Because　of　the　low　combustion　temperature　and　high　throttling　loss,　SI　(spark-ignited)　engines　always　encounter　dropped　performance　at　low　load　conditions.　This　paper　experimentally　investigated　the　co-effect　of　cylinder　cutoff　and　hydrogen　addition　on　improving　the　performance　of　a　gasoline-fueled　SI　engine.　The　experiment　was　conducted　on　a　modified　four-cylinder　SI　engine　equipped　with　an　electronically　controlled　hydrogen　injection　system　and　a　hybrid　electronic　control　unit.　The　engine　was　run　at　1400　rpm,　34.5　Nm　and　two　cylinder　cutoff　modes　in　which　one　cylinder　and　two　cylinders　were　closed,　respectively.　For　each　cylinder　closing　strategy,　the　hydrogen　energy　fraction　in　the　total　fuel　(beta(H2))　was　increased　from　0%　to　approximately　20%.　The　test　results　demonstrated　that　engine　indicated　thermal　efficiency　was　effectively　improved　after　cylinder　cutoff　and　hydrogen　addition,　which　rose　from　34.6%　of　the　original　engine　to　40.34%　of　the　engine　operating　at　two-cylinder　cutoff　mode　and　beta(H2)　=　20.41%.　Flame　development　and　propagation　periods　were　shortened　with　the　increase　of　the　number　of　closed　cylinders　and　hydrogen　blending　ratio.　The　total　cooling　loss　for　all　working　cylinders,　and　tailpipe　HC　(hydrocarbons),　CO　(carbon　monoxide)　and　CO2　(carbon　dioxide)　emissions　were　reduced　whereas　tailpipe　NOx　(nitrogen　oxide)　emissions　were　increased　after　hydrogen　addition　and　cylinder　closing.　(C)　2010　Elsevier　Ltd.　All　rights　reserved.