A Computational Study of the Combined Effects of EGR and Boost Pressure on HCCI Autoignition

This study computationally investigates the combined effects ofEGR and boost pressure on HCCI autoignition using iso-octane, PRF50and n-heptane. The computations were conducted using thesingle-zone model of CHEMKIN included in CHEMKIN-PRO with detailedchemical-kinetics mechanisms for iso-octane, PRF and n-heptane fromLawrence Livermore National Laboratory (LLNL). To better reproducethe state of EGR addition in real engine, the EGR composition isdetermined after several combustion cycles under the constantamount of fuel. All data points were acquired with a CA50 of 5°CAaTDC by adjusting initial temperature to remove the effect ofcombustion phasing, which can influence on HCCI autoignition fromany effect of the EGR and boost pressure themselves. The results show that EGR increases the burn duration andreduces the maximum pressure-rise rate with lower peak of maximumheat-release rates for all fuels even for a boost pressure, whichaccelerates a HCCI autoignition propensity. However, above acertain EGR ratio with no boost pressure, the LTHR of two-stageignition fuel PRF50 disappears, and its autoignition process ischanged to that of single-stage ignition fuel, which exhibits ahigh maximum pressure-rise rate and lower IMEP. Boost pressureincreases the autoignition reactivity, and especially pronounced onLTHR of two-stage ignition fuels n-heptane and PRF50. PRF50 turnsinto two-stage ignition fuel again by boost pressure even at highEGR ratio, and the autoignition process of PRF50 becomes similar tothat of n-heptane. Moreover, boost pressure could also eliminatethe need to increase the initial temperature for a fixed CA50 whenapplying EGR, which provides potential to allow higher power outputdue to the increased charge mass inducted. Finally, the combinedeffects of EGR and boost pressure provide substantial advantagesfor controlling low-temperature heat-release and reducing peak ofheat-release rate. These advantageous effects allow higher IMEPwith lower maximum pressure-rise rate across a wide EGR ratiorange, especially on the two-stage ignition fuel.