Because　of　the　increased　fuel-film　effect　and　dropped　combustion　temperature,　spark-ignited　(SI)　gasoline　engines　always　expel　large　amounts　of　HC　and　CO　emissions　during　the　cold　start　period.　This　paper　experimentally　investigated　the　effect　of　hydrogen　addition　on　improving　the　cold　start　performance　of　a　gasoline　engine.　The　test　was　carried　out　on　a　1.6-L,　four-cylinder,　SI　engine　equipped　with　an　electronically　controlled　hydrogen　injection　system.　A　hybrid　electronic　control　unit　(HECU)　was　applied　to　control　the　opening　and　closing　of　hydrogen　and　gasoline　injectors.　Under　the　same　environmental　condition,　the　engine　was　started　with　the　pure　gasoline　and　gasoline-hydrogen　mixture,　respectively.　After　the　addition　of　hydrogen,　gasoline　injection　duration　was　adjusted　to　ensure　the　engine　to　be　started　successfully.　All　cold　start　experiments　were　performed　at　the　same　ambient,　coolant　and　oil　temperatures　of　17　degrees　C.　The　test　results　showed　that　cylinder　and　indicated　mean　effective　pressures　in　the　first　cycle　were　effectively　improved　with　the　increase　of　hydrogen　addition　fraction.　Engine　speed　in　the　first　20　start　cycles　increased　with　hydrogen　blending　ratio.　However,　in　later　cycles,　engine　speed　varied　only　a　little　with　and　without　hydrogen　addition　due　to　the　adoption　of　close　loop　control　on　engine　speed.　Because　of　the　low　ignition　energy　and　high　flame　speed　of　hydrogen,　both　flame　development　and　propagation　durations　were　shortened　after　hydrogen　addition.　HC　and　CO　emissions　were　dropped　markedly　after　hydrogen　addition　due　to　the　enhanced　combustion　process.　When　the　hydrogen　flow　rate　increased　from　0　to　2.5　and　4.3　L/min,　the　instantaneous　peak　HC　emissions　were　sharply　reduced　from　57083　to　17850　and　15738　ppm,　respectively.　NOx　emissions　were　increased　in　the　first　5　s　and　then　reduced　later　after　hydrogen　addition.　Copyright　(C)　2011,　Hydrogen　Energy　Publications,　LLC.　Published　by　Elsevier　Ltd.　All　rights　reserved.