Editing Vehicle emissions control (section)

===Air injection===
{{Main|Secondary air injection}}
One of the first-developed exhaust emission control systems is secondary air injection. Originally, this system was used to inject air into the engine's exhaust ports to provide oxygen so unburned and partially burned hydrocarbons in the exhaust would finish burning. Air injection is now used to support the [[catalytic converter]]'s oxidation reaction, and to reduce emissions when an engine is started from cold.
After a cold start, an engine needs an air-fuel mixture richer than what it needs at [[operating temperature]], and the [[catalytic converter]] does not function efficiently until it has reached its own operating temperature. The air injected upstream of the converter supports combustion in the exhaust headpipe, which speeds catalyst warmup and reduces the amount of unburned hydrocarbon emitted from the tailpipe.