With　regard　to　the　improvement　of　efficiency,　combustion　stability,　and　emissions　in　a　gasoline　engine　at　idle　condition,　an　experimental　study　aimed　at　improving　engine　idle　performance　through　hydrogen　addition　was　carried　out　on　a　4-cylinder　gasoline-fueled　spark　ignited　(SI)　engine.　The　engine　was　modified　to　be　fueled　with　the　mixture　of　gasoline　and　hydrogen　injected　into　the　intake　ports　simultaneously.　A　self-developed　electronic　control　unit　(DECU)　was　dedicatedly　used　to　control　the　injection　timings　and　injection　durations　of　gasoline　and　hydrogen.　Other　parameters,　such　as　spark　timing　and　idle　valve　opening,　were　controlled　by　the　original　engine　electronic　control　unit　(OECU).　Various　hydrogen　enrichment　levels　were　selected　to　investigate　the　effect　of　hydrogen　addition　on　engine　speed　fluctuation,　thermal　efficiency,　combustion　characteristics,　cyclic　variation　and　emissions　under　idle　and　stoichiometric　conditions.　The　experimental　results　showed　that　thermal　efficiency,　combustion　performance,　NOx　emissions　are　improved　with　the　increase　of　hydrogen　addition　level.　The　HC　and　CO　emissions　first　decrease　with　the　increasing　hydrogen　enrichment　level,　but　when　hydrogen　energy　fraction　exceeds　14.44%,　it　begins　to　increase　again　at　idle　and　stoichiometric　conditions.　(C)　2009　International　Association　for　Hydrogen　Energy.　Published　by　Elsevier　Ltd.　All　rights　reserved.