Pressure Sensitivity of HCCI Auto-Ignition Temperature for Primary Reference Fuels

Some fuels with the same research octane number (RON) havedifferent HCCI engine performance. Therefore RON alone cannot beused for determining auto-ignition in HCCI combustion. The currentresearch focuses on creating an HCCI fuel index suitable forcomparing different fuels for HCCI operation. More thorough studiesare needed to map the fuel effects. One way to characterize a fuelis by using the Auto-Ignition Temperature (AIT). The AIT and theamount of Low Temperature Heat Release (LTHR) together describe theauto-ignition properties of the fuel. Both can be extracted fromthe pressure trace. The assumption is that the pressure andtemperature are known at inlet valve closing (IVC) and that themass in the cylinder does not change after IVC. The purpose of this study was to map the AIT of differentPrimary Reference Fuels (PRF) for HCCI combustion at differentcylinder pressures. Different pressure levels were achieved bychanging inlet air temperatures in 5 steps from 50°C to 150°C. ACooperative Fuel Research (CFR) engine with variable compressionratio was used. The compression ratio was varied from 5.5 to 15.5to keep combustion phasing, defined as 50% of total heat released,constant at 3±1° after TDC. The experiments were carried out inlean operation with a constant equivalence ratio of 0.33 and with aconstant engine speed of 600 rpm. The results showed that the AITs of the PRFs ranged from around580 K for PRF 0 to up to 800 K for PRF 100. Auto-ignition wasdefined as the point where the rate of heat released had reached0.2 J/CAD. At the lowest inlet air temperatures all fuels exceptPRF 100 showed Low Temperature Heat Release (LTHR) from about 3%LTHR/Total Heat Release for PRF 95 with up to 19% for PRF 0. Amountof LTHR was found to decrease linearly with increasing octanerating. The low octane PRFs ignited at almost the same temperatureindependent of the cylinder pressure. The high octane number PRFs(PRF 95 - 100) displayed a wide range of auto-ignition temperaturesresulting from the different inlet air temperatures. PRF 80, PRF 85and PRF 90 showed an intermediate behavior. A constantauto-ignition temperature was seen at the low inlet air temperaturebut at higher inlet air temperatures the IAT quickly raised whenthe LTHR disappeared.