Abstract: A highly efficient, pollution free thermodynamic engine employing a primary cold working fluid which is conducted in an open loop from a low temperature storage tank, through a succession of engine stages each comprising a constant volume heat exchanger and an expansion engine, and finally exhausted; and a secondary hot fluid which is circulated in a closed loop through successive engine stages and a secondary loop heat exchanger. A plurality of optional preliminary heat exchangers in the primary fluid loop initially heat the primary working fluid before entering the first engine stage to a temperature above the freezing temperature of the secondary fluid to eliminate freezing thereof. A secondary fluid heat exchanger enables transfer of heat between ambient tertiary fluid and the secondary fluid. Each engine stage employs a constant volume heat exchanger which provides an improved thermodynamic cycle with resulting high efficiency.