Experimental　study　focusing　on　the　combustion　and　cyclic　variation　characteristics　of　a　pure　hydrogen-fueled　engine　at　idle　and　lean　conditions　was　introduced.　The　experiment　was　carried　out　on　a　modified　four-cylinder　spark-ignition　engine　equipped　with　an　electronic　controlled　hydrogen　injection　system.　The　test　results　showed　that　when　excess　air　ratio　was　varied　within　2.08　to　3.20,　the　engine　original　electronic　control　unit　could　maintain　idle　speed　of　the　pure　hydrogen-fueled　engine　at　its　target　value　(790　r/min)　by　automatic　adjusting　the　spark　timing　and　idle　bypass　valve　opening.　The　engine　cooling　loss　was　decreased　with　the　increase　of　excess　air　ratio.　However,　the　cyclic　variation　in　indicated　mean　effective　pressure　and　combustion　duration　were　slightly　raised　when　the　engine　was　gradually　leaned　out.　When　excess　air　ratio　rose　from　2.08　to　3.20,　the　engine　fuel　energy　flow　rate　was　reduced　by　about　15.4%.