## 5 Conclusions  
Hydrogen-helium mixtures were created by increasing the volume fraction of helium in hydrogen fuel. Combustion experiments were carried out at a constant initial CoC value. Changes in the excess air ratio were realized by increasing the throttle opening (with a constant fuel injection time into the main chamber). The TJI system allowed an He/H\({}_{2}\) mixture to be injected into the main chamber and pure hydrogen into prechamber.  
Based on the research, conclusions were drawn about the engine's performance and the possibility of using hydrogen and helium as a dual fuel:  
* The increase in the proportion of helium in the hydrogen fuel (at the same throttle opening) resulted in a higher excess air ratio. Therefore, the maximum combustion pressure Pmx decreased (due to a reduction in the energy content of the fuel)--Figure 10;
* The average decrease in IMEP is about 10% for every 10% increase in He. It is worth mentioning that \(\lambda\) does not affect the changes in this value--Figure 6a;
* Increasing the proportion of helium in the hydrogen fuel reduces the unevenness of the engine's CoV(IMEP) from 2% to less than 1%;
* Increasing the proportion of He in the fuel significantly reduces the rate of heat release. Raising the percentage of He by 10% reduces dQmx by an average of 20%--Figure 8. This is significantly more than during the IMEP analysis;
* Increasing the proportion of helium by 10% in the hydrogen fuel reduces the maximum heat release in the main chamber by an average of 10% for every 10% increase in the helium content--Figure 9a; for the prechamber, the maximum heat release decreases by an average of 5%--Figure 9b;  
Figure 15: (**a**) Changes in exhaust gas temperature; (**b**) NOx concentration in exhaust gas (n = 1500 rpm; CoC = 8–10 deg aTDC).  
* The analysis shows that with a fixed throttle position and high He/H\({}_{2}\) ratio, the combustion process has to be accelerated (ignition time) to reach CoC = 8-10 deg aTDC;
* Knock combustion only occurs at low helium values in the fuel. In the main chamber, knock combustion does not occur when \(\lambda\) > 1.9 (MAPO < 1 bar). In the prechamber (into which pure hydrogen was injected), the absence of knock combustion was only noted at \(\lambda\) > 2.4;
* A temperature of 325 \({}^{\circ}\)C is the limit below which the concentration of NO\({}_{\rm x}\) in the exhaust gas decreases rapidly. This temperature reduces the NO\({}_{\rm x}\) concentration by more than 80% to a level of 30 ppm. At lower exhaust gas temperatures (with higher \(\lambda\)), the proportion of NO\({}_{\rm x}\) in the exhaust gas was below 10 ppm.  
Conceptualization, D.M. and I.P.; methodology, D.M. and I.P.; software, D.M. and I.P.; validation, D.M. and I.P.; formal analysis D.M. and I.P.; investigation, D.M. and I.P.; resources, D.M. and I.P.; data curation, D.M. and I.P.; writing--original draft preparation, D.M. and I.P.; writing--review and editing, D.M. and I.P.; visualization, D.M. and I.P.; supervision, D.M. and I.P.; project administration, D.M. and I.P.; funding acquisition, D.M. and I.P. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Poznan University of Technology, interdisciplinary grant number 0415/SIGR/7286.  
Not applicable.  
The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding author/s.  
The authors declare no conflicts of interest.