Editing Vehicle emissions control (section)

==Emissions control==
Engine efficiency has been steadily improved with improved engine design, more precise ignition timing and [[electronic ignition]], [[fuel injection|more precise fuel metering]], and [[electronic control unit|computerized engine management]].

Advances in engine and vehicle technology continually reduce the toxicity of exhaust leaving the engine, but these alone have generally been proved insufficient to meet emissions goals.  Therefore, technologies to detoxify the exhaust are an essential part of emissions control.

===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.

===Exhaust gas recirculation===
{{main|Exhaust gas recirculation}}
In the United States and Canada, many engines in 1973 and newer vehicles (1972 and newer in California) have a system that routes a metered amount of exhaust into the intake tract under particular operating conditions. Exhaust neither burns nor supports combustion, so it dilutes the air/fuel charge to reduce peak combustion chamber temperatures. This, in turn, reduces the formation of [[NOx|NO<sub>x</sub>]].

===Catalytic converter===
{{Main|Catalytic converter}}
The catalytic converter is a device placed in the exhaust pipe, which converts hydrocarbons, carbon monoxide, and NO<sub>x</sub> into less harmful gases by using a combination of platinum, palladium and rhodium as [[catalyst]]s.<ref>{{Cite journal|last1=Lathia|first1=Rutvik|last2=Dadhaniya|first2=Sujal|date=2019-01-20|title=Policy norms and proposed ways to achieve goals of Indian vehicle emission program|url=http://www.sciencedirect.com/science/article/pii/S0959652618332244|journal=Journal of Cleaner Production|language=en|volume=208|pages=1339–1346|doi=10.1016/j.jclepro.2018.10.202|s2cid=158500168 |issn=0959-6526|access-date=2020-05-06|archive-date=2021-10-06|archive-url=https://web.archive.org/web/20211006232544/https://www.sciencedirect.com/science/article/abs/pii/S0959652618332244|url-status=live|url-access=subscription}}</ref>

There are two types of catalytic converter, a two-way and a three-way converter. Two-way converters were common until the 1980s, when three-way converters replaced them on most automobile engines. See the [[catalytic converter]] article for further details.