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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 catalytic converter is 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?”

The Chemistry of Car Exhaust

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

Replacing a Catalytic Converter with Regular Hand ToolsReal-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. It enables 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.

What Does a Catalytic Converter Do to Performance?

Many believe catalytic converters reduce performance and efficiency. 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? If you do decide to remove your catalytic converter, you’ll need to keep your de-catted car off public roads.

Check out all the exhaust products available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on catalytic converters, 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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