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.
