An internal-combustion engine, comprises at least one cylinder, at least one intake pipe and at least one exhaust pipe associated to the cylinder, at least one intake valve and at least one exhaust valve, which control passage through the intake and exhaust pipes, wherein there are provided first sensor device(s) for detecting the temperature in the intake pipe, second sensor device(s) for detecting the pressure in the intake pipe, third sensor device(s) for detecting the engine r.p.m., and fourth device(s) for detecting or calculating the temperature in the exhaust pipe. An electronic control unit pre-arranged for receiving the signals at output from the first second and third sensor device(s) calculates the amount of fresh air taken in by the engine on the basis of a mathematical model that is of general applicability and that, in particular, is applicable irrespective of the technological implementation specifically used for a system for variable valve actuation with which the engine can be provided.