## 2 Materials and Methods  
### Optical Engine  
Measurements were carried out on a four-stroke single-cylinder optically accessible research engine (Figure 1a). Its main features are reported in Table 1. The pentroof head had four valves and a centrally located igniter (Figure 1b). The compression ratio (8.8:1) was 1-2 points lower than commercial SI engines. However, such a value belongs to a well-studied range in which a trade-off between power output and thermal efficiency occurs [39]. In Figure 2a, a schematic of the optical access can be found. It was realized by means of a Bowditch piston and a 45-degree mirror. The top of the piston was featured with a quartz crown of 60 mm radius, while the entire bore was 85 mm. Both quartz and mirror were designed to allow light transmission in the visible range (Figure 2a). This configuration required a dry contact between cylinder liner and piston rings [40], which were, therefore, realized in a self-lubricant material, i.e., a Teflon-graphite mix. Conventional mineral oil was used to lubricate all the other moving parts; its temperature, together with the coolant one, was set at \(343.0\pm 0.2\) K. This value was chosen to guarantee longer engine durability and reduced blow-by, with piston thermal expansion and crown temperature in the expected range of SI applications, despite the coolant being about 20 K colder than commercial power units [41]. The engine was coupled with a dynamic brake, namely, an AVL 5700 synchronous motor, which ensured speed control in both motoring and firing conditions. Airflow rate was regulated through an intake throttle valve. The injection, PFI type, was performed by means of a Weber IWP092 injector supplied with standard market gasoline (E5, RON 95, MON 85), pressurized at 5 bar (absolute). An Athena GET HPUH4 engine control unit (ECU) allowed controlling the energizing time (ET) of the injector with the air-fuel ratio (AFR) at the fixed intake throttle position. The same ECU was used also to set the ignition timing by sending a trigger signal to the dedicated control unit of the corona igniter. Indicating data were acquired and analyzed through a Kistler KiBox (Figure 3). The latter receives the in-cylinder signal from a Kistler 6061B, the flush installed in the combustion chamber, and the \begin{table}
\begin{tabular}{c c c} \hline \hline
**Feature** & **Unit** & **Value** \\ \hline Displaced volume & cm\({}^{3}\) & 500 \\ Stroke & mm & 88 \\ Bore & mm & 85 \\ Connecting rod length & mm & 139 \\ Compression ratio & - & 8.8:1 \\ Number of valves & - & 4 \\ Exhaust valve open & CAD bBDC & 13 \\ Exhaust valve close & CAD aTDC & 25 \\ Intake valve open & CAD bTDC & 20 \\ Intake valve close & CAD aBDC & 24 \\ In-cylinder residual gas mass fraction at the end of & \% & 9 \\ gas exchange process at \(\lambda=1\), 4.5 bar IMEP \({}^{1}\) & \% & 9 \\ \hline \hline \end{tabular} \({}^{1}\) Estimated by 3D-CFD simulation.  
\end{table} 
Table 1: Engine specifications. CAD—crank angle degree. Figure 2: (a) Details of Bowditch piston and optical access; (b) Advanced Corona Ignition System (ACIS) igniter, with an example of discharge in air. intake pressure signal from a Kistler 4075A5, used for in-cylinder pegging. Both these sensors were featured with an accuracy of \(\pm\)1%. The charge signal from the 6061B sensor was converted into voltage by a Kistler 5011 charge amplifier before Kibox acquisition. The crankshaft angular position was given by an AVL 365 C optical encoder with a temporal resolution of 0.1 crank angle degrees (CADs). In total, 101 consecutive combustion events were recorded and analyzed for each test point. The relative air-fuel ratio \(\lambda\) was measured via a fast probe (Horiba MEXA 720) at the exhaust pipe with an accuracy of \(\pm\)2.5%. Measurements of the amount of injected fuel were performed after engine tests.