Recent studies have shown that for a given RON, fuels with a
higher sensitivity (RON-MON) tend to have better antiknock
performance at most knock-limited conditions in modern engines. The
underlying chemistry behind fuel sensitivity was therefore
investigated to understand why this trend occurs. Chemical kinetic
models were used to study fuels of varying sensitivities; in
particular their autoignition delay times and chemical
intermediates were compared.
As is well known, non-sensitive fuels tend to be paraffins,
while the higher sensitivity fuels tend to be olefins, aromatics,
diolefins, napthenes, and alcohols. A more exact relationship
between sensitivity and the fuel's chemical structure was not
found to be apparent. High sensitivity fuels can have vastly
different chemical structures.
The results showed that the autoignition delay time (τ) behaved
differently at different temperatures. At temperatures below 775 K
and above 900 K, τ has a strong temperature dependence. However,
between 775 K and 900 K, τ has a decreased temperature dependence.
The change in temperature dependence in this region was found to
correlate with fuel sensitivity. The autoignition of fuels with a
higher sensitivity have a higher temperature dependence in that
region. A stronger temperature dependence on τ in this region
results in slower low temperature chemistry and faster high
temperature chemistry.
As a consequence, fuels behave differently depending on the
temperature regime of the end-gas. If two fuels have the same RON,
the autoignition integral for the two fuels approaches 1 at the
same time in the RON test. Lower end-gas temperatures would allow
sensitive fuels, which have slower low temperature chemistry, to
have better antiknock performance. However, higher end-gas
temperatures, such as those in the MON test, would allow
non-sensitive fuels to have better antiknock performance.
The fuels with larger sensitivities studied here were predicted
by kinetic models to produce large amounts of aldehydes, which are
relatively stable at low temperatures, but react rapidly at high
temperatures. These aldehydes appear to be an important cause of
the octane sensitivity.
