Concerning　the　throttling　loss　under　part　load　conditions,　it　is　feasible　to　further　improve　the　engine　thermal　efficiency　through　operating　the　engine　under　the　unthrottled　condition　and　controlling　its　load　by　changing　the　excess　air　ratio.　However,　the　narrow　flammability　of　ethanol　may　lead　the　ethanol　engine　to　encounter　high　cyclic　variations　under　unthrottled　and　lean　conditions.　The　addition　of　hydrogen　is　potentially　helpful　for　solving　this　problem.　In　this　test,　the　engine　was　run　under　an　speed　of　1400　rpm　and　unthrottled　conditions.　The　hydrogen　volume　fractions　in　the　intake　were　respectively　kept　at　0%　and　3%.　For　a　given　hydrogen　blending　level,　the　ethanol　flow　rate　was　reduced　to　enable　the　engine　to　run　under　lean　conditions.　The　results　showed　that　the　engine　efficiency　was　improved　with　the　blending　of　hydrogen.　The　highest　thermal　efficiency　was　improved　by　6.07%　after　blending　3%　hydrogen　to　the　intake　air.　The　addition　of　hydrogen　could　increase　the　engine　torque　output　at　lean　conditions.　Both　cooling　and　exhaust　losses　were　decreased　after　the　hydrogen　enrichment　while　adopting　the　lean　combustion　strategy.　The　hydrogen　addition　contributed　to　the　extended　lean　burn　limit　and　decreased　cyclic　variation　under　lean　conditions.　HC　and　CO　emissions　were　decreased　whereas　NOx　emissions　were　increased　after　the　blending　of　hydrogen.　(C)　2016　Elsevier　Ltd.　All　rights　reserved.