Spark-ignited　engines　always　suffer　high　specific　fuel　consumption,　emissions　and　cyclic　variation　at　idle　and　lean　conditions.　An　experiemental　investigation　aiming　at　extending　the　engine　improving　the　engine　economic　and　emissions　performance　with　hydrogen　addition　at　idle　conditions　was　carried　out　on　a　4-cylinder　spark-ignited　(SI)　gasoline　engine.　The　engine　was　modified　to　permit　hydrogen,and　gasoline　to　be　it　injected　into　the　intake　ports　simultaneously　to　realize　a　hybrid　hydrogen-gasoline　engine　(HHGE),　The　hydrogen　and　gasoline　flow　rates　were　governed　by　a　hybrid　electronic　control　unit　(HECU).　Three　hydrogen　volumetric　fractions　of　0,　3,　and　6%　in　the　intake　were　applied　to　investigate　the　effects　of　hydrogen　addition　on　engine　thermal　efficiency,　combustion　duration,　cyclic　variation,　and　emissions　at　idle　conditions　using　a　fixed　spark　advance.　Under　each　hydrogen　enrichment　level,　the　engine　was　gradually　leaned　by　reducing　the　gasoline　now　rate　until　the　coefficient　of　cyclic　variation　in　indicated　mean　effective　pressure　(1mep)　was　above　10%.　The　test　results　showed　that,　with　the　increase　of　hydrogen　enrichment　level,　engine-indicated　thermal　efficiency　was　improved,　and　the　lean　burn　limit　was　extended.　The　peak　in-cylinder　temperature　and　in-cylinder　temperature　at　exhaust　valve　opening　decrease　with　the　increase　of　excess　air　ratio　and　hydrogen　blending　level,　For　a　specified　excess　air　ratio,　HHGEs　tend　to　get　high　residual　gas　fraction　compared　with　tile　original　one.　The　combustion　duration　was　gradually　shortened　with　the　increase　of　hydrogen　addition　level　at　diluent　conditions.　Moreover,　CO　and　NO.,　emissions　were　reduced　at　lean　conditions　for　the　HHGE.,　RC　emissions　from　tile　HHGE　were　also　lower　than　tile　original　one　at　all　excess　air　ratios.