## Materials and Methods
### Mode Strategy Description and RCCI Engine Maps  
The dual-mode concept combines CDC and RCCI combustion regimes to enable operation along the whole engine map. The aim of this strategy is to promote RCCI operation whenever possible and switch to CDC to cover the engine map regions in which RCCI presents difficulties. Figure 3 illustrates the dual-mode engine map proposed by the authors in a previous work [43] in which the portions covered by both combustion regimes are differentiated and the ISFC values found for the RCCI region in the map. It can be seen that the low load-low speed region of the map leads to the worst performance, while the minimum indicated specific fuel consumption (ISFC) is achieved at 3000 rpm and 7 bar indicated mean effective pressure (IMEP), with values lower than 180 g/kWh. The influence of the engine speed on the ISFC is moderate, while the engine load has a large effect The RCCI diesel-gasoline region is covered with NOx and soot emission levels below 0.4 g/kWh and 0.01 g/kWh, respectively. The maximum load achievable with RCCI was limited by excessive MPRR (>10 bar/CAD) and/or excessive in-cylinder pressure peaks (P\({}_{\text{max}}\) > 160 bar). On the other hand, the lower operating region with RCCI was limited by excessive CO levels (>5000 ppm). Details regarding the dual-mode calibration procedure can be found in [31].  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Variable** & **Device** & **Manufacturer/Model** & **Accuracy** \\ \hline In-cylinder pressure & Piezoelectric transducer & Kistler/6125 BC & \(\pm\)1.25 bar \\ \hline Intake/exhaust pressure & Piezoresistive transducers & Kistler/4603B10 & \(\pm\)25 mbar \\ \hline Temperature in settling & & & \\ chambers and manifolds & Thermocouple & TC direct/type K & \(\pm\)2.5 \({}^{\circ}\)C \\ \hline Crank angle, engine speed & Encoder & AVL/364 & \(\pm\)0.02 CAD \\ \hline NOx, CO, HC, O\({}_{2}\), CO\({}_{2}\) & Gas analyzer &
\begin{tabular}{c} HORIBA/MEXA \\ 7100 DEGR \\ \end{tabular} & 4\% \\ \hline FSN & Smoke meter & AVL/415 & \(\pm\)0.025 FSN \\ \hline Gasoline/diesel fuel mass flow & Fuel balances & AVL/733S & \(\pm\)0.2\% \\ \hline Air mass flow & Air flow meter & Elster/RVG G100 & \(\pm\)0.1\% \\ \hline \hline \end{tabular}
\end{table}
Table 4: Accuracy of the instrumentation used in this work.  
Figure 2: Test cell scheme [43]. EGR: exhaust gas recirculation.  
Figure 4 shows the engine-out emissions results for the RCCI portion of the map, which will be used to implement the RCCI-SHV configuration. In terms of engine-out emissions, it is observed that NOx emissions are below 0.4 g/kWh across the entirety of the map, and the soot emissions are an order of magnitude lower than the limit imposed during the calibration process (0.01 g/kWh). This is possible by using gasoline fraction levels from 50% to 85% over the entirety of the map [28]. Regarding HC and CO emissions, the low load region shows unacceptable levels of both pollutants, while at upper loads the HC and CO levels are approximately 8-15 g/kWh.  
Figure 4: Engine-out emissions in the RCCI portion of the map.  
Figure 3: Illustration of the combustion strategies used to cover the engine map with the dual-mode reactivity controlled compression ignition (RCCI) diesel-gasoline/conventional diesel combustion (CDC) concept and indicated specific fuel consumption in the RCCI portion of the map. ISFC: indicated specific fuel consumption.