A method of reducing the nitrogen oxide emissions (Nox) of a supercharged piston engine (1), in which method the pressure of the combustion air is elevated to a level higher than that of the ambient pressure by means of a compressor apparatus (2) and in which the moisture level is elevated (4, 4') and heat obtained from the engine exhaust gases is introduced (5, 5', 5.1) to the combustion air prior to feeding it to the combustion chamber of the engine. The invention also relates to a piston engine arrangement (1) comprising a number of cylinders, a turbocharger (2) comprising a compressor part arranged in flow connection which each cylinder through the combustion air channel (3) and a turbine part arranged in flow connection with each cylinder through the exhaust gas channel (7), the arrangement comprising a nozzle arrangement (4, ') for introducing water to the combustion air channel (3) and a heat exchanger (5) arranged in heat transfer connection with the combustion air channel, the heat exchanger (5) being in heat transfer connection with the exhaust gases. ® KIPO & WIPO 2007
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