Intake air parameter estimating device for internal combustion engine

An intake parameter estimating device is provided for estimating a tuning frequency (NEK) of an internal combustion engine, which in turn can be used to estimate an intake air pressure of the internal combustion engine. The intake parameter estimating device basically has a fundamental frequency calculating section (ECU 41, step S12), an engine rotational speed detecting section (ECU 41, step S1) and a tuning frequency calculating section (ECU 41, step S2). The fundamental frequency calculating section (ECU 41, step S12) calculates a fundamental frequency (Fint) of a pressure wave inside an air intake pipe (15) based on a shape of the air intake pipe (15) and speed of sound (Spsd). The engine rotational speed detecting section (ECU 41, step S1) detects an engine rotational speed (NE). The tuning frequency calculating section (ECU 41, step S2) calculates a tuning frequency (NEK) of the pressure wave inside the air intake pipe (15) based on the fundamental frequency (Fint) and the engine rotational speed (NE). The tuning frequency (NEK) can then be used to estimate the intake air pressure of the internal combustion engine.