A Counterpersons Guide To Emission Sales

Air Injection Reaction 01

Air Injection Reaction (A.I.R.) - Theory




Typical A.I.R. system Layout


Air Injection Reaction Systems (A.I.R.)


The primary function of "Air Injection Reaction" is to promote the burning of any unburned HC or CO remaining in the exhaust, after the initial combustion process, by pumping air (oxygen) directly into the exhaust stream. The additional burning also helps to "light off" the catalytic converter and bring it to working temperature faster.

An additional function of the A.I.R. system, on oxygen sensor equipped vehicles, is to pump air into the exhaust system upstream of the O2 sensor during warm up. The additional burning brings the oxygen sensor to working temperature faster so that the system can go into closed loop sooner. This of course further reduces emissions and also improves fuel economy.


Pulse A.I.R. System - Does Not Use an Air Pump


The First A.I.R. Systems


Air injection systems were first introduced in the mid 1960’s. Earlier systems usually only inject air for the first few minutes of operation when the engine is cold and the fuel mixture is very rich. At that time, there is a lot of excess HC and CO in the exhaust stream. A system of thermal vacuum switches and air management valves is used to control when and where the air is injected. This is because under certain circumstances, air injection must be disabled to avoid backfiring in the exhaust system or overheating the catalytic converter on later systems.


Later AIR Systems


After the introduction of catalytic converters in the mid 1970’s, the air was injected directly into or just before the catalytic converter. Unlike the very first systems, these systems continued to inject air more or less continuously, even after the warm up period, to increase the oxidation capacity of the catalytic converter.


Electric A.I.R. Injection Pump


System Types


 Mechanical Air Pump: Most early systems used a belt-driven air pump to force the air into the exhaust system.

Pulse Air Systems: Some smaller engines used a passive "pulse air" injection system that did not use an air pump. Instead, they used negative pressure pulses (vacuum) that occur naturally in the exhaust gas stream to draw air into the system. This avoids the need for a bulky air pump.

Electric Air Pump Systems: As fuel injection and overall engine management technology improved, many manufactures phased out the use of air injection in the late 1980’s and early 1990’s. However, starting in the mid 1990’s, some manufacturers (e.g. Ford and GM) started using a version of A.I.R. that uses an electric air pump to force the air into the exhaust system. These systems usually only operate during the first few minutes of engine warm up to get both the oxygen sensor and the catalytic converter to working temperature quickly.

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