For　a　fuel　system　with　a　tangent　cam　or　a　constant-velocity　cam,　the　peak　injection　pressure　continues　to　rise　as　the　injection　duration　increases,　but　overly　high　peak　pressures　induce　mechanical　loads　and　wear,　limiting　the　maximum　engine　speed　and　injection　quantity.　To　improve　the　performance　of　an　EUP　(Electronic　Unit　Pump)　fuel　system　for　heavy-duty　diesel　engines,　this　work　proposes　a　new　pump　cam,　namely　the　constant-pressure　cam.　It　helps　the　EUP　run　at　a　higher　speed　and　deliver　larger　fuel　quantities　while　maintaining　a　constant　peak　injection　pressure,　which　improves　the　power　of　the　heavy-duty　diesel　engine.　A　model　based　on　the　EUP　was　built　to　determine　the　three　constraints　for　optimising　the　constant-pressure　cam:　1)　the　pump　pressure　should　equal　the　nozzle　pressure;　2)　the　cam　speed　should　decrease　with　the　increase　in　the　injection　duration;　and　3)　the　cam　acceleration　gradient　should　be　zero.　An　EUP　system　was　tested　with　the　tangent　cam　and　the　optimised　cam　under　different　conditions.　The　experimental　tesults　show　that　the　EUP　system　with　the　optimised　cam　delivers　more　injection　quantity　and　runs　at　higher　engine　speeds　while　maintaining　the　same　peak　pressure　as　the　tangent　cam.　(C)　2015　Elsevier　Ltd.　All　rights　reserved.