Hydrogen　is　the　most　promising　alternative　fuel　for　spark-ignited　engines.　This　paper　experimentally　investigated　the　performance　of　a　pure　hydrogen-fueled　SI　engine　at　idle　and　lean　conditions.　The　experiment　was　carried　out　on　a　four-cylinder　gasoline-fueled　SI　engine　equipped　with　an　electronically　controlled　hydrogen　port-injection　system　and　a　hybrid　electronic　control　unit　which　was　used　to　govern　the　hydrogen　injection　duration.　The　engine　original　electronic　control　unit　was　used　to　adjust　the　opening　of　idle　bypass　valve　and　spark　timing　to　enable　the　engine　to　be　run　around　its　target　idle　speed.　The　test　results　showed　that　the　fuel　energy　flow　rate　was　reduced　with　the　increase　of　excess　air　ratio　for　the　pure　hydrogen-fueled　engine　at　idle　and　lean　conditions.　When　excess　air　ratio　increased　from　2.08　to　3.2,　the　hydrogen　energy　flow　rate　was　decreased　from　11.79　to　9.97MJ/h.　Both　the　flame　development　and　propagation　periods　were　increased　with　the　increase　of　excess　air　ratio.　Because　of　the　increased　opening　of　idle　bypass　valve　and　dropped　cylinder　temperature,　both　pumping　and　cooling　losses　were　reduced　when　the　engine　was　leaned　out.　NOx　and　CO　emissions　were　negligible,　but　HC　and　CO2　were　still　existed　for　the　pure　hydrogen-fueled　SI　engine　due　to　the　possible　burning　of　the　blow-by　lubricant　oil　gas.　Copyright　(c)　2013　John　Wiley　&　Sons,　Ltd.