Intake air meter for an internal combustion engine

Claim 1

An intake air meter for an internal combustion engine comprising: a Karman vortex shedder which is disposed in the main air intake passageway of said engine; ultrasonic transmitter means for generating ultrasonic waves and ultrasonic receiver means for receiving said ultrasonic waves and producing a corresponding electrical output signal, said ultrasonic transmitter means and said ultrasonic receiving means being disposed on opposite sides of said main air intake passageway downstream of said Karman vortex shedder such that a Karman vortex street which is shed by said Karman vortex shedder will pass therebetween; oscillator means for driving said ultrasonic transmitter so as to generate said ultrasonic waves; first waveform shaping means for shaping the output signal of said ultrasonic receiver; phase comparator means for producing an output signal corresponding to the phase difference between a first input signal which is the output signal from said first waveform shaping means and a second input signal; loop filter means for removing unwanted frequency components from the output signal of said phase comparator; voltage-controlled phase shifting means for shifting the phase of the output signal from said oscillator means in accordance with the voltage of the output signal from said loop filter means and providing a phase-shifted output as said second input signal to said phase comparator means; low-pass filter means for removing a carrier frequency component from the output signal of said phase comparator means; compensating means for removing a pressure-dependent component from the output signal of said low-pass filter means, said pressure-dependent component being dependent on the air pressure fluctuations within said main intake air passageway, said compensating means comprising: pressure sensor means for measuring the air pressure in the vicinity of said Karman vortex shedder and producing a corresponding output signal, amplifier means for amplifying the AC component of the output signal from said pressure sensor means and producing an output signal having the same amplitude as said pressure-dependent component, inverter means for producing an output signal which is the inverse of the output signal from said amplifier means, and second waveform shaping means for summing the output signal from said low-pass filter means and the output signal from said inverter means; and filter means for removing the AC component from the output signal of said pressure sensor and producing a DC output signal which is proportional to the average air pressure within said main intake air passage.