Catalytic converter

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 engines as well as kerosene heaters and stoves.

Catalytic converters require temperature of 800 degrees Fahrenheit to efficiently convert harmful exhaust gases into inert ones, such as carbon dioxide and water vapor. So, first catalytic converters were placed close to the engine to ensure fast heating. However, such placing caused several problems, such as "vapor lock".

Many vehicles have a close-coupled catalytic converter located near the engine's exhaust manifold. The converter heats up quickly, due to its exposure to the very hot exhaust gases, enabling it to reduce undesirable emissions during the engine warm-up period. This is achieved by burning off the excess hydrocarbons which result from the extra-rich mixture required for a cold start.

When catalytic converters were first introduced, most vehicles used carburetors that provided a relatively rich air-fuel ratio. Oxygen (O₂) levels in the exhaust stream were therefore generally insufficient for the catalytic reaction to occur efficiently. Most designs of the time therefore included secondary air injection, which injected air into the exhaust stream. This increased the available oxygen, allowing the catalyst to function as intended.

Some three-way catalytic converter systems have air injection systems with the air injected between the first (NO_x reduction) and second (HC and CO oxidation) stages of the converter. As in two-way converters, this injected air provides oxygen for the oxidation reactions. An upstream air injection point, ahead of the catalytic converter, is also sometimes present to provide additional oxygen only during the engine warm up period. This causes unburned fuel to ignite in the exhaust tract, thereby preventing it reaching the catalytic converter at all. This technique reduces the engine runtime needed for the catalytic converter to reach its "light-off" or operating temperature.

Most newer vehicles have electronic fuel injection systems, and do not require air injection systems in their exhausts. Instead, they provide a precisely controlled air-fuel mixture that quickly and continually cycles between lean and rich combustion. Oxygen sensors are used to monitor the exhaust oxygen content before and after the catalytic converter, and this information is used by the Electronic Control Unit to adjust the fuel injection so as to prevent the first (NO_x reduction) catalyst from becoming oxygen-loaded, while simultaneously ensuring the second (HC and CO oxidation) catalyst is sufficiently oxygen-saturated.