NOVELTY - A power system (10) comprises an energy conversion device (12) (e.g. fuel cell) and a volume expansion engine (14). The energy conversion device receives a fuel; implements an electro-chemical process to generate an electrical output; and produces a first exhaust flow. The volume expansion engine receives the first exhaust flow; combusts the first exhaust flow to generate a power output; and produces a second exhaust flow directed into the energy conversion device.    USE - As a power system for producing power (claimed); useful as a fuel-cell/engine hybrid power system.    ADVANTAGE - In the power system, the exhaust flow from the volume expansion engine is redirected into energy conversion device (e.g. fuel cell), and the normally wasted fuel in the exhaust flow can be utilized in the fuel cell to generate additional useful energy. As a result, the cost of power system is reduced, and design flexibility of the power system is increased. In the power system, the exhaust flow from the energy conversion device (e.g. fuel cell) contains hydrogen, carbon monoxide and other constituents, hence the volume expansion engine can be operated in a low-polluting mode, which can be accomplished by operating the engine with an exceptionally lean mixture of fuel and air. The combustion of lean mixture is facilitated by the hydrogen, carbon monoxide or other constituents present in the exhaust flow obtained from the fuel cell. In the power system, the exhaust from the engine is directed back to the energy conversion device or fuel cell. Hence, heat and constituents of the engine's exhaust, facilitate economical use of variety of fuel cell types (e.g. molten carbonate fuel cells, phosphoric-acid fuel cells and solid oxide fuel cells). The power system exhibits high-efficiency, low exhaust emissions, and design flexibility. It is contemplated that volume expansion engine can operate on low-sulfur oil to reduce/eliminate the presence of sulfur compounds in fluid passageway, thus preventing damage to energy conversion device due to the presence of sulfur compounds.    DETAILED DESCRIPTION - A power system (10) comprises an energy conversion device (12) (e.g. fuel cell) and a volume expansion engine (14). The energy conversion device receives a fuel; implements an electro-chemical process to generate an electrical output; and produces a first exhaust flow. The volume expansion engine receives the first exhaust flow; combusts the first exhaust flow to generate a power output; and produces a second exhaust flow directed into the energy conversion device. The energy conversion device includes a carbonate electrolyte (24). The power system further comprises a supplemental fuel source (28) in communication with the volume expansion engine; a reformer (40) to condition the fuel; and a blower (50). The blower is configured to mix air with the second exhaust flow, before entering of the second exhaust flow into the energy conversion device, and to regulate the temperature of the resulting mixture. The heat resulting from the electro-chemical process is directed towards the reformer.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a power system.    Power system (10)    Energy conversion device (12)    Volume expansion engine (14)    Carbonate electrolyte (24)    Supplemental fuel source (28)    Reformer (40)    Blower. (50)