NOVELTY - A hybrid energy conversion system comprises an internal combustion engine operated with a working fluid of air containing system fuel in excess of the stoichiometric quantity, with the result that hydrogen, carbon monoxide, hydrocarbons and/or reduced sulfur are present in the internal combustion engine product gas exhaust.    USE - Used for generating power from hydrocarbon and alcohol fuels.    ADVANTAGE - The hybrid energy conversion system achieves high overall energy conversion efficiency.    DETAILED DESCRIPTION - A hybrid energy conversion system comprises an internal combustion engine operated with a working fluid of air containing system fuel in excess of the stoichiometric quantity, with the result that hydrogen, carbon monoxide, hydrocarbons and/or reduced sulfur are present in the internal combustion engine product gas exhaust; fuel cells that produce electric power through electrochemical oxidation of product gas containing hydrogen, carbon monoxide, hydrocarbons and/or reduced sulfur at the anode surfaces and reduction of air at the cathode surfaces; gas transfer mechanism to move at least a portion of the product gas from the internal combustion engine to the anode surfaces of the fuel cells; supply mechanism to bring air to the cathode surfaces of the fuel cells; exhaust mechanism to remove spent product gas and air from the fuel cell anode and cathode; compressor (20) to raise the pressure of the air entering the internal combustion engine and the fuel cells from a first lower to a second higher pressure; afterburner (28) to mix and combust the spent product gas and air from the fuel cell anode and cathode to form a hot exhaust gas stream at the second higher pressure; and expander (24) to reduce the hot exhaust gas stream from the second higher pressure to a lower pressure and produce shaft power.    An INDEPENDENT CLAIM is also included for heat engine using air as a working fluid comprising a compressor that operates to raise air from a lower pressure to a higher pressure, a heat source for heating the compressed air, and an expander that expands the heated compressed air to a lower pressure to produce shaft power.    DESCRIPTION OF DRAWING(S) - The figure illustrates a power plant.    Compressor (20)    Fuel stream (23)    Expander (24)    Afterburner (28)    Generator (33)