Simultaneous measurements of absorption and emission in preflame reaction under knocking operation 2000-05-0159
There is an urgent need today to improve the thermal efficiency
of spark- ignition (SI) engines in order to reduce carbon dioxide
emission and conserve energy in an effort to prevent global
warming. However, a major obstacle to improving thermal efficiency
by raising the compression ratio of SI engine is the easily
occurrence of engine knocking.
The result of studies done by numerous researchers have shown
that knocking is an abnormal combustion in which the unburned gas
in the end zone of the combustion chamber autoignites. However, the
combustion reaction mechanism from autoignition to the occurrence
of knocking is still not fully understood.
The study deals with the light absorption and emission behavior
in the preflame reaction interval before hot flame reactions. With
the aim of shedding some light on this mechanism, the present study
focused on light emission and absorption behavior of certain
intermediate products of combustion which are known to play a key
role in the combustion reactions of hydrocarbon fuels. Emission and
absorption spectroscopy were used to measure the behavior of
intermediate products of combustion at wavelengths thought to
correspond to formaldehyde (HCHO, characteristic spectra of 395.2
and 293.1 nm), which shows cool flame reactions, the OH radicals
(306.4 nm) and HCO (329.8 nm), which exhibits blue flame reactions.
The light emission behavior measured for the three radicals by
emission spectroscopy was introduced into a polychromator and the
absorption behavior measured by absorption spectroscopy was
introduced into a monochromator for each species individually.
The HCHO emission intensity and absorbance waveforms recorded
under normal combustion on n-heptane show behavior indicative of
the passage and degeneracy of cool flame in the preflame reaction
interval. The OH radicals and a HCO also show similar light
emission behavior. It was observed that radical behavior differed
between normal combustion and knocking operation.