Experimental Investigation and 1D Simulation of a Turbocharger Compressor Close to Surge Operation 2015-01-1720

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. Measurements are carried out at the test facility of the University of Genoa. The compressor is included in the in-series intake circuit and a pulsating flow is generated by a motor-driven cylinder head fitted with a variable valve actuation system. Different rotational speeds and various valve opening strategies, characterized by different opening durations, are considered. Among the tested operating conditions, close-to-surge operation is also analyzed. The unsteady compressor behavior is investigated in terms of instantaneous inlet and outlet static pressures, and mass flow rate.

The numerical activity is performed at the University of Naples. The experimental test-rig is schematized in a commercial 1D code. The compressor is described following an enhanced map-based approach, where proper capacities are placed upstream and downstream the compressor to take into account the mass and energy storage effects. The model is validated for various rotational speeds and valve lift strategies, the numerical/experimental comparisons denoting a quiet good agreement.

After being validated, the methodology is employed to analyze the effect of the equivalent engine speed and valve strategy on the surge resistance. The results show that, for the higher speeds, the compressor only experiences soft-surge phenomena. On the contrary, below a specific speed, flow reversal within the intake system occurs and typical deep-surge cycles arise. In addition, valve lift strategies characterized by shorter opening durations seem to most likely promote the surge onset.