Secondary air injection is also known as air injection which is the vehicle emission control strategy that was introduced first in the mid60s. This takes place when fresh air is injected to exhaust stream to produce fuller combustion of the exhaust gases. The mechanism wherein the exhaust emissions are controlled mostly depend on the injection method as well as the point at which the air is entering the exhaust system and varies during technological advancements.
The first air injected system is close to the engine either in the exhaust ports of the cylinder head or in the exhaust manifold. These systems are providing oxygen in order to oxidize or burn both the unburned as well as partially burned fuel in exhaust before ejecting it to the tailpipe.
There was actually a massive number of partially burned and unburned fuel in the 60s and 70s cars and for this reason, secondary air injection reduced tailpipe emissions a lot. The extra heat of recombustion on the other hand particularly with excessively rich exhaust may caused maladjusted carburetor or misfiring, has a tendency to bring damage to the exhaust valves and be seen to make the exhaust manifold to incandesce.
Emission control strategies have become more effective and sophisticated the volume of the partially burned as well as unburned fuel in exhaust stream shrunk and especially when the catalytic converter was made, the function of secondary air injection shifted as well.
So rather than being the primary emission control tool, the secondary air injection was used instead as a support to the catalytic converter. The original air injection point became known as being the upstream injection point. When the catalytic converter is cold, air is injected to the upstream point that’s burnt with deliberately rich exhaust. As soon as the catalyst is warmed up, air will be injected to the downstream location, the catalytic converter itself which will help with catalysis of unburned hydrocarbons.
Pumped air injection system uses a vane pump referred as air pump that is turned by the engine using an electric motor or belt. The air intake of the pump is being filtered by the rotating screen in excluding dirt particles big enough to bring damage to the system. Then after that, the air is delivered under light pressure to injection point/s. A check valve is going to prevent the exhaust to force its way back into the air injection system which otherwise damage other parts and components. This is the reason why the secondary air injection systems play an important role in cars both in the past and today.