An index to relate fuel properties to HCCI auto-ignition would
be valuable to predict the performance of fuels in HCCI engines
from their properties and composition. The indices for SI engines,
the Research Octane Number (RON) and Motor Octane Number (MON) are
known to be insufficient to explain the behavior of oxygenated
fuels in an HCCI engine. One way to characterize a fuel is to use
the Auto-Ignition Temperature (AIT). The AIT can be extracted from
the pressure trace. Another potentially interesting parameter is
the amount of Low Temperature Heat Release (LTHR) that is closely
connected to the ignition properties of the fuel.
A systematic study of fuels consisting of gasoline surrogate
components of n-heptane, iso-octane, toluene, and ethanol was made.
21 fuels were prepared with RON values ranging from 67 to 97. Five
different inlet air temperatures ranging from 50°C to 150°C were
used to achieve different cylinder pressures and the compression
ratio was changed accordingly to keep a constant combustion
phasing, CA50, of 3 ± 1° after TDC. The experiments were carried
out in lean operation with a constant equivalence ratio of 0.33 and
with a constant engine speed of 600 rpm. The effect of ethanol and
toluene on the LTHR was evaluated. The conclusion was that ethanol
had a quenching effect, which is consistent with other studies.
Toluene was found only to have a quenching effect at the lowest
inlet air temperature. The AIT and the amount of LTHR for different
combinations of n-heptane, iso-octane, ethanol and toluene were
charted.