A system for estimating fresh air mass flow into a turbocharged engine comprising: a compressor located in an inlet flow path of the engine and at least a pressure sensor in an inlet of the compressor and a pressure sensor in an outlet of the compressor; an air treatment device located in the flow path of the engine; at least a pressure sensor in an inlet of the air treatment device and a pressure sensor in an outlet of the air treatment device; a controller arranged to determine an actual fresh air mass flow in subsequent time frames by ∘ measuring, in an actual time frame, a pressure drop over the compressor and ▪ using a first calculated fresh air mass flow as a starting value for deriving a second fresh air mass flow in said time frame from a compressor model using the measured pressure drop and compressor rotational speed; and ∘ measuring in a previous time frame, before said actual time frame, a pressure drop over the air treatment device; ▪ estimating a pressure drop over the air treatment device using the second fresh air mass flow and an estimated flow resistance of the air treatment device; ▪ correcting the second fresh air mass flow by comparing the estimated pressure drop with the measured pressure drop over the air treatment device and using the corrected second fresh air mass flow as an actual fresh air mass flow in said time frame, and ▪ using the actual fresh air mass flow in said time frame as first calculated fresh air mass flow in a next time frame of a subsequent iteration. a compressor located in an inlet flow path of the engine and at least a pressure sensor in an inlet of the compressor and a pressure sensor in an outlet of the compressor; an air treatment device located in the flow path of the engine; at least a pressure sensor in an inlet of the air treatment device and a pressure sensor in an outlet of the air treatment device; a controller arranged to determine an actual fresh air mass flow in subsequent time frames by ∘ measuring, in an actual time frame, a pressure drop over the compressor and ▪ using a first calculated fresh air mass flow as a starting value for deriving a second fresh air mass flow in said time frame from a compressor model using the measured pressure drop and compressor rotational speed; and ∘ measuring in a previous time frame, before said actual time frame, a pressure drop over the air treatment device; ▪ estimating a pressure drop over the air treatment device using the second fresh air mass flow and an estimated flow resistance of the air treatment device; ▪ correcting the second fresh air mass flow by comparing the estimated pressure drop with the measured pressure drop over the air treatment device and using the corrected second fresh air mass flow as an actual fresh air mass flow in said time frame, and ▪ using the actual fresh air mass flow in said time frame as first calculated fresh air mass flow in a next time frame of a subsequent iteration. ∘ measuring, in an actual time frame, a pressure drop over the compressor and ▪ using a first calculated fresh air mass flow as a starting value for deriving a second fresh air mass flow in said time frame from a compressor model using the measured pressure drop and compressor rotational speed; and ▪ using a first calculated fresh air mass flow as a starting value for deriving a second fresh air mass flow in said time frame from a compressor model using the measured pressure drop and compressor rotational speed; and ∘ measuring in a previous time frame, before said actual time frame, a pressure drop over the air treatment device; ▪ estimating a pressure drop over the air treatment device using the second fresh air mass flow and an estimated flow resistance of the air treatment device; ▪ correcting the second fresh air mass flow by comparing the estimated pressure drop with the measured pressure drop over the air treatment device and using the corrected second fresh air mass flow as an actual fresh air mass flow in said time frame, and ▪ using the actual fresh air mass flow in said time frame as first calculated fresh air mass flow in a next time frame of a subsequent iteration. ▪ estimating a pressure drop over the air treatment device using the second fresh air mass flow and an estimated flow resistance of the air treatment device; ▪ correcting the second fresh air mass flow by comparing the estimated pressure drop with the measured pressure drop over the air treatment device and using the corrected second fresh air mass flow as an actual fresh air mass flow in said time frame, and ▪ using the actual fresh air mass flow in said time frame as first calculated fresh air mass flow in a next time frame of a subsequent iteration.