What Is a Catalytic Converter?

What Is a Catalytic Converter?

Last Updated April 29, 2024 | C.J. Tragakis

A catalytic converter is an emissions control device that helps remove harmful greenhouse gases from a vehicle’s exhaust. The catalytic converter (or "cat") is the second component in a vehicle’s exhaust system (behind the exhaust manifold). It’s followed by the mid-pipes, resonator, muffler, and tailpipe.

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Patented in 1950 by Eugene Houdry, the catalytic converter wasn’t required on U.S. vehicles until 1975. This was due to stricter environmental regulations put in place during the ‘70s to cut down on smog and the depletion of the ozone layer. Catalytic converters greatly help with both of these environmental issues.

Labeled Diagram of Exhaust System

What Does a Catalytic Converter Do?

A catalytic converter is the main cleaning component of an exhaust system. It converts dangerous compounds into less harmful substances before they are released into the atmosphere.

Outside of a Catalytic Converter

The fuel that is burned in a combustion engine is composed of hydrocarbons. Hydrocarbons are a major cause of ozone layer depletion and health problems in humans. Other byproducts of internal combustion are carbon monoxide and toxic nitrogen oxides.

In a perfectly efficient chemical equation, all hydrocarbons would be burned. In reality, they often make it to the exhaust system. The catalytic converter removes hydrocarbons and dangerous gases by changing them into other molecules through chemical processes.

Overview of Catalytic Converter Process

How Does a Catalytic Converter Work?

When heated to around 600 degrees F, a catalytic converter begins its conversion process. It speeds up chemical reactions that would otherwise take a long time. Rare earth metals are used to induce the necessary reactions.

The process is assisted by oxygen sensors. The first oxygen sensor is used to determine how much oxygen is making it to the catalytic converter. If not enough oxygen is entering, that means too many hydrocarbons are making it through the system. The computer will then automatically adjust the engine’s fuel-air mixture.

A second oxygen sensor, after the catalytic converter, let’s the car know if it’s taking in enough oxygen for the reactions to properly occur.

What’s Inside a Catalytic Converter?

Inside a catalytic converter’s stainless steel body is a honeycomb structure made of ceramic. The ceramic doesn’t react chemically, but is used to retain the heat needed to set off the reactions.

Inside Honeycomb of Catalytic Converter

The honeycomb is covered with three rare earth metals: platinum, rhodium, and palladium. These materials are what actually induce the conversions. Using a honeycomb shape maximizes the surface area on which the reactions can happen.

Chemical Reactions

There are three primary reactions that occur inside a catalytic converter. In modern systems, all three are used. Older cats may only use the two oxidation catalyst reactions.

Oxidation Catalyst - Two reactions are part of the oxidation catalyst. These are done with platinum and palladium.

The first oxidation reaction converts carbon monoxide (CO1) to carbon dioxide (CO2) by "adding" oxygen atoms to CO1.

Carbon Monoxide Oxidation in Catalytic Converter

The second oxidation reaction oxidizes unburnt hydrocarbons. Hydrocarbons (HC) and oxygen (O2) are converted to become water (H2O) and carbon dioxide (CO2).

Hydrocarbon Oxidation in Catalytic Converter

These two reactions are used in two-way catalytic converters, which are more common in diesel vehicles and cars made before 1981.

Reduction Catalyst - Most modern catalytic converters add an additional reaction using platinum and rhodium. These three-way catalytic converters were implemented in the U.S. in 1981 and are used in virtually every gasoline vehicle today.

In addition to the oxidation catalyst, three-way cats use a reduction catalyst. It converts nitrogen oxide (a harmful greenhouse gas) to nitrogen (N2) by removing oxygen atoms. Nitrogen comprises 78% of our atmosphere, making it a harmless emission. The oxygen atoms can be used in the oxidation reactions.

Nitrogen Reduction Catalyst in Catalytic Converter

How Long Do Catalytic Converters Last?

Many cats will last the lifetime of a car or at least 100,000 miles. However, they will eventually wear out and go bad. Check out our guide on the symptoms of a bad catalytic converter for a full run-down of what to look for.

Unfortunately, it’s not possible to really "fix" a bad cat. Most of the time, they’re replaced outright. The cost of replacing a catalytic converter can easily be $2,000 or more, including both parts and labor. Their high price and precious metal components can also make them a target for theft.

Performance Catalytic Converters

Contrary to popular belief, catalytic converters don’t sap power. In fact, modern ones are so efficient, they can actually help your vehicle be more powerful.

However, there’s no denying that early cats from the '70s and '80s were quite restrictive. They dragged horsepower levels down, which is part of the reason why so many big V8s from that time made so little power. Today, factory catalytic converters have much better exhaust flow and are designed to be as unobtrusive as possible.

The cat can be swapped out for an upgrade if you really want. High-flow catalytic converters are engineered to have better exhaust flow. Though designed to be EPA-compliant, these aftermarket catalytic converters are often not legal under the stricter California exhaust laws. Performance downpipes often include high-flow cats as part of the kit.

Going Catless

You can take the catalytic converter out completely. However, it’s illegal to remove or modify the catalytic converter in a vehicle used on public roads. Removing a catalytic converter should solely be done by hardcore racers who will only drive the car on a private track.

The technology has gotten so good in modern vehicles, there simply isn’t a need to mess with your cat otherwise. As more cars go electric, we’ll see a diminishing role for catalytic converters. While battery hybrid electric vehicles (BHEVs) still have them, full EVs don’t need a cat at all.

Sources: Catalytic Converter: How It Works, Donut Media | What You Should Know About Using, Installing, or Buying Aftermarket Catalytic Converters, EPA | Frequently Asked Questions Emissions Laws and Regulations, Racing Beat | Catalytic Converters, Explain That Stuff

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