## Materials and Methods  
### Test Cell, Fuels, and Engine Description  
The single-cylinder diesel engine (SCE) used to acquire the experimental data that was subsequently utilized to perform the simulations is based on a production light-duty 1.9 L engine, the main characteristics of which are reported in Table 1. The engine has four valves driven by dual overhead cams. The production piston, with a re-entrant bowl and a geometric compression ratio of 17.1:1 was used. The swirl ratio was fixed at 1.4 to be representative of the value used in the production engine. To set the desired swirl ratio, the SCE was equipped with tangential and helical valves located in the intake port [42]. Figure 1 shows the fuel injection systems used for RCCI operation, the main characteristics of which are depicted in Table 2. A centrally located solenoid direct injector (DI) was used to inject the EN590 diesel fuel into the cylinder. The fuel was supplied to the injector by means of a common-rail fuel injection system. A DRIVVEN controller was used to manage the different injection parameters. The gasoline fuel was injected into the intake manifold by means of a port fuel injection (PFI) governed through a Genotec unit. The PFI was located 160 mm from the intake valves to avoid pooling of the \begin{table}
\begin{tabular}{c c} \hline \hline
**Engine Type** & **Four Stroke, Four Valves, Direct Injection** \\ \hline Number of cylinders [\(-\)] & 1 \\ Displacement [L] & 0.477 \\ Bore \(\times\) stroke [mm] & 82 \(\times\) 90.4 \\ Compression ratio [\(-\)] & 17.1:1 \\ Bowl type & Re-entrant \\ Rated power [kW] & 27.5 \(\oplus\) 4000 rpm \\ Rated torque [Nm] & 80 \(\oplus\) 2000–2750 rpm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Engine characteristics. fuel. The mass flow rate of both fuels was measured using dedicated AVL 733S fuel balances (AVL LIST GmbH, Graz, Styria, Austria). Table 3 summarizes the main properties of the diesel and gasoline fuels used in this work. Figure 2 shows the layout of the test cell used in this work. During the experimental tests, the engine speed and load are controlled by an electric dynamometer. To feed the engine with fresh air, several devices are used. The first element is a screw compressor, which is used to compress the air up to a pressure of 3 bar. Afterward, a heat exchanger and an air dryer are used to set the desired temperature and relative humidity of the air. The mass air flow is measured using an airflow meter. Finally, a settling chamber is connected to the intake ducts to attenuate the pulsating flow induced by the cylinder movement. As Figure 2 shows, several pressure and temperature sensors are instrumented along the intake line to control their values. The exhaust gas recirculation (EGR) line consists of a heat exchanger to modulate the intake charge temperature, a settling chamber to promote a regular EGR mass flow, and a regulation valve to modify the EGR rate as required. As for the fresh air, the EGR temperature is controlled in various locations along the EGR line. Lastly, the temperature and pressure of the intake charge (air-EGR mixture) is controlled in the intake manifold before entering into the cylinder. The exhaust line has a similar disposition to the intake line. First of all, the pressure and temperature of the exhaust stream are monitored through transducers located in the exhaust manifold. Figure 1: Fuel injection systems scheme [43]. PFI: port fuel injection; DI: direct injector.