Homogeneous charge compression ignition (HCCI) combustion with
fully premixed charge is severely limited at high-load operation
due to the rapid pressure-rise rates (PRR) which can lead to engine
knock and potential engine damage. Recent studies have shown that
two-stage ignition fuels possess a significant potential to reduce
the combustion heat release rate, thus enabling higher load without
knock.
This study focuses on three factors, engine speed, intake
temperature, and fuel composition, that can affect the pre-ignition
processes of two-stage fuels and consequently affect their
performance with partial fuel stratification. A model fuel
consisting of 73 vol.% isooctane and 27 vol.% of n-heptane (PRF73),
which was previously compared against neat isooctane to demonstrate
the superior performance of two-stage fuels over single-stage fuels
with partial fuel stratification, was first used to study the
effects of engine speed and intake temperature. The results for
PRF73 show that increasing engine speed from 1200 to 1600 rpm
causes almost no change in φ-sensitivity, which is defined by the
advancement of combustion phasing for an increase in equivalence
ratio. Consequently, the maximum combustion pressure rise rate
(PRRmax) can be reduced
substantially with partial fuel stratification at this higher speed
as it was at 1200 rpm. In contrast, increasing intake temperature
from 60°C to 174°C eliminates the low temperature heat release of
PRF73. Despite the single-stage ignition at this temperature, PRF73
still shows a weak but definitive φ-sensitivity, likely due to the
relatively strong intermediate temperature heat release before hot
ignition. As a result, PRRmax was
reduced modestly with partial fuel stratification. This PRF73
result is distinctively different from that of isooctane at the
same intake temperature. To study the importance of fuel
composition, PRF73 is compared with a low-octane, gasoline-like
distillate fuel, termed Hydrobate, which could be readily produced
from petroleum feedstocks. With the similar HCCI reactivity to
PRF73, Hydrobate shows little difference in φ-sensitivity and
performs similarly with partial fuel stratification compared to
PRF73. This result indicates that it is the overall fuel HCCI
reactivity, rather than the exact fuel composition, that determines
the φ-sensitivity and the consequent performance with partial fuel
stratification.