Rotary　engine　has　flat　chamber　and　longs　for　fuel　with　high　flame　speed　and　small　quenching　distance.　Hydrogen　has　many　excellent　characteristics　that　are　suitable　for　the　rotary　engine.　In　this　paper,　the　performance　of　a　rotary　engine　fueled　with　pure　hydrogen　at　different　excess　air　ratios　was　experimentally　investigated.　The　investigation　was　carried　out　on　a　single-rotor　hydrogen-fueled　rotary　engine　equipped　with　port　fuel　injection　system.　An　online　electronic　control　module　was　used　to　govern　the　hydrogen　injection　duration　and　excess　air　ratio.　In　this　study,　the　engine　was　operating　at　the　idle　speed　of　3000　rpm　and　different　excess　air　ratios　varied　from　0.993　to　1.283.　The　test　results　demonstrated　that　the　fuel　energy　flow　rate　of　the　hydrogen　rotary　engine　and　engine　stability　were　reduced　with　the　increase　of　excess　air　ratio.　When　the　excess　air　ratio　increased　from　0.993　to　1.283,　the　hydrogen　energy　flow　rate　was　decreased　from　14.91　to　11.55　MJ/h.　Both　the　flame　development　and　propagation　periods　were　increased　with　excess　air　ratio.　CO　emission　was　negligible,　but　HC,　CO2　and　NOx　emissions　were　still　detected　due　to　the　evaporation　and　possible　burning　of　the　lubrication-used　gasoline,　and　oxidation　reaction　of　nitrogen　of　the　intake　air.　(C)　2017　Hydrogen　Energy　Publications　LLC.　Published　by　Elsevier　Ltd.　All　rights　reserved.