# Experimental Activities on a Hydrogen-Fueled Spark-Ignition Engine for Light-Duty Applications
## 4 Results and Discussion
### Effects of Air Dilution  
Initially, we delve into the outcomes pertaining to the air dilution effect. This parameter holds a position of utmost importance within the realm of engine parameters. The scientific literature extensively attests to the merits of lean combustion, encompassing the reduction in thermal losses, NOx emissions, and decreased susceptibility to knock, among other advantages. This acquires heightened significance in the context of hydrogen-fueled engines due to hydrogen's remarkable combustion properties, facilitating the achievement of ultra-lean combustion while maintaining burning stability. Figure 2 depicts the cycle-averaged profiles of the in-cylinder pressure and heat release rate (HRR). These profiles were obtained from the low-load operating point (4.96 mg/str H\({}_{2}\)) at 1500 rpm as the dilution rate was increased. The achievement of conditions for maximum brake torque (MBT) involved spark timing optimization. For this purpose, the characterization of the air dilution was accomplished through the air-fuel equivalence ratio parameter (\(\lambda\)) using the PFI system. In these results, it is evident that higher \(\lambda\) values correspond to elevated pressure profiles. With the fuel quantity held constant, increasing \(\lambda\) necessitates an increased intake pressure when using a PFI system. The high values, which reach almost 1.8 bar even under these low load conditions, were necessary to sustain the dilution conditions when using hydrogen as a fuel, especially at high values of \(\lambda\) (>2), to restrict the operation of PFI-based engines at high load conditions due to boosting system limitations or maximum in-cylinder pressure peaks. The impact on the HRR profiles reveals that as the dilution increases, the curve flattens and widens. The extended duration of combustion is attributed to deteriorated combustion propagation conditions. These trends are more effectively illustrated in Figure 3, where the maximum incylinder pressure and the HRR peaks are displayed alongside the pertinent combustion periods as the dilution rate varies. As can be observed, this figure reaffirms the previously noted observation, showcasing a rising trend in the in-cylinder pressure peaks with \(\lambda\). Notably, at \(\lambda=4\), combustion instabilities impact the average pressure profile, resulting in \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Engine Speed [rpm]** & **Hydrogen [mg/str]** & \(\lambda\) & **Injection System** \\ \hline
1500 & 4.96 & [1.40.2:4.0] & PFI/DI \\
1500 & 8.08 & [2.40.2:3.4] & PFI/DI \\
1500 & \(10.92/11.37\) & 2.6/1.8 & PFI/DI \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Details of the experimental tests carried out in this investigation. Figure 2: In-cylinder pressure and HRR profiles at 1500 rpm engine speed and low-load conditions, obtained using the PFI system, showcasing variations with different \(\lambda\) values under the MBT conditions. a reduction in the maximum peak value due to unburned cycles and partial combustion. This cyclic variation establishes the stability limit for air dilution. As illustrated in Figure 2, the results in Figure 3 also corroborate that the maximum HRR value decreases as more diluted conditions are reached. This decline in the HRR peaks exhibits two distinct slopes: an initial region where increasing dilution significantly reduces the maximum peaks, particularly up to \(\lambda\approx\) 2.6, and a subsequent region where further dilution exerts less influence, albeit still leading to a decrease in the maximum value. Figure 3 also presents crucial combustion parameters, encompassing the combustion phasing and duration. Under MBT conditions, the combustion phasing (CA50) falls within the approximate range of 8 to 10 CAD. It is noteworthy that CA10 experiences only marginal effects from increasing \(\lambda\), whereas CA90 exhibits a significant dependency on \(\lambda\), rising from approximately 15 CAD to values surpassing 30 CAD.