re: RE: Air pumps (no tattoos or rear engined cars)

[Jay Negrin <jmnegrin@domain.elided>]

Joe Elliot asks more about air pumps: (sorry if this has been covered,
but I've been out of town a couple of days)

<<Where's Eric Storhok??  I don't understand why you'd have an air pump
on a car with no cat.  Was the idea just to dilute the exhaust with
fresh air to lower the percentage of pollutants coming out the
tailpipe?  That's pretty lame.>>  SNIP the rest about tattoos and
Porsches....

The idea, Joe, is to add oxygen, carried in the air, to react with
unburned hydrocarbons  (HCs)in the exhaust stream.  Air pumps were in
use as early as 1966 in some doemestic autos, and 1968 in imported.
Depends on how easy a go the mfg had with initial design of the motor.
If you were BMW, Honda or several others, you had a thermal reactor, no
catalyst involved, as a place for the high temp chemistry to take
place.  The car has to run a tad rich for the system to work, though
(loss of fuel economy).  Too hot, and you make less HCs, but more oxides
of nitrogen (NOx) which are more difficult to get rid of with a
single-stage cat, which is what SPICA cars have.

Later cars, with 3-way cats and electronic fuel controls, help control
all of the above with nearly stoichimetric (perfect chemistry) air-fuel
mixtures.  To do that you need (are we ready for this?) and oxygen
sensor (ta Dah!) in the exhaust.  Small changes in the amount of oxygen
in the exhaust signal the need for equally small changes in the fuel
charge, raising or lowering the air-fuel ratio as needed.  The three-way
cat can oxidize small amounts of HC and reduce small amounts of NOx.  If
all else is clean, and in good-running order, you have an exhaust that
we all can breathe.

That is how it works, in theory.  OK?

Jay Negrin
AROSC
76 Alfetta GT -which, in spite of extensive exhaust mods, still has to
meet smog regs in CA until 2006, the earliest

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