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The flow in engine turbocharger compressors and turbines is highly unsteady in nature, as it responds to the intake and exhaust manifolds of the internal combustion engine. The optimization of the turbocharger system is therefore a very difficult task, since the devices operate at off-design conditions for most of the engine cycle. Experimental studies allow for improving the understanding on the behavior of the engine components, in particular when tests are performed under real engine operating conditions; however, the experimental tests can be more efficient if they are combined with theoretical simulation tools, which help to select significant engine operating conditions. In this paper, experimental investigations were performed on a flexible component test rig (expressly suited to perform tests on automotive turbochargers) at ICE Laboratory of the University of Genoa (ICEG-DIMSET).

The transient response of an engine with both High Pressure (HP) and Low Pressure (LP) EGR loops was compared by conducting step changes in EGR fraction at a constant engine speed and load. The HP EGR loop performance was shown to be closely linked to turbocharger performance, whereas the LP EGR loop was relatively independent of turbocharger performance and vice versa. The same experiment was repeated with the variable geometry turbine vanes completely open to reduce turbocharger action and achieve similar EGR rate changes with the HP and LP EGR loops. Under these conditions, the increased loop volume of the LP EGR loop prolonged the response of intake O2 concentration following the change in air-fuel ratio. The prolonged change of intake O2 concentration caused emissions to require more time to reach steady state as well. Strong coupling between the HP EGR loop and turbochargers was again observed using a hybrid EGR strategy.