## 3 Experimental Setup and Test Procedure  
As aforementioned, the tested engine is provided by an electro-hydraulic variable valve actuation (VVA) device. In spite of the VVA potentials, in the experimental tests the load is adjusted only acting on the throttle valve opening or waste-gate valve opening, without modifying the intake valve lift profile which is set to the \"Full Lift\" configuration. The intake air is constantly supplied to the engine at 293 \(\pm\) 1 K by an air conditioning unit. Each engine cylinder is equipped with piezo-quartz pressure transducer (accuracy of \(\pm\)0.1%) to detect the in-cylinder pressure signal. The instantaneous pressure signals are acquired by the AVL INDICOM over 270 consecutive pressure cycles for the combustion analysis, assuming a polytropic thermodynamic process. A resolution of 0.1 CAD within the angular window between \(-\)90 and 90 CAD AFTDC is chosen, while outside this angular interval the sampling resolution is set at 1 CAD. An automatic post-processing tool, based on a thermodynamic model, computes the ensemble averages of in-cylinder pressure and burn rate profiles and the combustion characteristics data. Furthermore, the boost pressure and the upstream turbine pressure are acquired through piezo-resistive low pressure indicating sensors located at the compressor outlet and at the outlet of the exhaust manifold, respectively. The engine is also equipped with thermocouples to monitor intake and exhaust temperatures, with particular attention to the control of the turbine inlet temperature in order to avoid unacceptable levels for the turbine blades. A prototype driver, managed within LabView environment, is capable to switch from the commercial ECU to an external user control of the main engine variables: fuel injection timing and duration, boost pressure, spark timing, external EGR flow and temperature. EGR-related variables are managed acting on valve opening and cooler device within the EGR circuit. Exhaust gas emissions are sampled upstream of the three-way catalyst. An ultra-violet gas analyzer (ABB UV Limas 11) measures NO\({}_{\rm x}\). A cold extractive IR gas analyzer (ABB URAS) detects CO, CO\({}_{2}\) and O\({}_{2}\) while a FID analyzer (Siemens, Milano, Italy) is used for THC. A non-dispersive infrared sensor measures the CO\({}_{2}\) concentration at the inlet, downstream of the throttle valve to monitor the EGR rate. The sensor error is set at 0.07% for the measured CO\({}_{2}\) concentration. The engine is tested in steady-state conditions at two different speeds (1800 and 3000 rpm) and various loads (from low to medium/high levels), also including increasing external EGR rates. In particular, EGR is modified by keeping constant the engine IMEP and the relative A/F ratio, \(\lambda\). This means that at increasing the EGR rate the IMEP is restored to the nominal value with engine boosting; in a similar way, \(\lambda\) is maintained to the stoichiometric value by a modulation of gasoline injection duration. These operating points are selected because they are representative of the WLTC cycle for a segment A vehicle equipped with the engine under investigation. Performance in these points, such as fuel consumption and CO\({}_{2}\) emissions, are relevant for the vehicle homologation. Various operating parameters are collected during the experiments, including torque/power, air flow rate, fuel flow rate, boost and exhaust pressures and temperatures, exhaust emissions, etc. The overall considered operating conditions are reported in Table 2. Test grid collects 31 points which were gathered into 7 groups, each one characterized by the same nominal load level and rotational speed. For a single group, a label is defined (Table 2), referring to the nominal net IMEP and speed, which is utilized in the following figures. The table also shows the measured values of EGR rate, relative A/F ratio, \(\lambda\), and the spark advance (SA).