The invention relates to a method and an arrangement in connection with a piston engine provided with a turbocompressor. In the method, combustion air from the engine is pressurized by the compressor of the turbocompressor, pressurized combustion air is humidified, fuel is combusted in the cylinder of the engine by means of combustion air and the exhaust gases generated during the combustion are conveyed by means of combustion air to the turbine of the turbocompressor. A partial flow is separated from the pressurized combustion air flow, conveyed past the engine and combined with the exhaust gas flow going to the turbine.