A computational study of the combined effects of EGR and boost pressure on HCCI autoignition

This study computationally investigates the combined effects of EGR and boost pressure on HCCI autoignition using iso-octane, PRF50 and n-heptane. The computations were conducted using the single-zone model of CHEMKIN included in CHEMKIN-PRO with detailed chemical-kinetics mechanisms for iso-octane, PRF and n-heptane from Lawrence Livermore National Laboratory (LLNL). To better reproduce the state of EGR addition in real engine, the EGR composition is determined after several combustion cycles under the constant amount of fuel. All data points were acquired with a CA50 of 5°CA aTDC by adjusting initial temperature to remove the effect of combustion phasing, which can influence on HCCI autoignition from any effect of the EGR and boost pressure themselves. The results show that EGR increases the burn duration and reduces the maximum pressure-rise rate with lower peak of maximum heat-release rates for all fuels even for a boost pressure, which accelerates a HCCI autoignition propensity. However, above a certain EGR ratio with no boost pressure, the LTHR of two-stage ignition fuel PRF50 disappears, and its autoignition process is changed to that of single-stage ignition fuel, which exhibits a high maximum pressure-rise rate and lower IMEP. Boost pressure increases the autoignition reactivity, and especially pronounced on LTHR of two-stage ignition fuels n-heptane and PRF50. PRF50 turns into two-stage ignition fuel again by boost pressure even at high EGR ratio, and the autoignition process of PRF50 becomes similar to that of n-heptane. Moreover, boost pressure could also eliminate the need to increase the initial temperature for a fixed CA50 when applying EGR, which provides potential to allow higher power output due to the increased charge mass inducted. Finally, the combined effects of EGR and boost pressure provide substantial advantages for controlling low-temperature heat-release and reducing peak of heat-release rate. These advantageous effects allow higher IMEP with lower maximum pressure-rise rate across a wide EGR ratio range, especially on the two-stage ignition fuel.