Obtaining accurate measures of the turbocharger rotational speed is a key task to achieve good powertrain control performance in turbocharged combustion engines. However, direct access to the rotating parts of a turbocharger requires expensive sensors that present long-term reliability issues. In view of this, this article focuses on the design of measurement architectures for the estimation of the turbocharger shaft rotating speed via the numerical processing of the overall sound emissions acquired by a microphone placed in the vehicle hood. This kind of signal represents an extremely rich source of information about the operating conditions of all noisy powertrain subsystems. The core of the scheme is represented by an adaptive discrete-time nonlinear frequency locked-loop (FLL) filter that is properly designed to extract the useful frequency content from the acquired audio signal. The whole architecture is innovative, flexible, and extremely low cost by requiring, for its implementation, the additional installation of a single microphonic capsule only. Moreover, it exhibits such a modest computational burden to be directly implementable in commercial engine control units (ECUs) without requiring additional computing hardware. Reported experimental assessments show that the accuracy of the estimate is excellent in all allowed rotational speed regimes.
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