To　reduce　the　fuel　consumption　and　emissions　of　spark-ignited　(SI)　engines,　hydrogen　enrichment　was　used　to　improve　the　performance　of　a　lean　burn　SI　engine　operating　at　low　speed　and　load　conditions.　A　hydrogen　port-injection　system　was　mounted　on　the　intake　manifolds　to　introduce　hydrogen　into　the　intake　ports　sequentially　while　keeping　the　original　gasoline　injection　system　unchanged.　A　hybrid　electronic　control　unit　(HECU)　was　adopted　to　control　injection　timings　and　durations　of　gasoline　and　hydrogen,　accomplishing　four　excess　air　ratios　of　1.00,　1.18,　1.43　and　1.67　and　three　hydrogen　volume　fractions　in　the　intake　of　3%,　5%,　8%.　The　experimental　results　showed　that　engine　brake　thermal　efficiency　and　torque　output　were　increased,　combustion　durations　were　shortened,　cyclic　variation　and　HC　emissions　were　reduced,　but　NO(x)　emissions　were　increased　with　the　increase　of　hydrogen　addition.　CO　emission　was　also　reduced　under　lean　conditions　with　hydrogen　enrichment.　(C)　2010　Elsevier　Ltd.　All　rights　reserved.