The　addition　of　hydrogen　is　an　effective　way　for　improving　the　gasoline　engine　performance　at　lean　conditions.　In　this　paper,　an　experiment　aiming　at　studying　the　effect　of　hydrogen　addition　on　combustion　and　emissions　characteristics　of　a　spark-ignited　(SI)　gasoline　engine　under　various　loads　and　lean　conditions　was　carried　out.　An　electronically　controlled　hydrogen　port-injection　system　was　added　to　the　original　engine　while　keeping　the　gasoline　injection　system　unchanged.　A　hybrid　electronic　control　unit　was　developed　and　applied　to　govern　the　spark　timings,　injection　timings　and　durations　of　hydrogen　and　gasoline.　The　test　was　performed　at　a　constant　engine　speed　of　1400　rpm,　which　could　represent　the　engine　speed　in　the　typical　city-driving　conditions　with　a　heavy　traffic.　Two　hydrogen　volume　fractions　in　the　total　intake　of　0%　and　3%　were　achieved　through　adjusting　the　hydrogen　injection　duration　according　to　the　air　flow　rate.　At　a　specified　hydrogen　addition　level,　gasoline　flow　rate　was　decreased　to　ensure　that　the　excess　air　ratios　were　kept　at　1.2　and　1.4,　respectively.　For　a　given　hydrogen　blending　fraction　and　excess　air　ratio,　the　engine　load,　which　was　represented　by　the　intake　manifolds　absolute　pressure　(MAP),　was　increased　by　increasing　the　opening　of　the　throttle　valve.　The　spark　timing　for　maximum　brake　torque　(MBT)　was　adopted　for　all　tests.　The　experimental　results　demonstrated　that　the　engine　brake　mean　effective　pressure　(Bmep)　was　increased　after　hydrogen　addition　only　at　low　load　conditions.　However,　at　high　engine　loads,　the　hybrid　hydrogen　gasoline　engine　(HHGE)　produced　smaller　Bmep　than　the　original　engine.　The　engine　brake　thermal　efficiency　was　distinctly　raised　with　the　increase　of　MAP　for　both　the　original　engine　and　the　HHGE.　The　coefficient　of　variation　in　indicated　mean　effective　pressure　(COVimep)　for　the　HHGE　was　reduced　with　the　increase　of　engine　load.　The　addition　of　hydrogen　was　effective　on　improving　gasoline　engine　operating　instability　at　low　load　and　lean　conditions.　HC　and　CO　emissions　were　decreased　and　NOx　emissions　were　increased　with　the　increase　of　engine　load.　The　influence　of　engine　load　on　CO(2)　emission　was　insignificant.　All　in　all,　the　effect　of　hydrogen　addition　on　improving　engine　combustion　and　emissions　performance　was　more　pronounced　at　low　loads　than　at　high　loads.　(C)　2010　Professor　T.　Nejat　Veziroglu.　Published　by　Elsevier　Ltd.　All　rights　reserved.