An arrangement in connection with a piston engine provided with a turbocompressor, which arrangement comprises a flow duct for conveying combustion air pressurized by a compressor to cylinders of the piston engine, an exhaust gas duct for conveying exhaust gas from the piston engine to a turbine, and means adapted in conjunction with the turbine for feeding a cleansing agent to the turbine. There are feeding means arranged in connection with the exhaust gas duct for introducing air and/or water into the exhaust gas duct for cooling the exhaust gas.