Homogeneous charge compression ignition (HCCI) is a promising concept that can be used to reduce NOx and soot emissions in combustion engines, keeping efficiency as high as for diesel engines. To be able to accurately control the combustion behavior, more information is needed about the auto-ignition of fuels. Many fuels, especially those containing n-paraffins, exhibit pre-reactions before the main heat release event, originating from reactions that are terminated when the temperature in the cylinder reaches a certain temperature level. These pre-reactions are called low temperature heat release (LTHR), and are known to be affected by engine speed. This paper goes through engine speed effects on auto-ignition temperatures and LTHR for primary reference fuels. Earlier studies show effects on both quantity and timing of the low temperature heat release when engine speed is varied. In this study, these effects are further explored by looking at the auto-ignition temperatures and the pressure and temperature evolution in the cylinder.Four primary reference fuels (PRF, blends of n-heptane and iso-octane) were used, from PRF70 to PRF100. All fuels were tested in a CFR engine with variable compression ratio, running in HCCI operation. Engine speed was varied from 600 to 1200 rpm. An equivalence ratio of 0.33 was used, and a constant combustion phasing of 3 degrees after TDC was maintained by changing the compression ratio for each operating point. Different pressure and temperature evolutions were achieved by varying the inlet air temperature in three steps from 50°C to 150 °C.At higher engine speeds the LTHR decreased or disappeared. Auto-ignition temperature increased at higher engine speeds due to the shorter residence time in the LTHR temperature zone. The temperature range where LTHR was detected was shifted to higher temperatures with increased engine speed.