Szilard proposes three engine models in 1929 to resolve several paradoxes arising from Maxwell's Demon. We analyze Szilard's second demon models. We show that the second one,apparently employs distinct molecular species and semipermeable membranes. On this basis, we propose a fourth model of Szilard's engines. The mechanism of this model is based on Raoult's law and Van't Hoff's law in colligative properties of aqueous solutions, coupling the evaporating process of water molecules with the reverse osmosis process of water molecules, forms a spontaneous thermodynamic cycle composed.We find:(i) The cycle can proceed spontaneously in a gravitational field, without the need for an external force to do its work;(ii) The continuous flow of water molecules in the cycle, like the continuous current in a superconducting ring, can continue for a long time, but it is not a perpetual motion machine and does not violate the second law of thermodynamics;(iii) The cycle is capable of doing what Maxwell's Demon does, it is capable of producing a temperature difference in a single hot bath heated at temperature equilibrium. (iv) Its transitory functioning as an engine that converts disorganized heat energy to work is governed by the Onsager reciprocal relations. Taken together, this Szilard's fourth model is a new self-consistent, non-equilibrium thermodynamic cycle that provides a new theoretical model for understanding that quasi-perpetual motion processes and autonomous mechanical Maxwell's Demon do exist in nature.