Experimental Investigation and 1D Simulation of a Turbocharger Compressor Close to Surge Operation

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 engine speeds. In fact, in the above conditions, the boost pressure and the engine performance are 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 behavior of a small in-series turbocharger compressor. Measurements are carried out on 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 intake valve opening strategies, characterized by different opening durations, are considered. In each investigated operating point, close-to-surge operating conditions are promoted. The unsteady compressor behavior is investigated in terms of instantaneous inlet and outlet static pressures, mass flow rate and turbocharger rotational speed. 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 phenomena inside the device. The model is validated for various rotational speeds and valve lift strategy, the numerical/experimental comparisons denoting a satisfactory agreement. After the above validation, the methodology is employed to analyze the effect of the engine rotational speed and of the valve lift strategy on the surge resistance. The results show that, for the higher engine speeds, the compressor simply experiences soft-surge phenomena. On the contrary, below a well specified engine speed, flow reversal within the intake system occur and typical deep-surge cycles arise. In addition, valve lift strategies characterized by shorter opening durations seem to most likely promote the surge onset.