## 2 Engine System  
The engine used for the experimental activity is a twin-cylinder turbocharged spark ignition engine, equipped with two port injectors, one for each cylinder, to supply the gasoline just upstream of the intake valves. Engine's main characteristics are reported in the following Table 1. It is provided by a conventional pent-roof combustion chamber, a centered spark plug and a standard ignition system. Each cylinder presents 4 valves, two intake valves and two exhaust valves. An electro-hydraulic Variable Valve Actuation (VVA) module is mounted on the intake side, allowing for a flexible control of the lift profile. This device provides the actuation of both Early Intake Valve Closure (EIVC) and Full Lift valve strategies. On the exhaust side, a fixed valve lift strategy is employed. Engine boosting is realized by a small waste-gated turbocharger. The compressor operating domain bounds the engine performance because of surge and choke phenomena and of a maximum allowable rotational speed (255,000 rpm). In particular, at low speeds, the boost pressure is limited to avoid the compressor surging occurrence [17]. An additional constraint is imposed by the manufacturer to the maximum boost level, achieved at high speeds, to guarantee the mechanical integrity of the intake plenum. The engine is designed following the so-called \"downsizing\" concept. At high loads the engine works under knock-limited conditions and this requires to delay the combustion phasing (50% of mass fraction burned--MFB 50%), especially at lower speeds where a higher knock tendency usually occurs. On the other hand, at high speeds the A/F mixture has to be particularly enriched to maintain the Temperature at Turbine Inlet below a certain maximum allowable level. These control strategies, although mandatory for the examined engine, greatly penalize the fuel consumption at high loads. For this reason, the base engine configuration is properly modified for research purpose through the installation of an external low-pressure (LP) exhaust gas recirculation (EGR) circuit as depicted in Figure 1. \begin{table}
\begin{tabular}{c c} \hline \hline
**Model** & **2 Cylinders, Turbocharged PFI** \\ \hline Compression ratio, - & 9.9 \\ Displacement, cm\({}^{3}\) & 875 \\ Valve number, - & 4 per cylinder \\ Bore/Stroke, mm & 80.5/86 \\ Connecting rod length, mm & 136.85 \\ Max Brake Power, kW & 64.6@5500 \\ Max Brake Torque, Nm & 146.1@2500 \\ IVO-IVC at 2 mm lift, CAD AFTDC & 342/356–420/624 \\ EVO-EVC at 2 mm lift, CAD AFTDC & 134–382 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Engine characteristics. Figure 1: Engine layout with LP EGR circuit. EGR rate and temperature are controlled through the throttle EGR valve and the cooler device, respectively. An enhanced cooling system, requiring a water cooled heat exchanger, is utilized to cool down the recycled gas. The LP EGR system was preferred over a high-pressure (HP) device, because it allows to avoid the pressure fluctuations and the counter-flow within the EGR circuit, due to the turbocharger dumping effect. As known, EGR is essentially considered for knock control purposes at high loads, although certain EGR-related efficiency advantages are also obtained at part loads, due to the engine de-throttling. The engine at test bench is also equipped along the exhaust line with a Three-way Catalytic converter (TWC) to guarantee the pollutant emissions (HC, CO and NOx) abatement, provided that the air/fuel ratio window very close to the stoichiometric value is maintained. The exhaust system was opportunely modified to extract a portion of the exhaust gas from a single cylinder. In particular, a thin metallic tube is inserted inside of the exhaust manifold, through a properly realized hole, and oriented towards the exhaust side of one cylinder. The internal end of the above tube is located very close the exhaust valves. In this way, the exhaust gases of the selected cylinder (Cyl #1 in the following) are extracted and externally derived to measure the individual cylinder-out noxious emissions.