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According to the Environmental Protection Agency’s estimates, in 2019, 7% of the light-duty vehicles in the United States did not comply with their mandated vehicle emission regulations. Even more astonishing, is the fact that one specific component on these vehicles accounts for about 68% of these compliance failures.
HISTORY OF SMOG
Though the catalytic converter has become the primary mechanism of the automobile industry for controlling exhaust emissions in internal combustion engines, its origin is a byproduct of industrialization as a whole. During the turn of the 20th century, the smog created in urban areas by factory smokestacks triggered the first concerns for air quality. As the automobile and the internal combustion engine became more abundant, their impact on air quality grew more worrisome., During the 1940s, In the United States the growing problem of urban smog, specifically in the Los Angeles area prompted the French mechanical engineer Eugene Houdry, to take interest in the problem. Houdry was an expert in catalytic oil refining and had developed techniques for catalytically refining heavy liquid tars into aviation gasoline.
WHAT IS SMOG
The exhaust of all internal combustion engines used on vehicles is composed primarily of three constituent gases, nitrogen, carbon dioxide, and water vapor. In lean operating modes of gasoline engines and in diesel engines, oxygen is also present. Diesel engines by design generally operate with excess air, which always results in exhausted oxygen, especially at low engine loads.
The nitrogen and oxygen are primarily pass-throughs of atmospheric gases. While the carbon dioxide and water vapor are the direct products of the combustion process. Depending on the engine type and configuration, these harmless gases form 98-99% of an engine’s exhaust. However, the remaining 1-2% of combustion products comprise thousands of compounds, all of which to some degree, create air pollution.
The primary components of these pollutants, carbon monoxide, and nitrogen oxides, are formed within the highly reactive, high-temperature flame zone of the combustion cycle. While unburned and partially oxidized hydrocarbons tend to form near the cylinder walls where the combustion flame is quenched. Particulate matter, especially in diesel engines, is also produced in the form of soot. In addition to this, engine exhaust also contains partially burned lubricating oil, and ash from metallic additives in the lubricating oil and wear metals.
WHY CATALYTIC CONVERTERS
In 1970, the United States passed the Clean Air Act, which required all vehicles to cut its emissions by 75% in only five years and the removal of the antiknock agent, tetra-ethyl lead from most types of gasoline.
THE FIRST CONVERTER
Modern automotive catalytic converters are composed of a steel housing containing catalyst support called a substrate, that’s placed inline with an engine’s exhaust stream. Because the catalyst requires a temperature of over 450 degrees C to function, they’re generally placed as close to the engine as possible to promote rapid warm-up and heat retention.
On early catalytic converters, the catalyst media was made of pellets, placed in a packed bed. These early designs were restrictive, sounded terrible, and wore out easily. During the 1980s, this design was superseded by a cubic ceramic-based honeycomb monolithic substrate, coated in a catalyst. These new cores offered better flow and because of their much larger surface area, exposed more catalyst material to the exhaust stream. The ceramic substrate used is primarily made of a synthetic mineral known as cordierite.
TYPES OF CATS
The first generation of automotive catalytic converters worked only by oxidation. These were known as two-way converters as they could only perform two simultaneous reactions - the oxidation of carbon monoxide to carbon dioxide and the oxidation of hydrocarbons to carbon dioxide and water.
By 1981, "three-way" catalytic converters had superseded their two-way predecessor. Three-way catalytic converters induce chemical reactions that reduce nitrogen oxide to harmless nitrogen. This reaction can occur with either carbon monoxide, hydrogen, or hydrocarbons within the exhaust gas.
While three-way catalytic converters are more efficient at removing pollutants, their effectiveness is highly sensitive to the air-fuel mixture ratio. For gasoline combustion, this ratio is between 14.6 and 14.8 parts air to one part fuel. Furthermore, they need to oscillate between lean and rich mixtures within this band in order to keep both reduction and oxidation reactions running. Because of this requirement, computer-controlled closed-loop electronic fuel injection is required for their effective use.
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24 сен 2024
