A　throttle　can　be　installed　on　the　intake　pipe　of　a　natural　gas　(NG)/diesel　dual-fuel　engine　to　control　the　excess　air　ratio　of　the　air-fuel　mixture　by　adjusting　the　air　intake.　Building　on　a　previously　proposed　NG/diesel　dual-fuel　supply　strategy　using　the　adjustment　of　excess　air　ratio,　this　work　further　studied　the　effects　of　different　injection　timing　schemes　on　output　power,　fuel　efficiency,　and　pollutant　emissions　of　a　dual-fuel　engine　under　low　to　medium　load　conditions.　In　the　experiment,　the　engine　was　operated　at　a　speed　of　1600　r/min,　under　either　low　(27.1　Nm)　or　medium　(50.6　Nm)　loads,　and　the　NG　substitution　rate　was　either　40%,　60%,　or　80%.　The　effect　of　different　injection　timing　schemes　on　the　combustion　performance　of　the　engine　under　low　to　medium　load　conditions　was　studied　based　on　in-cylinder　pressure　changes　detected　by　a　pressure　sensor.　Experimental　results　showed　that　under　medium-speed　low-load　conditions　and　a　NG　substitution　rate　of　40%,　setting　the　diesel　injection　timing　to　27　degrees　CA　BTDC　increased　the　engine　output　power　by　9.03%,　reduced　the　brake　specific　energy　consumption　(BSEC)　by　13.33%,　and　effectively　reduced　CO,　CO2,　and　HC　emissions.　Under　medium-speed　medium-load　conditions　with　a　NG　substitution　rate　of　80%,　setting　the　diesel　injection　timing　to　25　degrees　CA　BTDC　increased　the　engine　output　power　by　14.62%,　reduced　the　BSEC　by　11.73%,　and　significantly　reduced　CO,　CO2,　and　HC　emissions.