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What Does a Catalytic Converter Do?

A mechanic points out the catalytic converter's location underneath a vehicle

There may be no single piece of automotive equipment more maligned than the catalytic converter, but it might also be that this important part is just misunderstood. The purpose of a catalytic converter is to help clean up the exhaust gases created by the internal combustion process. Is fits right in the middle of automotive emissions, on-board diagnostics and exhaust repair, so if you’re battling engine performance issues or diagnosing emissions problems, you may be wondering, “What does a catalytic converter do?” Let’s take a look at this often misunderstood but highly important component.

How Does a Catalytic Converter Work?

The catalytic converter is part of the exhaust system as well as the emissions system. It is connected to the engine exhaust ports via the exhaust manifold, typically very close to the engine to take advantage of higher exhaust gas temperatures. Depending on the engine layout and vehicle design there may be one catalytic converter or as many as four. It may look like a muffler, but there are no internal baffles to control sound, only a composite matrix containing special compounds. The exhaust enters the catalytic converter, passes through the matrix where it is treated, then exits into the exhaust pipe where it may pass through an exhaust resonator, then a muffler, and finally out into the atmosphere. An oxygen sensor before the catalytic converter and one after it read the composition of the exhaust gases. These readings then tell the ECU if the fuel mixture is adjusted correctly as well as if the catalytic converter is functioning correctly.

The Chemistry of Car ExhaustReplacing a Catalytic Converter with Regular Hand Tools

The most important reason for the addition of one or more catalytic converters to your vehicle is to help everyone breathe better. How? First, consider a bit of chemistry: To get you from Point A to Point B, the engine combines oxygen with hydrocarbon-rich fuel. The sudden oxidation of gasoline or diesel fuel releases energy that drives the pistons, crankshaft, transmission, differentials and wheels. Ideally, when precise amounts of oxygen and fuel combine, the only waste products are water and carbon dioxide, as is shown in this gasoline combustion equation:

2 C8H18 (fuel) + 25 O2 (oxygen) → 18 H2O (water) + 16 CO2 (carbon dioxide) + energy

Real-world combustion is hardly ideal, though. Undesirable variations in gasoline composition, fuel contaminants, engine condition, atmospheric conditions or even driver demand can result in incomplete combustion and unwanted chemical reactions. For example, high cylinder temperatures lead to the formation of nitrogen oxides (NOx), and poor combustion leads to the formation of carbon monoxide (CO) and unburned fuel (HC), which means these harmful and toxic compounds can make their way into the air.

What Does a Catalytic Converter Do for Emissions?

Air-fuel ratio sensor feedback and exhaust gas recirculation can mitigate some of the inefficiencies of real-world combustion, but not all. The catalytic converter is the last defense before engine exhaust escapes into the world. How a catalytic converter works is by enabling specialized chemical reactions, using heat and rare earth metals such as platinum, palladium and rhodium to combine unwanted emissions into safer compounds:

  • Nitrogen oxides and carbon monoxide produce carbon dioxide and nitrogen: 2 CO + 2 NOx → 2 CO2 + N2
  • Fuel and nitrogen oxides produce carbon dioxide, water and nitrogen: HC + NOx → CO2 + H2O + N2
  • Hydrogen and nitrogen oxides produce water and nitrogen: 2 H2 + 2 NOx → 2 H2O + N2
  • Carbon monoxide oxidizes, producing carbon dioxide: 2 CO + O2 → 2 CO2
  • Fuel oxidizes, producing carbon dioxide and water: HC + O2 → H2O + CO2

Heat is important for catalytic converter function — reactions start at 800°F (426°C) — so some catalytic converters are built into the exhaust manifold. This helps get them up to operating temperature sooner, reducing or eliminating undesirable emissions. These reactions all happen inside the metal body of the catalytic converter.

What Does a Catalytic Converter Do to Performance?

Many believe catalytic converters reduce performance and efficiency. This myth can be traced back to the early days of automotive emissions equipment. Early catalytic converters did use restrictive designs (like layers of treated pellets) which also had a tendency to clog up. But modern catalytic converters use much better flowing matrix designs which also last longer. People who have gutted their catalytic converters or simply put in straight piping have indeed reported improved performance in the 5 to 10% range. If your aim is to extract every bit of power on track day, then a high-flow catalytic converter or straight pipes might be highly desirable, but for such small gains on a daily driver, why risk failing emissions inspections or receiving fines? Removing a catalytic converter also runs afoul of the 1990 Clean Air Act, so you’ve been warned. If you do decide to remove your catalytic converter, you’ll need to keep your de-catted car off public roads.

What In a Catalytic Converter Makes It Valuable?

No doubt you have heard about the problem of catalytic converter theft. Those specials rare earth metals mentioned earlier that work so well in creating those chemical reactions are also valuable for other uses. Platinum makes excellent shiny jewelry. Palladium and rhodium have many uses in the electronics industry. Ironically though the stolen and resold catalytic converter are likely to have their valuable metals recovered and recycled right back into new catalytic converters. Certain vehicles like the Toyota Prius are especially prone to catalytic convert theft due to the high concenveration of precious metals required to make the catalyst process work with lower engine temperatures.

Check out all the exhaust products available on NAPAOnline or trust one of our 17,000 Auto Care locations for routine maintenance and repairs. For more information on what is a catalytic converter and what it does, chat with a knowledgeable expert at your local NAPA Auto Parts store.

Photo courtesy of Royal Air Force Mildenhall.


Benjamin Jerew View All

Ben has been taking things apart since he was 5, and putting them back together again since he was 8. After dabbling in DIY repairs at home and on the farm, he found his calling in the CGCC Automobile Repair program. After he held his ASE CMAT for 10 years, Ben decided he needed a change. Now, he writes on automotive topics across the web and around the world, including new automotive technology, transportation legislation, emissions, fuel economy and auto repair.

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