Producing　the　syngas　by　onboard　ethanol　steam　reforming　is　an　effective　way　for　recovering　the　exhaust　heat　in　the　engine　tailpipe.　Besides,　as　hydrogen　is　contained　in　the　syngas,　the　addition　of　syngas　is　also　capable　of　improving　engine　combustion　and　emissions　characteristics.　In　this　paper,　an　experimental　study　was　carried　out　on　a　four-cylinder　1.6　L　spark-ignited　engine　to　explore　the　effect　of　syngas　addition　on　the　engine　performance.　A　fuel　reforming　reactor　with　the　copper　based　catalysts　was　designed　and　mounted　on　the　engine　tailpipe,　so　that　the　ethanol　solution　could　be　decomposed　to　be　syngas　which　is　mainly　composed　of　hydrogen　and　carbon　monoxide　when　the　catalysts　were　heated　by　the　exhaust　gas.　The　intake　manifolds　was　also　modified　to　permit　syngas　to　be　injected　into　the　fourth　cylinder　of　the　engine.　The　engine　was　run　at　1800　rpm　and　a　manifolds　absolute　pressure　of　61.5　kPa.　The　spark　timing　for　the　maximum　brake　torque　was　adopted　for　each　testing　point.　The　syngas　volume　fraction　in　the　total　intake　gas　was　gradually　increased　from　0%　to　2.43%.　Meanwhile,　the　gasoline　injection　duration　governing　by　a　hybrid　electronic　control　unit　was　adjusted　to　keep　the　excess　air　ratio　of　the　fuel-air　mixture　in　the　fourth　cylinder　at　about　1.00.　The　experimental　results　demonstrated　that　the　syngas　volume　flow　rate　was　markedly　enhanced　from　90　to　240　L/h　when　the　feedstock　flow　rate　was　increased　from　18　to　54　ml/min.　The　peak　ethanol　conversion　efficiency　reached　81.16%　at　a　feedstock　flow　rate　of　36　mL/min.　The　hydrogen　concentration　was　increased　whereas　carbon　monoxide　concentration　was　decreased　in　the　syngas　with　the　increase　of　the　feedstock　supply.　The　engine　indicated　thermal　efficiency　was　raised　to　be　39.01%　at　the　syngas　volume　fraction　of　2.43%.　The　flame　development　and　propagation　durations　were　shortened;　HC　and　NOx　emissions　were　reduced　whereas　CO　emission　was　increased　after　the　syngas　addition　at　the　stoichiometric　condition.　Copyright　(C)　2012,　Hydrogen　Energy　Publications,　LLC.　Published　by　Elsevier　Ltd.　All　rights　reserved.