## 4 Conclusions  
The analysis of the literature studying hydrogen combustion has resulted in the following conclusions and recommendations:  
* The density of hydrogen is almost an order of magnitude lower than the density of natural gas, whereas the net calorific value per cubic meter is much lower. Thus, while maintaining the parameters of the power plant when switching to hydrogen, it is necessary to pay attention to the flow sections of the fuel supply system, especially to the fuel injectors. * The addition of hydrogen leads to an increased intensity of chemical processes per unit volume in the flame front. This results in the compactification of the flame, a change in its shape, and a decrease in its size. * The adiabatic combustion temperature of hydrogen flame is 5-15% higher than that of methane; however, the operating range of hydrogen covers some lower minimum temperatures of up to 1050 K, compared to \(\approx\)1400-1500 K for methane, due to the high concentration of active radicals (H, HO). * The addition of hydrogen results in a significant increase in the flame propagation speed. The greatest effect is achieved at a hydrogen concentration in the fuel of 65% or more. In this case, the maximum velocity shifts to the region of rich mixtures. On a basis of high hydrogen velocity, the combustion chamber can theoretically be shortened, which will reduce both the engine weight and NOx emissions by cutting the residence time of the air-fuel mixture in the high-temperature zones. * To ensure combustion with no flashbacks, it is necessary to increase the flow rate, which requires an increase in the pressure drop across the flame tube head and, in general, an increase in pressure losses in the combustion chamber. * The ignition delay time of hydrogen is of several orders of magnitude shorter than that of methane. The minimum autoignition temperature shifts to the region of rich mixtures and is approximately 70-150 degrees lower than that of methane. Thus, for combustion chambers with the air temperature T\({}_{\text{k}}\)\(>\) 700 K, it is advisable to use a combustion system without premixing, for example, LDI or a multicluster. * The CO emission increases when hydrogen is added to the fuel, while CO\({}_{2}\) decreases for stoichiometric and rich mixtures. Burning lean mixtures of hydrogen with air allows a reduction in the level of CO and CO\({}_{2}\) emissions compared to the combustion of methane. * The emission of NOx when hydrogen is added to the fuel increases at the same adiabatic temperature of the flame, which indicates different mechanisms for the formation of nitrogen oxides during the combustion of methane and methane-hydrogen mixtures. However, nitrogen oxide emissions can be achieved by lowering the combustion temperature. * An increase in the concentration of hydrogen in the mixtures with methane and natural gas is accompanied by an increase in resistance to flame blowout but increases the tendency of the mixture to flashback [58]. * The change in the amplitude of pulsations with increased concentration of hydrogen in the fuel is determined by the initial mechanism of their formation. In this case, the pulsation frequency usually increases. * As a rule, the effect of increased pressure is more pronounced for the hydrogen-air flame than for the methane-air flame, which complicates the process of interpolating the data obtained under atmospheric conditions for the engine parameters. Thus, further research into the influence of pressure on the combustion of hydrogen presents a crucial task. Supervision, E.A.S. ; Writing--original draft, I.A.Z., A.Y.K. and M.H.M. ; Writing--review & editing, D.V.A., P.A.S., A.G.U. and V.A.D. ; Visualization, K.D.T. and D.V.Y. All authors have read and agreed to the published version of the manuscript.