Catalytic converter facts for kids

A three-way catalytic converter on a 1996 Dodge Ram truck engine.

A computer model showing how gases flow inside a catalytic converter.

A catalytic converter is a special device in a vehicle's exhaust system. Its main job is to clean up harmful gases from the engine before they go into the air. It changes these bad gases into less harmful ones using a special chemical process called catalysis.

You'll find catalytic converters mostly in cars and trucks that run on gasoline or diesel fuel. They are also used in other machines like electrical generators, forklifts, and even some wood stoves to help keep our air cleaner.

The first widely used catalytic converters appeared in the United States in 1975. These early versions, called "two-way" converters, helped turn carbon monoxide (CO) and unburned fuel (hydrocarbons) into carbon dioxide (CO₂) and water (H₂O).

Later, "three-way" converters were developed. These could do even more! They also reduced harmful nitrogen oxides (NO
x ). By 1981, most cars in the U.S. had these advanced three-way converters to meet stricter air quality rules.

How Catalytic Converters Were Invented

The idea for catalytic converters started in France in the late 1800s. Early designs used special metals like platinum, rhodium, and palladium on clay materials.

A French engineer named Eugene Houdry later patented a catalytic converter. He was worried about air pollution from cars and factories. In the 1950s, he began working on converters for car engines.

Many engineers, including Carl D. Keith and John J. Mooney, helped improve these devices. The first ones for cars were produced in 1973.

By 1975, most new cars in the United States had catalytic converters. This was because of new rules to reduce harmful exhaust gases. These early converters were "two-way" and cleaned up carbon monoxide and unburned fuel.

An important change happened around this time: lead was removed from gasoline. Lead was bad for people and also damaged catalytic converters. Removing lead helped the converters work much better.

In the 1970s, engineers also developed "three-way" converters. These could reduce even more harmful gases, like nitrogen oxides. This was a big step forward in making cars cleaner.

How Catalytic Converters Are Built

A view inside a metal-core catalytic converter.

A ceramic-core catalytic converter, showing its honeycomb design.

A catalytic converter has a few key parts:

• The Core (Substrate): This is usually a ceramic block with a special honeycomb structure. This design creates a very large surface area. Sometimes, metal cores are used for extra heat resistance. This ceramic core was invented by Rodney Bagley, Irwin Lachman, and Ronald Lewis.
• The Washcoat: This is a special layer applied to the core. It's made of materials like aluminum oxide. The washcoat makes the surface rough and uneven, which further increases the area where chemical reactions can happen.
• Oxygen Storage: Materials like ceria are added to help store and release oxygen. This is important for the cleaning process.
• The Catalysts: These are the most important parts! They are usually tiny amounts of precious metals like platinum, palladium, and rhodium. These metals speed up the chemical reactions without being used up themselves. Platinum and palladium help with oxidation (adding oxygen), while rhodium helps with reduction (removing oxygen).

When a catalytic converter stops working, it can be recycled. The valuable precious metals inside, like platinum, palladium, and rhodium, can be recovered and reused.

Where Catalytic Converters Are Placed

For a catalytic converter to work well, it needs to be very hot, about 400 degrees Celsius (750°F). Because of this, they are usually placed very close to the engine. This helps them heat up quickly from the hot exhaust gases.

Sometimes, smaller converters, called "pre-cats," are placed even closer to the engine. This helps them start cleaning the exhaust gases almost immediately after the engine starts.

Types of Catalytic Converters

Two-Way Converters

A two-way catalytic converter does two main jobs at the same time:

• It changes carbon monoxide (CO) into carbon dioxide (CO₂).
• It changes unburned fuel (hydrocarbons) into carbon dioxide and water (H₂O).

These converters are often used in diesel engines. They were also common in gasoline cars in North America until 1981.

Three-Way Converters

Three-way catalytic converters are even more advanced. They do the two jobs of a two-way converter, plus one more important task:

• They reduce harmful nitrogen oxides (NO
  x ) into harmless nitrogen gas (N₂). Nitrogen oxides cause acid rain and smog.

Since 1981, most gasoline cars in many countries use three-way converters. These converters work best when the engine's air-fuel mixture is just right. Car computers use oxygen sensors to keep this mixture balanced.

Converters for Diesel Engines

For diesel engines, a common type is the diesel oxidation catalyst (DOC). This converter uses palladium or platinum to change harmful particles, hydrocarbons, and carbon monoxide into carbon dioxide and water.

DOCs are very good at reducing diesel smell and visible smoke. However, they don't reduce nitrogen oxides. For that, diesel vehicles often use other systems like selective catalytic reduction (SCR). SCR systems use a liquid called diesel exhaust fluid (DEF) to turn nitrogen oxides into nitrogen and water.

Diesel engines also produce tiny soot particles. Catalytic converters only remove some of these. So, many diesel vehicles also have a diesel particulate filter (DPF) to catch the soot.

How Catalytic Converters Are Installed

Many cars have a catalytic converter placed right next to the engine's exhaust manifold. This spot helps it get hot very quickly. A fast warm-up means the converter starts cleaning exhaust gases sooner, especially when the engine is cold.

Older cars sometimes used a system called "secondary air injection." This pumped extra air into the exhaust. The added oxygen helped the converter work better.

Newer cars with electronic fuel injection don't usually need air injection. Their computers precisely control the fuel mixture. Oxygen sensors before and after the converter help the computer make sure the converter is working efficiently.

What Can Harm a Catalytic Converter

Catalytic converters can be damaged by certain substances in the exhaust. The most well-known harmful substance is lead. That's why cars with converters must only use unleaded fuel.

Other things that can harm a converter include sulfur, manganese, and silicon. These can come from fuel additives or engine leaks. For example, if engine coolant leaks into the engine, it can damage the converter.

If an engine has problems that cause too much unburned fuel to reach the converter, it can get extremely hot. This extreme heat can melt the inside of the converter, making it stop working and blocking the exhaust flow.

Rules About Catalytic Converters

Many countries have rules about vehicle emissions. In the United States, for example, it's against the law to remove or disable a catalytic converter unless you're replacing it. This is to make sure vehicles continue to clean their exhaust.

Cars without working catalytic converters usually fail emission tests. There are also special high-performance converters available for vehicles with modified engines.

In the European Union and the United Kingdom, catalytic converters have been required on all new gasoline cars since 1993.

How Converters Affect Exhaust Flow

Early catalytic converters could sometimes block the exhaust flow, which might reduce a car's power or fuel efficiency. However, modern catalytic converters are designed to allow exhaust gases to flow through easily. This means they don't significantly affect a vehicle's performance.

Important Considerations

Warm-Up Time

Cars produce most of their pollution during the first few minutes after starting. This is because the catalytic converter needs time to heat up and become fully effective.

To solve this, car makers often place converters very close to the engine. Some vehicles even have small "pre-cats" or electrically heated converters to warm up even faster. This helps reduce pollution right from the start.

Environmental Impact

Catalytic converters are very good at reducing harmful tailpipe emissions. However, there are a few things to consider:

• Engines with three-way converters use slightly more fuel than some other engine types. This means they produce a little more carbon dioxide (CO₂), which is a greenhouse gas.
• Producing catalytic converters requires precious metals like palladium and platinum. Mining these metals can have environmental effects.
• The converters themselves get extremely hot. In dry areas, this heat can sometimes cause wildfires if the hot converter touches flammable materials.

Protecting Catalytic Converters

Catalytic converters contain valuable precious metals, such as platinum, palladium, and rhodium. Because of this, they can sometimes be targeted by people who want to remove and sell these metals.

This issue is more common with certain types of vehicles, like pickup trucks and SUVs, because they are higher off the ground, making the converter easier to reach. Hybrid cars also have converters that are often targeted because they contain more of these valuable metals.

To help protect these important parts, new laws have been proposed. For example, in the U.S. Senate, the PART Act was introduced in 2023. This law would require new converters to have traceable identification numbers. It would also make it a federal crime to remove them illegally.

The number of reported incidents involving converters increased significantly in the U.S. from the late 2010s to the early 2020s. Replacing a converter can be expensive, often costing over $1,000.

How Converters Are Checked

Modern cars have on-board diagnostics (OBD-II) systems. These systems constantly check how well the emissions control system, including the catalytic converter, is working.

If the system detects a problem, it will turn on the "check engine" light on the dashboard. For example, if the engine is misfiring, which could damage the converter, the light might flash.

Oxygen sensors are key to this system. Cars usually have one oxygen sensor before the converter and one after. The car's computer compares the readings from these two sensors. If the readings are too similar, it means the converter isn't cleaning the exhaust properly, and the "check engine" light will come on.

NO
x sensors are also used, especially in diesel engines with SCR systems. They work similarly to oxygen sensors but monitor nitrogen oxide levels to ensure the system is reducing these harmful gases effectively.

See Also

In Spanish: Convertidor catalítico para niños

• Catalytic heater
• Cerium(III) oxide
• List of auto parts
• NO
  x adsorber
• Roadway air dispersion modeling