## 4 Evaluation of Hydrogen Combustion with the Addition of Helium
### Thermodynamic Parameters
It should be remembered that during the tests, 1) there were fluid flows between PC and MC and 2) pure hydrogen was injected into the prechamber. Therefore, the maximum decrease in heat release in the prechamber is approximately 20% (at \(\mathrm{He/H_{2}}\) = 40/60). This shows that, on average, for every 10% of helium addition, the decrease in the maximum amount of heat released in the PC is about 5%. Figure 8: Heat release rate dQ: (**a**) in the main chamber; (**b**) in the prechamber; fueled with hydrogen without helium (\(\mathrm{He/H_{2}}\) = 0/100), variable excess air ratio and n = 1500 rpm; red line—passive prechamber data; other—active combustion chamber with increased fuel dose in PC. An analysis of the effect of helium on engine operation is presented in relation to the cylinder pressure (at a constant TH throttle opening and at a constant amount of fuel injected into the prechamber)--Figure 10.  
Figure 10: Cylinder pressure: (**a**) in the main chamber; (**b**) in the prechamber, with changes in the He/H2 ratio at different throttle opening (go_PC = 2 ms, active prechamber; CoC = 8–10 deg). Figure 9: Heat release: (**a**) in the main chamber; (**b**) in the prechamber; fueled with hydrogen without helium (He/H2 = 0/100), variable excess air ratio and n = 1500 rpm; red line—passive prechamber; other—active combustion chamber with increased fuel dose in PC. An analysis of the data for the same fuel doses to the main chamber (qo_MC = 9.5 ms) and the prechamber (qo_PC = 2 ms) at different helium percentages shows that the excess air ratio varied (Figure 10a). The study shows that a 10% change in the ratio of hydrogen to helium in the fuel does not always result in a constant change in the \(\lambda\). On average, the change is about \(\Delta\lambda\) = 0.3. At higher throttle openings, these changes are more variable. An analysis of the combustion process at a constant throttle position and changing He/H\({}_{2}\) ratio shows changes in CoC. Despite the acceleration of the start of combustion, about 6-7% lower maximum combustion pressures were obtained (with TH = 15% and qo_PC = 2 ms, \(\lambda\) = 1.52). Greater throttle opening (and directly higher \(\lambda\)) results in greater variations in maximum pressure. Increasing the helium content by 10% at a high throttle opening (TH = 20% and \(\lambda\) = 1.90), the changes in maximum pressure are about 8%. At the highest values (TH = 25%; \(\lambda\) = 2.42), the change in maximum pressure reaches more than 13%. Similar changes were observed during analysis of the combustion pressure in the prechamber (Figure 10b). Again, as the helium content increased by 10%, there was a decrease in maximum pressure of around 6%. Relating the changes in Pmx to the corresponding changes in IMEP in Figure 6, it can be seen that the changes in IMEP at 10% helium addition are 11-13% (larger changes were observed at higher throttle openings). Increasing the proportion of helium in the fuel also reduces the maximum heat release (Figure 11a). With a 10% increase in the amount of helium in the fuel, heat release decreases by 28%. Further increases in the proportion of He result in smaller decreases compared to the reference (pure hydrogen injection). At 20% He addition, a 50% reduction in Qmx was achieved. It is worth mentioning that there was a 30% decrease in heat release from the maximum at He/H\({}_{2}\) = 10/90 to heat release at He/H\({}_{2}\) = 20/80. It is noteworthy that the decreases in heat release caused by increasing the throttle opening (at the same He/H\({}_{2}\) ratio value) are proportional during the combustion of fuels with different helium contents. When analyzing the Qmx values in the prechamber (Figure 11b), lower maximum values are observed (which is due to the flows between the main chamber and the prechamber). With regard to PC, the change in heat release is linear and depends on the proportion of helium in the fuel. With an increased He/H\({}_{2}\) ratio, Qmx in both chambers is similar.