# Experimental Activities on a Hydrogen-Fueled Spark-Ignition Engine for Light-Duty Applications
## 4 Results and Discussion
### Effects of Air Dilution
Additionally, the duration of different combustion phases, derived from the aforementioned parameters, are also depicted in Figure 3. CA50-10 displays the least sensitivity to air dilution, characterized by the smallest slope. Both the end of combustion (CA90-50) and the combustion initiation (spark timing-CA10) are equally influenced, demonstrating identical slopes and initial values. The impact of \(\lambda\) on the engine performance and emissions is illustrated in Figure 4. These figures depict the results for both low-load and mid-load conditions (8.08 mg/str H\({}_{2}\)). The results presented here were obtained using the PFI injection system, and they represent the optimal combustion phasing or MBT. For the low-load point, the following trends emerge. Instabilities, as indicated by COVIMEP, begin to increase more steeply at around \(\lambda\) = 3.2. The specific fuel consumption reaches its minimum value at \(\lambda\) = 3.2, coinciding with the air dilution ratio that maximizes the engine performance and efficiency. At this point, the engine achieves an IMEP of 5.2 bar, corresponding to 40% of the gross indicated efficiency (GIE). For the mid-load point, the operating range for dilution spans from \(\lambda\) = 2.4 to 3.4. The lowest achievable dilution value is \(\lambda\) = 2.4. Higher dilutions are necessary to reduce the risk of abnormal combustion (knocking), which can lead to engine damage, especially considering the higher pressure and temperatures reached in the combustion chamber. The engine performance starts to decline when the air dilution equivalence ratio exceeds 3.4, consistent with the trend observed at low-load conditions. At \(\lambda\) = 3, a peak in GIE and IMEP is observed. This trend holds true for both low- and mid-load conditions, where the combustion process becomes less efficient as \(\lambda\) increases. The minimum specific fuel consumption drops below 74 g/kWh, with an efficiency and engine performance improvement, with IMEP exceeding 8.7 bar. In earlier studies, it was observed that NOx emissions peaked at \(\lambda\) = 1.3 [13]. Values above 2 significantly reduce this pollutant by impeding the thermal formation mechanism. The results obtained in this investigation are consistent with previous findings, notably  
Figure 3: Maximum in-cylinder pressure and HRR peaks, characteristic combustion periods (CA10, CA50, and CA90), spark timing, combustion duration periods (CA-Spark timing, CA50-CA10, and CA90-CA10) obtained at 1500 rpm engine speed and low-load conditions for various \(\lambda\) values. Combustion phasing was optimized to achieve MBT conditions, and the results were obtained using the PFI system. reducing its value for \(\lambda>2\). In the case of mid-load conditions, due to limitations related to engine knock, the first \(\lambda\) value that ensures safe operation brings NOx emissions levels below 1 g/kWh. However, although higher dilution can minimize NOx emissions stemming from the thermal mechanism, they never reach zero because other formation pathways persist. During engine operation, certain limitations reported in the literature on PFI-based H2-engines were identified. Figure 5 includes all the measured data points to provide an overview of the operational limits as a function of the air dilution and engine load. The available engine operating region is constrained by two primary factors that affect the performance and efficiency. The knocking tendency increases as the quantity of the injected hydrogen rises and the level of dilution decreases, whereas the combustion stability is compromised mainly due to dilution. With an increase in the quantity of fuel, the effective range of \(\lambda\) decreases. Additionally, elevated in-cylinder pressure and temperature increase the likelihood of knocking and intensify its impact. Conversely, combustion stability due to an excessively diluted mixture can lead to abnormal combustion phenomena, such as pre-ignition, backfire, or even knocking.