## 3 Results and Discussion  
### Methane and Methane-Hydrogen Blends  
Combustion in CAI engines strongly depends on the initial gas temperature, as reported in Figure 2 for methane. At 458 K the charge does not burn, while increasing the temperature combustion start angle moves towards the top dead center because in-cylinder gases reach earlier the thermodynamic conditions for auto-ignition. Engine indicated efficiency consequently changes: If the auto-ignition occurs too late, peak cylinder pressure is positioned later in the expansion stroke and is reduced in magnitude with negative effects on impe; if it is too advanced, peak cylinder pressure occurs \begin{table}
\begin{tabular}{c c c} \hline \hline  & **CH\({}_{4}\)** & **H\({}_{2}\)** \\ \hline Density [kg/Nm\({}^{3}\)] & 0.7064 & 0.0888 \\ \hline Stoichi AFR [mass air/mass fuel] & 17.2 & 34.3 \\ \hline Stoichi AFR [vol air/vol fuel] & 9.54 & 2.39 \\ \hline Lower Heating Value [MJ/kg] & 50 & 119.9 \\ \hline Lower Heating Value vol. [MJ/Nm\({}^{3}\)] & 35.32 & 10.64 \\ \hline Auto-ignition temperature [\({}^{\circ}\)C] & 813 & 858 \\ \hline Thermal conductivity at 300 K [W/(m K)] & 0.034 & 0.182 \\ \hline Octane Number & \textgreater{}120 & \textgreater{}130 \\ \hline Laminar flame speed at stoichi. conditions [m/s] & 0.38 \(\div\) 0.40 & 2 \(\div\) 2.2 \\ \hline Flammability limits in air [\%vol.] & 5.3–15 & 4–75 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Methane and hydrogen properties [45]. too early so increasing the compression work [19]. Figure 3 shows this trend: The best combustion phasing for methane is obtained at 468 K initial temperature, where the indicated efficiency attains its maximum value, with the engine delivering 8.4 bar impe with 47.6% efficiency. Adding hydrogen to methane causes the ignition to happen earlier (closer to the top dead center) for a constant initial temperature, as depicted in Figure 4.  
Peak cylinder pressure with hydrogen blends is higher than that obtained using methane, but this does imply neither higher engine efficiency, nor higher impe, as shown in Figure 3. Indeed, the combustion for CH\({}_{4}\)/H\({}_{2}\) blends at 468 K starts too early, and as discussed above, this reduces both impe and efficiency. However, since hydrogen has a wider flammability, which limits its addition and  
Figure 3: Effect of initial gas temperature on indicated efficiency and impe (methane and methane-hydrogen blends). The best operating conditions are circled. Figure 2: Pressure traces versus crank angle at different initial gas temperatures (methane). The thicker line shows the best operating conditions. extends the CAI operational range allowing the engine to run with lower initial gas temperatures. The best operative conditions can be found maximizing both impe and efficiency and it can be observed from Figure 3 that if the engine is operated at temperature between 443 K and 463 K, it would deliver a higher impe with engine efficiency slightly higher than methane. Using methane-hydrogen blends, ER is adjusted to get 8.4 bar as maximum impe, the same value as for methane. While the maximum impe for methane is attained at 468 K, the initial gas temperature requirement for CH4H2_1 drops to 448 K with an ER of 0.38, Figure 5. It is worthwhile noting that by varying the ER, the efficiency remains practically constant and CH4H2_1 showed an efficiency at the optimal operative conditions similar to that of methane, Figure 5.  
Figure 4: Pressure traces versus crank angle (methane and methane-hydrogen blends). Figure 5: Effect of initial gas temperature on indicated efficiency and impe (methane and CH4H2_1). The best operating conditions are circled. Results are similar with CH4H2_2 blend and also in this case the equivalence ratio is adjusted to keep the maximum imp equal to that of methane. In this case, the optimal initial temperature is 443 K so that engine delivers a maximum impe of 8.6 bar with ER = 0.38, Figure 6. Additionally, in this case the maximum indicated efficiency does not change significantly.