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Downsizing is widely considered one of the main path to reduce the fuel consumption of spark ignition internal combustion engines. As known, despite the reduced size, the required torque and power targets can be attained thanks to an adequate boost level provided by a turbocharger. However, some drawbacks usually arise when the engine operates at full load and low speeds. In fact, in the above conditions, the boost pressure and the engine performance is limited since the compressor experiences close-to-surge operation. This occurrence is even greater in case of extremely downsized engines with a reduced number of cylinders and a small intake circuit volume, where the compressor works under strongly unsteady flow conditions and its instantaneous operating point most likely overcomes the steady surge margin. In the paper, both experimental and numerical approaches are followed to describe the unsteady behavior of a small in-series turbocharger compressor.

In the paper the difficulties related to measurement of instantaneous fluid dynamic parameters in the turbocharging circuit of automotive engines under unsteady flow conditions are highlighted, referring to pressure, mass flow rate, temperature and turbocharger rotational speed. Frequency response of transducers, sensor position, measuring procedures and filtering techniques need to be taken into account to correctly describe the unsteady behaviour of tested component. The results of an experimental campaign developed on a small turbocharger for downsized gasoline engines are presented, focusing on the evaluation of the instantaneous parameters that allowed to estimate unsteady turbine performance. Measurements were performed by using a flexible test rig, operating at the University of Genoa, particularly suitable for investigations under unsteady flow operation.