## 2 Combustion of Hydrogen in an Internal Combustion Engine  
### Combustion of Pure Hydrogen in an Internal Combustion Engine  
Hydrogen is the most common element on earth. The small and light hydrogen molecule is very mobile (high mass diffusivity) and leads to a very low atmospheric density. Hydrogen as an engine fuel has a wide flammability range falling within the excess air ratio of 0.14 to 10 [5]. The advantage of the hydrogen-air mixture is the low minimum energy required to ignite it under atmospheric conditions, as low as 0.017 mJ at \(\lambda\) = 1.2-1.5 [5]. This value is an order of magnitude less than the energy required to ignite a mixture of air and iso-octane or methane. An important property of hydrogen in a stoichiometric fuel-air mixture is that it has a large volume fraction of 29.5%. This value is greater than the volume fraction of methane in the mixture with air, which is only 9.5%. An important aspect of determining the safe use of all fuels is their auto-ignition temperature. Hydrogen has an auto-ignition limit located in the temperature range from about 773 K (500 \({}^{\circ}\)C) to about 858 K (585 \({}^{\circ}\)C) [6]. The mass density of hydrogen is much lower than that of gasoline and methane; at a pressure of 1 atm and at a temperature of 0 \({}^{\circ}\)C, it is 0.09 kg/Nm\({}^{3}\). This relation means that hydrogen accumulates 2.6 times more energy per unit mass than petrol. It is important to note that the flame propagation velocity during the combustion of a hydrogen-air mixture is an order of magnitude greater than the flame propagation velocity occurring due to the combustion of petrol-air [7].  
The properties of hydrogen make it beneficial for use in modern internal combustion engines with two-stage turbulent combustion of the fuel-air mixture. A comparison of the single and split injection of pure hydrogen into a TJI engine with a passive prechamber was undertaken by [PERSON] et al. [8]. During the study, it was observed that, as the start of hydrogen injection was delayed, both the average useful pressure and the thermal efficiency of the engine gradually increased. The maximum values for BMEP and brake thermal efficiency were 3.2 bar and 33.4%. According to CFD simulations carried out by [PERSON] et al. [9], high thermal efficiency of the engine can be achieved while burning an ultra-lean hydrogen-air mixture. This is due to the high rate of flame propagation that occurs during hydrogen combustion.  
Optimum thermodynamic parameters are possible to achieve during the combustion of lean hydrogen-air mixtures, but the formation of nitrogen oxides is an obstacle to the use of this fuel. The solution to this problem was undertaken by [PERSON] et al. [10] by investigating the effect of using two selective catalytic reactors to reduce NO\({}_{\rm x}\) emissions in a hydrogen-fueled engine. Two catalyst reactors were used in the study, copper-zeolite and VWT (V\({}_{2}\)O\({}_{5}\)--WO\({}_{3}\)--TiO\({}_{3}\)). The results show that the use of a VWT-based catalyst reactor in the first position and a Cu-based catalyst reactor in the second position effectively reduces N\({}_{2}\)O emissions, at a similar NO\({}_{\rm x}\) conversion rate. An alternative solution to this problem may be to use hydrogen with helium as a dual-component fuel.