> This subject needs expert analysis. I just cannot believe that the air
> pump results in an after burn as this would mean very hot exhaust
> pipes and presumably a noticeable belching out of flames.
Barrie, it does, the process is called Thermal Afterburning and I'll just
now quote right from the Bosch Auto Handbook (pg 521):
<begin quote>
Thermal Afterburning
Before today's catalytic treatment of exhaust emissions became standard,
initial attempts to reduce emissions utilized thermal afterburning. The
method employs a specific residence time at high temperatures for burning
the exhaust-gas components which failed to combust during normal combustion
in the engine cylinders. In the rich range, the process must be supported
with supplementary air injection (secondary air) in lean-burn engines, the
residual oxygen in the exhaust gas supports the afterburning process.
In the past, mechanical pumps driven by belts directly from the engine were
used for secondary-air injection. Since such air injection is only required
during the engine's warm-up period, these pumps are switched using
electro-magnet couplings. Lower-priced blower pumps powered by electric
motors are quickly superseding the former mechanical versions.
Due to its lack of potential, particularly in maintaining low NOx <me:
collective oxides of Nitrogen> limits, thermal afterburning alone is
currently considered to be of little significance. It can be used, however,
to reduce emissions of HC and CO during the operating phase in which the
catalytic converter has not yet reaches its operating temperature. Because
it greatly reduces the time taken for the catalytic converter to reach its
operating temperature, thermal afterburning with air injection during the
engine warm-up phase in combination with catalytic aftertreatment will play
a major role in achieving compliance with more stringent emission limits in
the future.
<end quote>
Quoted from the 5th Edition, last updated in 2000.
Kai
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