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Some fuels with the same research octane number (RON) have different HCCI engine performance. Therefore RON alone cannot be used for determining auto-ignition in HCCI combustion. The current research focuses on creating an HCCI fuel index suitable for comparing different fuels for HCCI operation. More thorough studies are needed to map the fuel effects. One way to characterize a fuel is by using the Auto-Ignition Temperature (AIT). The AIT and the amount of Low Temperature Heat Release (LTHR) together describe the auto-ignition properties of the fuel. Both can be extracted from the pressure trace. The assumption is that the pressure and temperature are known at inlet valve closing (IVC) and that the mass in the cylinder does not change after IVC. The purpose of this study was to map the AIT of different Primary Reference Fuels (PRF) for HCCI combustion at different cylinder pressures.

An index to relate fuel properties to HCCI auto-ignition would be valuable to predict the performance of fuels in HCCI engines from their properties and composition. The indices for SI engines, the Research Octane Number (RON) and Motor Octane Number (MON) are known to be insufficient to explain the behavior of oxygenated fuels in an HCCI engine. One way to characterize a fuel is to use the Auto-Ignition Temperature (AIT). The AIT can be extracted from the pressure trace. Another potentially interesting parameter is the amount of Low Temperature Heat Release (LTHR) that is closely connected to the ignition properties of the fuel. A systematic study of fuels consisting of gasoline surrogate components of n-heptane, iso-octane, toluene, and ethanol was made. 21 fuels were prepared with RON values ranging from 67 to 97.

HCCI combustion can be enabled by many types of liquid and gaseous fuels. When considering what fuels will be most suitable, the emissions also have to be taken into account. This study focuses on the emissions formation originating from different fuel components. A systematic study of over 40 different gasoline surrogate fuels was made. All fuels were studied in a CFR engine running in HCCI operation. Many of the fuels were blended to achieve similar RON's and MON's as gasoline fuels, and the components (n-heptane, iso-octane, toluene, and ethanol) were chosen to represent the most important in gasoline; nparaffins, iso-paraffins, aromatics and oxygenates. The inlet air temperature was varied from 50°C to 150°C to study the effects on the emissions. The compression ratio was adjusted for each operating point to achieve combustion 3 degrees after TDC. The engine was run at an engine speed of 600 rpm, with ambient intake air pressure and with an equivalence ratio of 0.33.