# Experimental Activities on a Hydrogen-Fueled Spark-Ignition Engine for Light-Duty Applications
## 4 Results and Discussion
### Effects of Injection Strategy  
In this section, a comparison is conducted between the PFI and DI injection systems with the objective of identifying and understanding the potential differences in terms of performance and emissions. To do this, the analysis focuses on the low-load point, where the injection system has been switched to DI while maintaining the amount of hydrogen. Initially, the start of injection (SOI) was kept constant to \(-70\) CAD aTDC while sweeping the air-to-fuel ratio. The results of the combustion parameters obtained from this study are presented in Figure 10. There are no remarkable differences in terms of the combustion parameters. Both the duration and the maximum HRR peak are similar to those of the PFI case. The only substantial difference lies in the maximum pressure reached in the cylinder, which is reasonably lower in the DI case (between 1.5 and 3.5 bar). This effect is attributed to the lower intake pressure required to operate the engine under the same conditions. Since  
Figure 8: IMEP, GIE, COVIMEP and ISFC at 1500 rpm engine speed, under low-load conditions and different spark timings. Results obtained using the PFI system. Figure 7: 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 spark timing values. The results were obtained using the PFI system and multiple dilution conditions. Figure 9: ISOx emissions at 1500 rpm engine speed, under low-load conditions and different spark timings. Results obtained using the PFI system. hydrogen is injected during the compression stroke, no additional work is necessary to force the gas into the cylinder, reducing the demand on the boosting system. Figure 11 shows the impact of the DI system on the engine performance and the NOx emissions. As can be seen, the main difference is focused on the thermal efficiency and subsequently the specific fuel consumption levels achieved. While the maximum GIE levels for PFI are around 40%, they increase up to 42.5% when using DI. This results in 4 g/kWh of ISFC gain, decreasing up to 70 g/kWh in the low-load case. However, the degree of dilution at which these values are reached is slightly lower. In the case of PFI, the maximum GIE is centered around lambda 3, whereas this value decreases to 2.6 in DI. This is a direct consequence of the deterioration of the combustion stability as the dilution increases in the combustion chamber. In the case of DI, the combustion stability deteriorates earlier with lower degrees of dilution. Note that the combustion duration is remarkably larger for DI (Figure 10) than PFI (Figure 3 at \(\lambda=3.2\)). This effect could be attributed to the interaction of the hydrogen jet and the flow movement inside the combustion chamber. The coupling between these two flows determines the level of turbulence in the chamber and the degree of mixture stratification, both of which are crucial factors in combustion stability and pollutants generation. Low levels of turbulence, resulting from a negative interaction between the tumble motion and the H2 jet, can potentially lead to unfavorable flame propagation conditions, culminating in a partial or complete misfire. Likewise, the injection timing (SOI) dictates the duration for the hydrogen to achieve homogeneity within the cylinder. Variations in the local \(\lambda\) within the combustion chamber contribute to elevated flame temperatures and augmented NOx production. In Figure 12, we depict the main engine performance parameters while varying the SOI and maintaining a constant air-to-fuel ratio (\(\lambda=2.6\)). Two distinct trends can be discerned associated with the timing of the intake valve closing event. With injection before IVC, the engine performance exhibits minimal fluctuations with SOI retardation.