An HCCI combustion has a low temperature heat release (LTHR) and a high temperature heat release (HTHR). During the LTHR period, fuel chemicals break down into radicals and small hydrocarbons, and they assist an initial reaction of HTHR. This is an important role of LTHR. On the contrary, LTHR has a negative aspect. In general, a heating value of LTHR changes depending on HCCI engine load due to the difference of the injected fuel quantity. The heating value of LTHR is low under low load condition, and the heating value of LTHR is high under high load condition. This leads to the changes of the starting crank angle of HTHR against engine load and it is a nuisance problem for the control of HCCI engine operation. Therefore, a fuel which exhibits the constant LTHR phasing against engine load would be preferable. It was reported that the n-heptane and toluene blended fuel exhibited the dual phase high temperature heat release (DP-HTHR) combustion and some fuels exhibited the uniform LTHR phasing against engine load in previous SAE papers [
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,
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]. The LTHR had been expected to have become “saturated” at a certain amount of heat release, but the details were uncertain. Further research has been reported in this paper. Seven different surrogate fuels (G01-G07), which had the same research octane number of 75, were blended from n-heptane, iso-octane and toluene, by changing the blending ratio of those three chemicals. An effect of fuel components on HCCI engine operational range was investigated from the point of maximum pressure rise rate. Then, the HCCI engine was re-operated in a 390-450 kPa IMEP range with G01-G07 fuels. Those data were integrated, and the stabilizations of LTHR and HTHR with the dosage of toluene in fuel were investigated from the points of engine load (IMEP) and maximum pressure rise rate. It was found that the dosage of toluene in fuel had two effects on HTHR combustion stability against engine load. One effect was the stabilization of LTHR and the other effect was the stabilization of the combustion period against engine load. To investigate the effects of combustion period and CA50 on HTHR phasing and maximum pressure rise rate, the engine was operated with G07, NMP85, NTL70 and NCP75 fuels at a very similar IMEP and CA50 condition. The test results showed that the maximum pressure rise rate was changed by the CA50 and combustion period as well.