Ammonia　and　hydrogen　are　two　carbon-free　alternative　fuels　for　engines.　Considering　that　there　are　few　studies　on　improving　an　ammonia-hydrogen　engine＇s　lean　combustion　performance,　this　investigation　based　on　an　engine　with　the　Miller　cycle　employs　hydrogen　direct　injection　(HDI)　and　NH3　port　injection　and　then　proposes　an　adjustment　strategy　to　vary　the　start　of　injection　(SOI)　of　HDI　for　affecting　the　engine　combustion　process.　When　ammonia　volume　share　(AVS)　is　greater　than　50%,　the　engine　shows　poor　performance.　Nevertheless,　the　stratification　effect　of　hydrogen　jets　may　be　used　to　elevate　the　hydrogen　concentration　around　spark　plug　by　properly　delaying　SOI　and　then　creating　the　locally　rich　mixture　with　a　high　hydrogen　share,　which　prominently　promotes　the　flame　kernel　formation,　shortens　CA0-90,　reduces　CoVPmax,　and　enables　the　engine　to　reach　approximately　36%　BTE.　It　is　notable　that　very　late　SOI　can　slightly　deteriorate　the　heat　release　process　and　lower　engine　performance.(c)　2023　Hydrogen　Energy　Publications　LLC.　Published　by　Elsevier　Ltd.　All　rights　reserved.