Regarding　the　limited　fossil　fuel　reserves,　the　renewable　ethanol　has　been　considered　as　one　of　the　substitutional　fuels　for　spark　ignition　(SI)　engines.　But　due　to　its　high　latent　heat,　ethanol　is　usually　hard　to　be　well　evaporated　to　form　the　homogeneous　fuel-air　mixture　at　low　temperatures,　e.g.,　at　idle　condition.　Compared　with　ethanol,　hydrogen　possesses　many　unique　combustion　and　physicochemical　properties　that　help　improve　combustion　process.　In　this　paper,　the　performance　of　a　hydrogen-enriched　SI　ethanol　engine　under　idle　and　stoichiometric　conditions　was　investigated.　The　experiment　was　performed　on　a　modified　1.6　L　SI　engine　equipped　with　a　hydrogen　port-injection　system.　The　ethanol　was　injected　into　the　intake　ports　using　the　original　engine　gasoline　injection　system.　A　self-developed　hybrid　electronic　control　unit　(HECU)　was　adopted　to　govern　the　opening　and　closing　of　hydrogen　and　ethanol　injectors.　The　spark　timing　and　idle　bypass　valve　opening　were　governed　by　the　engine　original　electronic　control　unit　(OECU),　so　that　the　engine　could　operate　under　its　original　target　idle　speed　for　each　testing　point.　The　engine　was　first　fueled　with　the　pure　ethanol　and　then　hydrogen　volume　fraction　in　the　total　intake　gas　was　gradually　increased　through　increasing　hydrogen　injection　duration.　For　a　specified　hydrogen　addition　level,　ethanol　flow　rate　was　reduced　to　keep　the　hydrogen-ethanol-air　mixture　at　stoichiometric　condition.　The　test　results　showed　that　hydrogen　addition　was　effective　on　reducing　cyclic　variations　and　improving　indicated　thermal　efficiency　of　an　ethanol　engine　at　idle.　The　fuel　energy　flow　rate　was　reduced　by　20%　when　hydrogen　volume　fraction　in　the　intake　rose　from　0%　to　6.38%.　Both　flame　development　and　propagation　periods　were　shortened　with　the　increase　of　hydrogen　blending　ratio.　The　heat　transfer　to　the　coolant　was　decreased　and　the　degree　of　constant　volume　combustion　was　enhanced　after　hydrogen　addition.　HC　and　CO　emissions　were　first　reduced　and　then　increased　with　the　increase　of　hydrogen　blending　fraction.　The　acetaldehyde　emission　from　the　hydrogen-enriched　ethanol　engine　is　lower　than　that　from　the　pure　ethanol　engine.　However,　the　addition　of　hydrogen　tended　to　increase　NOx　emissions　from　the　ethanol　engine　at　idle　and　stoichiometric　conditions.　(C)　2010　Professor　T.　Nejat　Veziroglu.　Published　by　Elsevier　Ltd.　All　rights　reserved.