Turbocharging technique, together with engine downsizing, will play a fundamental role in the near future as a way to reach the required maximum performance while reducing engine displacement and, consequently, CO2 emissions. However, performing an optimal control of the turbocharging system is very difficult, especially for small engines fitted with a low number of cylinders. This is mainly due to the high turbocharger operating range and to the fact that the flow through compressor and turbine is highly unsteady, while only steady-flow maps are usually provided by the manufacturer. In addition, in passenger cars applications, it is usually difficult to optimize turbocharger operating conditions because of the lack of information about pressure/temperature in turbine upstream/downstream circuits and turbocharger rotational speed. This work presents a methodology suitable for instantaneous turbocharger rotational speed determination through a proper processing of the signal coming from an accelerometer mounted on the compressor diffuser or a microphone faced to the compressor. The presented approach can be used to evaluate turbocharger speed mean value and turbocharger speed fluctuation (due to unsteady flow in turbine upstream and downstream circuits), which can be correlated to the power delivered by the turbine. The whole estimation algorithm has been developed and validated for a light-duty turbocharged common-rail diesel engine mounted in a test cell. Nevertheless, the developed methodology is general and can be applied to different turbochargers, both for spark ignited and diesel applications.

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