The purpose is the same in both cases - excess oxygen is needed in the exhaust stream after combustion in the cylinder in order for further oxidation to occur transforming harmful emissions (NOx, CO and unburned Hydrocarbons) into harmless CO2 and H2O. The ecu is programmed to determine whether the air (source of the excess oxygen) needs more or less time to 'work' it's chemical reactions on the exhaust stream. When more time is needed, the air is injected to the head so the presence of excess oxygen begins earlier during the exhausts route to the converters.
when the engine is cold the air goes upstream(cylinder heads). when the engine is warm(closed loop) or when the O2 sensor inputs are being used the air is diverted downstream(to the cats).
if the air went to the cyl heads during warm (closed loop) operation then the O2s would show full lean therefore the computer would adjust to full rich and trip a check engine light.
the reason for downstream is that the 3-way cats use most of the oxygen(left over from combustion) in the first chamber so it needs additional oxygen for the 2nd chamber to work properly.
Thanks for the additional detail cjones - I'm learning something here too. So, now you've made me start thinking more about it.
What's the need for the upstream injection when the engine is cold? Perhaps when cold without the O2 signals, it's running a bit richer, and in that situation, all three cat chambers need additional O2, hence the injection further upstream? I know that the toughest part of the passing the EPA tests was getting through the 'cold' start up portion. If messing with the O2 sensor readings is truly at issue, I wonder why they didn't simply didn't arrange the plumbing to inject downstream of the O2, but upstream of the cat; seems that plumbing would be a lot easier/less expensive than the crossover tube, casting the heads with the air injection passages, machining them, etc.
during warmup (before engine is warm) the air is put upstream to introduce more oxygen in the exhaust manifolds to continue combustion and reduce HC and CO. the exhaust manifold temp is high enough to promote combustion but needs additional oxygen.
keep in mind that cats need to be around 500* F to function so when an engine is cold (just started) the cats don't operate.
Makes all the since in the world - forgot about light-off time for the cats; so when cold, put it in as far upstream as possible to provide as much time as possible for further oxidation. Then once O2's and cats are up and running, just inject at the cats to provide excess O2. Thanks for the education - Michael