Development
The mechanism by which exhaust emissions are controlled depends on the method of injection and the point at which air enters the exhaust system, and has varied during the course of the development of the technology.
The first systems injected air very close to the engine, either in the cylinder head's exhaust ports or in the exhaust manifold. These systems provided oxygen to oxidize (burn) unburned and partially burned fuel in the exhaust before its ejection from the tailpipe. There was significant unburned and partially burned fuel in the exhaust of 1960s and early 1970s vehicles, and so secondary air injection significantly reduced tailpipe emissions. However, the extra heat of recombustion, particularly with an excessively rich exhaust caused by misfiring or a maladjusted carburetor, tended to damage exhaust valves and could even be seen to cause the exhaust manifold to incandesce.
As emission control strategies grew more sophisticated and effective, the amount of unburned and partially burned fuel in the exhaust stream shrank, and particularly when the catalytic converter was introduced, the function of secondary air injection shifted. Rather than being a primary emission control device, the secondary air injection system was adapted to support the efficient function of the catalytic converter. The original air injection point became known as the upstream injection point. When the engine is cold, air injected at this point cleans up the extra-rich exhaust and raises the temperature of the exhaust so as to bring the catalytic converter to operating temperature quickly. Once the engine is warm, air is injected to the downstream location — the catalytic converter itself — to assist with catalysis of unburned hydrocarbons and carbon monoxide.
Read more about this topic: Secondary Air Injection
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