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Automobile emissions are regulated in the U.S. by the Environmental Protection Agency on the national level, though states can elect to follow an even more stringent set of rules enacted by the California Air Resources Board. In fact, enough states have adopted these standards that the EPA is collaborating with CARB on its vehicle emissions rules for the 2022–2025 model years. Although cars aren’t the biggest polluters, cleaning up their emissions has paid dividends in the air quality of places like Los Angeles. Here are some of the devices that help keep California’s San Bernardino Mountains visible from the L.A. basin and the air we all breathe clean.
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Exhaust Gas Recirculation: EGR redirects a portion of an engine’s inert exhaust gas back into the intake tract to dilute the fresh air that’s fed to the cylinders. This reduces the combustion temperature and the production of nitrogen oxides (NOx). The amount of exhaust sent back into the engine is regulated by either an electronically controlled or vacuum-actuated valve. Diesel engines often water-cool the recirculated gases to increase the mass and the effect of the recycled gas.
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Catalytic Converter: This exhaust component reduces engine emissions by passing the exhaust through catalytic material made of a combination of platinum, rhodium, and palladium. Once heated up by the exhaust, the catalytic material sparks a chemical reaction that changes carbon monoxide and hydrocarbons into carbon dioxide and water. Since 1975, nearly every car has been fitted with a catalytic converter.
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Evaporative Emissions Control: Gasoline and diesel evaporate in a car’s fuel system, releasing harmful hydrocarbon gases into the atmosphere. To combat this, all modern fuel systems are fitted with a charcoal canister that traps the evaporated fuel and stores it until it can be burned in the engine.
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Diesel Particulate Filter: This works a bit like a paper air filter in that it traps particulate matter (i.e. the black soot that diesel engines can produce). Unlike a paper filter, a particulate filter isn’t disposable. Instead, when the filter reaches a full state, the engine injects extra fuel after combustion to increase the exhaust temperature and burn off the trapped particulates (mostly carbon), resulting in the production of carbon dioxide and water.
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Selective Catalytic Reduction: Like the catalytic converter, an SCR catalyst contains precious metals to drive exhaust-cleaning chemical reactions in diesels. Unlike a conventional cat, the SCR process requires an injection of urea into the exhaust to convert NOx into water, carbon dioxide, and nitrogen. Volkswagen famously cheaped out and didn’t fit an SCR system in some versions of its diesel models, instead relying on cheaty software to pass federal emissions tests.
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