Biodiesels
have generally been found to exhibit a shorter duration of ignition delay (the
interval between fuel injection and autoignition).
An increase in
the fatty acid ester alcohol moiety alkyl chain length can reduce the duration
of ignition delay, while the presence of carbon chain branches in the alcohol
moiety can increase the duration ignition.
However, the
impact of the alcohol moiety on ignition delay is less than the fatty acid
moiety, and so where a specific ignition delay of a biodiesel is required, this
is best achieved through modification of the fatty acid profile.
In direct
injection common rail compression ignition combustion, the primary influence of
the biodiesel composition on the levels of NOx emitted is through the duration
of ignition delay.
Longer
ignition delays result in a larger premixed burn fraction and peak heat release
rates, which increase the rates of thermal NOx production.
In
mechanically actuated fuel injection, biodiesel bulk modulus, which increases
with alkyl chain length and degree of unsaturation, has a significant influence
on the time at which fuel injection commences and thus the residence time of
in-cylinder gases at elevated temperatures at which NOx formation occurs.
Therefore, it
be may be advantageous to utilize different biodiesels of compositions optimized
for lower NOx emissions in common rail and mechanical fuel systems
respectively.
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