity of each fluid is entrapped within each convolution of the conduit resulting in an unbalanced fluid condition producing a gravitational turning moment. Pressure differential across the coiled conduit between the inlet and the outlet maintains fluid flow through the conduit from the inlet to the outlet thereby sustaining the unbalanced condition which causes conduit rotation. The required differential may be provided by a head of water or may be created by evaporating a liquid or by thermally expanding a gas.