In　order　to　study　factors　affecting　lean　combustion　limit,　an　experimental　study　was　carried　out　on　a　spark-ignited　engine　fueled　with　compressed　natural　gas　and　hydrogen　blends.　Effects　of　ignition　timing,　hydrogen　fraction,　engine　speed,　throttle　opening,　coolant　and　oil　temperature　were　investigated.　Experiments　were　conducted　at　a　low　and　moderate　level　of　engine　load　and　speed　with　a　wide　range　of　hydrogen　fraction,　varying　from　0　to　40　percent　by　volume.　The　results　indicated　that　lean　combustion　limit　could　be　obviously　extended　by　adding　hydrogen　into　compressed　natural　gas.　In　addition,　leaner　but　more　stable　combustion　can　be　acquired　under　higher　throttle　opening　or　lower　engine　speed　conditions.　Ignition　timing　was　studied　separately,　a　series　of　timings　were　adjusted　around　lean　combustion　limit　under　various　conditions.　Experiments　also　demonstrated　that　both　over-advanced　and　over-retarded　ignition　could　lead　to　a　reduction　in　lean　operating　area　of　the　engine.　Further　experiments　on　coolant　and　lubricant　oil　pointed　out　that　lean　combustion　limit　was　positively　correlated　with　coolant　temperature;　while　an　oil　temperature　increase　corresponded　to　an　initial　decrease　followed　by　an　increase　in　lean　combustion　limit.　(C)　2012　Elsevier　Ltd.　All　rights　reserved.