The combustion of hydrocarbon fuels in a
spark-ignition engine does not happen instantaneously. The fuel burns
over the course of milliseconds, involving various oxygen-consuming
reactions triggered by heat, pressure and the spark source. The
resultant explosive oxidation process converts the vaporized fuel into
gaseous products that produce the pressure that drives the engine.
under certain conditions, and particularly when pushing the limits of
engine efficiency and performance, the fuel can auto-ignite too early,
which affects efficiency and can damage the engine. Researchers and
engineers have been unable to predict the auto-ignition behavior with
sufficient accuracy to break new ground in performance and fuel
Zhandong Wang and S. Mani Sarathy from the
University's Clean Combustion Research Center (CCRC) proposed an
extended oxidation mechanism that could explain the discrepancy between
simulation and experiments.
“We identified some unexplained
reactive species in the combustion of the typical alkanes found in
transportation fuels in an advanced collaborative experiment conducted
recently at the Lawrence Berkeley National Laboratory in the U.S.,” said
Wang. “We postulated that these species indicate an alternative series
of reactions involving a third oxygen addition process that is an
extension of the well-known first and second oxygen addition processes.”
the classical low-temperature oxidation mechanism, alkane
hydrocarbons—open chains of carbon and hydrogen atoms—react with oxygen
to form branched structures. This is known to occur by the addition of
oxygen in two stages: the first and second oxygen additions.
and Sarathy identified a pathway for a third oxygen addition in alkanes
with six or more carbon atoms to form another series of chain-branching
intermediate compounds, explaining the shorter-than-expected delay
before auto-ignition in these hydrocarbons.
“Understanding how auto-ignition occurs can help design fuels that prevent it from occurring,” said Sarathy.
researchers also expect their third oxygen addition reaction scheme to
help in the development of low-temperature combustion engine concepts
such as homogeneous charge compression ignition and reactivity
controlled compression ignition engines.
“These new technologies
exploit a two-stage heat release process based on low-temperature
oxidation chemistry for which optimal combustion timing is critical for
engine power and efficiency,” they noted.