## 4 Evaluation of Hydrogen Combustion with the Addition of Helium
### Knock Combustion  
Knock combustion must be considered during hydrogen combustion, especially for stoichiometric fuel-air mixtures. In the current research, the minimum value of the excess air factor is \(\lambda\geq 1.45\); however, as found in many publications [27; 28], even such values cause knock combustion in both (or one) combustion chambers.  
Knock combustion is determined using the MAPO method (Section 3.4) and is graphically represented in Figure 13. From the presented results, it is possible to identify the areas of the highest intensity of knock combustion. It occurs during the combustion of pure hydrogen at low throttle opening (up to 20%) and an excess air ratio. The MAPO limit, above which knock combustion occurs, is 1 bar. This limit was determined on the basis of research work [16; 28; 29]. This is a subjective value, which depends on the engine's displacement or load. The assumption of a limit value of 1 bar indicates the existence of knock combustion also for a 10% helium content in the fuel. This occurs only at the lowest throttle opening (TH = 15%). In general, knock combustion occurs when the engine is running up to \(\lambda<1.9\) (based on Figure 13a). Similar conclusions were obtained by other researchers determining the limit of knock combustion in this range of an excess air ratio [27].  
A characteristic feature is the occurrence of knock combustion in the prechamber (Figure 13b). The range of its occurrence is much larger. In this range, knock combustion was recorded not only for 10% helium but also at much higher contents--20% (for qo_PC = 8 ms). Such a relationship is due to the fact that pure hydrogen was injected into the prechamber. Its high concentration in the prechamber, with a low excess air ratio, leads to knock combustion.  
It is worth mentioning that in both chambers, when using a passive prechamber (qo_PC = 0 ms), knock combustion did not occur (MAPO < 1 bar).  
Based on Figure 13 and knowing that 100 engine cycles were recorded at each test point, the probability of knock combustion was determined. It was defined as the number of knock combustion cycles in relation to 100 engine cycles analyzed.  
Figure 12: Heat release rate: (**a**) in MC; (**b**) in PC (n = 1500 rpm; CoC = 8–10 deg aTDC).  
The results are shown in Figure 14. An analysis of the data in Figure 13 shows that knock combustion is more probable in the prechamber than in the main chamber. The results in Figure 14 relate to the number of knock combustion cycles, while the results in Figure 13 relate to the intensity of knock combustion. Therefore, these figures cannot be directly compared. The data in Figure 14a show that during pure hydrogen combustion at the lowest throttle opening (TH = 15%), the probability of knock combustion in the main chamber is above 80%. This means that the knock occurred in more than 80 out of 100 cycles. The addition of helium reduces this probability to an average value of about 70%. This means that although the intensity of knock combustion does not exceed 1 bar, the possibility of it occurring is not zero. In the range of \(\lambda\) = 2.0, the probability of the knock combustion is up to 20%, and the data in Figure 13a show that MAPO \(<<\) 1 bar.  
An analysis of the probability of the knock combustion in the prechamber (Figure 14b) shows much larger ranges of its occurrence. Even when \(\lambda\) = 1.9, the knock combustion can occur in the prechamber in half of the engine cycles. When \(\lambda\) > 2.4, the unfavorable combustion is practically non-existent.  
Figure 14: Percentage of knock combustion cycles depending on \(\lambda\): (**a**) in MC; (**b**) in PC (n = 1500 rpm; CoC = 8–10 deg aTDC).  
Figure 13: MAPO variation (knock combustion) depending on \(\lambda\): (**a**) in MC; (**b**) in PC (n = 1500 rpm; CoC = 8–10 deg aTDC).