NOVELTY - The system has a fuel cell (25) coupled to an engine exhaust, and a battery (36) coupled to the fuel cell. A controller i.e. engine control unit (48), receives a signal indicative of electric output of a fuel cell output circuit and adjusting one of fuel amount and air amount supplied to an internal combustion engine (24) in response to the signal to affect air-fuel ratio of the exhaust of the engine. The controller adjusts the fuel and air amount according to exhaust ratio, which is inferred from the electrical output and exhaust temperature.    USE - System for operating a fuel cell e.g. solid oxide fuel cell, molten carbonate fuel cell and proton exchange membrane fuel cell (all claimed), in an exhaust of an internal combustion engine e.g. petrol engine or diesel engine, of a hybrid electric vehicle (HEV) in parallel/series split hybrid electric vehicle configuration. Can also be used for series configuration, parallel configuration and integrated starter-alternator configuration.    ADVANTAGE - The fuel cell serves as an emission control device to decrease carbon monoxide (CO), hydrogen (H2), hydrocarbon (HC) emission by oxidizing the CO, H2, HC into carbon dioxide (CO2) and water while generating power, thus eliminating oxidizing catalysts or three way catalyst converters, and reducing system cost. The fuel cell decreases nitrogen oxides (NOx) emission by reducing NOx to nitrogen (N2) and oxygen (O2) when current from a battery is applied to the fuel cell, so that the fuel cell serves multiple functions such as power generation, sensing and/or emission control.    DETAILED DESCRIPTION - The electric output is one of current, voltage and impedance. An INDEPENDENT CLAIM is also included for a method for operating an internal combustion engine of a vehicle.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram of an engine in a hybrid power train.    Internal combustion engine (24)    Fuel cell (25)    Generator motor (30)    Battery (36)    Vehicle system controller (48)