Abstract:　Numerous　experimental　works　concerning　the　comparative　analysis　of　heat　transfer　processes　in　hydrogen　and　gasoline　engines　have　confirmed　the　noticeable　increase　in　heat　loss　in　the　case　of　applying　hydrogen.　Without　denying　the　influence　of　higher　values　of　the　flame　propagation　velocity　and　the　hydrogen　combustion　temperature　contributing　to　intensification　of　the　convective　heat　transfer　in　the　combustion　chamber,　the　present　article　shows　that,　when　using　hydrogen　as　a　fuel,　the　main　problem　consists　in　penetration　of　the　flame　into　the　gap　between　the　heat　belt　of　the　piston　and　the　sleeve.　We　analyze　this　phenomenon　using　the　flame　extinguishing　theory　by　Ya.B.　Zeldovich.　On　the　base　of　the　unsteady　heat　flux　measurements　in　the　gap,　as　well　as　of　3D　simulation　of　both　the　thermophysical　processes　in　the　combustion　chamber　and　the　thermal　state　of　the　piston　of　the　experimental　gasoline　engine　converted　to　hydrogen,　we　explain　the　phenomenon　of　relative　increase　in　heat　losses　in　the　hydrogen　engine,　its　existence　previously　having　been　shown　experimentally　without　theoretical　justification.　©　2022,　Allerton　Press,　Inc.