Hydrogen　has　many　excellent　combustion　properties　that　can　be　used　for　improving　combustion　and　emissions　performance　of　gasoline-fueled　spark　ignition　(SI)　engines.　In　this　paper,　an　experimental　study　was　carried　out　on　a　four-cylinder　1.6　L　engine　to　explore　the　effect　of　hydrogen　addition　on　enhancing　the　engine　lean　operating　performance.　The　engine　was　modified　to　realize　hydrogen　port　injection　by　installing　four　hydrogen　injectors　in　the　intake　manifolds.　The　injection　timings　and　durations　of　hydrogen　and　gasoline　were　governed　by　a　self-developed　electronic　control　unit　(DECU)　according　to　the　commands　from　a　calibration　computer.　The　engine　was　run　at　1400　rpm,　a　manifold　absolute　pressure　(MAP)　of　61.5　kPa　and　various　excess　air　ratios.　Two　hydrogen　volume　fractions　in　the　total　intake　of　3%　and　6%　were　applied　to　check　the　effect　of　hydrogen　addition　fraction　on　engine　combustion.　The　test　results　showed　that　brake　thermal　efficiency　was　improved　and　kept　roughly　constant　in　a　wide　range　of　excess　air　ratio　after　hydrogen　addition,　the　maximum　brake　thermal　efficiency　was　increased　from　26.37%　of　the　original　engine　to　31.56%　of　the　engine　with　a　6%　hydrogen　blending　level.　However,　brake　mean　effective　pressure　(Bmep)　was　decreased　by　hydrogen　addition　at　stoichiometric　conditions,　but　when　the　engine　was　further　leaned　out　Bmep　increased　with　the　increase　of　hydrogen　addition　fraction.　The　flame　development　and　propagation　durations,　cyclic　variation,　HC　and　CO2　emissions　were　reduced　with　hydrogen　addition.　When　excess　air　ratio　was　approaching　stoichiometric　conditions,　CO　emission　tended　to　increase　with　the　addition　of　hydrogen.　However,　when　the　engine　was　gradually　leaned　out,　CO　emission　from　the　hydrogen-enriched　engine　was　lower　than　the　original　one.　NOx　emissions　increased　with　the　increase　of　hydrogen　addition　due　to　the　raised　cylinder　temperature.　(C)　2009　Professor　T.　Nejat　Veziroglu.　Published　by　Elsevier　Ltd.　All　rights　reserved.