The optimal fuel for partially premixed combustion (PPC) is considered to be a gasoline boiling range fuel with an octane number around 70. Higher octane number fuels are considered problematic with low load and idle conditions. In previous studies mostly the intake air temperature did not exceed 30 °C. Possibly increasing intake air temperatures could extend the load range. In this study primary reference fuels (PRFs), blends of iso-octane and n-heptane, with octane numbers of 70, 80, and 90 are tested in an adapted commercial diesel engine under partially premixed combustion mode to investigate the potential of these higher octane number fuels in low load and idle conditions. During testing combustion phasing and intake air temperature are varied to investigate the combustion and emission characteristics under low load and idle conditions. The results show that PRF70, 80 and 90 present stable combustion when an intake temperature higher than 60 °C is used at low load and idle conditions. The coefficient of variations of the gross indicated mean effective pressure (COVIMEPgross) is below 3% and 4.5% at low load and idle condition respectively, which is well below the 10% limit. PRF90 has positive ignition dwells regardless of combustion phasing and intake temperature for all the measurements, and the oxygen concentration is sufficient at both low load and idle conditions to ensure near zero soot emissions. The difference in ignition delay between PRF 70 and 80 is smaller than that between PRF80 and 90, and PRF70 and 80 display similar emission behavior.