Importance

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Abstract

An acoustic tachometer for diesel engines includes a microphone which is responsive to the sounds generated by the engine combustion occurrences. The microphone signal is passed through a band-pass filter having a variable center frequency which is determined by a control signal from a sweep generator. The control signal has a known relationship to the operating speed of the engine. As the band-pass filter is swept through its frequency spectrum, a peak detector identifies the signals of maximum amplitude, which occur at the fundamental frequency of the combustion occurrences. The control signal voltage at the maximum amplitude and hence at the fundamental frequency is stored. At the end of each sweep, the stored control signal voltage is fed to an oscillator having a frequency which is dependent upon the stored control signal voltage, and is related to revolutions per minute (RPM). The output of the oscillator excites a counter-display circuit which displays the operating speed in RPM. In an alternate embodiment, the stored control signal voltage is connected to a second band-pass filter having a variable center frequency which is dependent upon the stored control signal voltage. The microphone signal is fed through the second filter, which passes only the fundamental frequency to a clipper and phase locked loop, from which is driven a counter and digital display showing RPM. Error detection circuitry alerts the operator if the sound to the microphone is inadequate or excessive for accurate measurement.

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First Claim