Lean　combustion　is　an　effective　way　for　improving　the　spark-ignited　(SI)　engine　performance.　unfortunately,　due　to　the　narrow　flammability　of　gasoline,　the　pure　gasoline-fueled　engines　sometimes　suffer　partial　burning　or　misfire　at　very　lean　conditions.　Hydrogen　has　many　excellent　combustion　properties　that　can　be　used　to　extend　the　gasoline　engine　lean　bum　limit　and　improve　the　gasoline　engine　performance　at　lean　conditions.　In　this　paper,　a　1.6　L　port　fuel　injection　gasoline　engine　was　modified　to　be　a　hybrid　hydrogen-gasoline　engine　(HHGE)　fueled　with　the　hydrogen-gasoline　mixture　by　mounting　an　electronically　controlled　hydrogen　injection　system　on　the　intake　manifolds　while　keeping　the　original　gasoline　injection　system　unchanged.　A　self-developed　hybrid　electronic　control　unit　(HECU)　was　used　to　flexibly　adjust　injection　timings　and　durations　of　gasoline　and　hydrogen.　Engine　tests　were　conducted　at　1400　rpm　and　a　manifolds　absolute　pressure　(MAP)　of　61.5　kPa　to　investigate　the　performance　of　an　HHGE　at　lean　burn　limits.　Three　hydrogen　volume　fractions　in　the　total　intake　gas　of　1%,　3%　and　4.5%　were　adopted.　For　a　specified　hydrogen　volume　fraction,　the　gasoline　flow　rate　was　gradually　reduced　until　the　engine　reached　the　lean　burn　limit　at　which　the　coefficient　of　variation　in　indicated　mean　effective　pressure　(COVimep)　was　10%.　The　test　results　showed　that　COVimep　at　the　same　excess　air　ratio　was　obviously　reduced　with　the　increase　of　hydrogen　enrichment　level.　The　excess　air　ratio　at　the　lean　bum　limit　was　extended　from　1.45　of　the　original　engine　to　2.55　of　the　4.5%　HHGE.　The　engine　brake　thermal　efficiency,　CO,　HC　and　NO(x)　emissions　at　lean　burn　limits　were　also　improved　for　the　HHGE.　(C)　2009　Professor　T.　Nejat　Veziroglu.　Published　by　Elsevier　Ltd.　All　rights　reserved.