An experimental study was performed to develop a more
fundamental understanding of the effects of secondary air injection
(SAI) on exhaust gas emissions and catalyst light-off
characteristics during cold start of a modern SI engine. The
effects of engine operating parameters and various secondary air
injection strategies such as spark retardation, fuel enrichment,
secondary air injection location and air flow rate were
investigated to understand the mixing, heat loss, and thermal and
catalytic oxidation processes associated with SAI. Time-resolved
HC, CO and CO₂ concentrations were tracked from the cylinder exit
to the catalytic converter outlet and converted to time-resolved
mass emissions by applying an instantaneous exhaust mass flow rate
model. A phenomenological model of exhaust heat transfer combined
with the gas composition analysis was also developed to define the
thermal and chemical energy state of the exhaust gas with SAI.
The study found that significant emissions reduction can be
achieved with SAI by the thermal oxidation process prior to the
catalyst, which results in higher exhaust gas temperatures and
therefore enhances the chemical process inside the catalyst by
faster catalyst light-off. The engine operation, with a relative
air/fuel ratio 20% rich of stoichiometric and 100% secondary air,
yielded the fastest catalyst light-off time of 4.2 sec. The SAI
system reduced HC emissions by 46% to 88% and CO emissions by 37%
to 93% compared with the normal operating conditions. The analysis
showed that the post-catalyst HC emissions levels were optimized
with secondary air flow rates corresponding to an overall exhaust
lambda of 1.3. The improvement in the thermal oxidation reaction
with the increased mixing rates upstream of the catalyst decreased
the catalytic oxidation reaction due to the increased consumption
of reactants upstream of the catalyst. Therefore, the post-catalyst
HC emission levels were not strongly affected by the mixing
rates.
