Types Of Catalytic Converter: Exploring the Various Types

As an automotive engineer, I find the variations in catalytic converter design or different types of catalytic converter that have emerged over decades of emissions control innovations fascinating. While converters employ different configurations, active materials, and chemical processes to reduce pollution from internal combustion engines, they all serve the same purpose.

Catalytic converters are available in many “types” depending on the vehicle type and target emissions. There are converters optimized for diesel engines, lean-burn gasoline engines, and conventional engines. Configurations range from single-stage to dual-stage setups. Active metals span from platinum to palladium, rhodium, and more.

This guide will explore modern vehicles’ different type of catalytic converter varieties. Understanding the strengths of each design provides insight into their best applications and how they aid the ultimate goal – preserving air quality for all. A peek into the diverse catalytic converter family reveals the multi-faceted approach to managing emissions.

Different Types of Catalytic Converter

  1. Traditional Two-Way Catalytic Converters
  2. Three-Way Catalytic Converters (TWCs)
  3. Diesel Oxidation Catalysts (DOCs)
  4. Selective Catalytic Reduction (SCR) Converters
  5. Diesel Particulate Filters (DPFs)
  6. Lean NOx Traps (LNT)
  7. Dual-bed catalytic converter
  8. Close-coupled catalytic converter
  9. Underfloor catalytic converter

Traditional Two-Way Catalytic Converters

Two-way catalytic converters were the earliest variety, introduced in 1975. They are best suited for reducing:

Carbon monoxide (CO) – With their platinum and palladium metals, two-way converters oxidize this toxic gas into carbon dioxide. Hydrocarbons (HC) – The catalysts break these unburnt fuel remnants into carbon dioxide and water vapor.

Read More: How Catalytic Converter Work

However, two-way converters have limitations for reducing:

Nitrogen oxides (NOx) require different catalyst chemistry and operating conditions. Two-way converters allow NOx to pass through unconverted. While an upgrade from unmitigated pollution, two-way converters struggle to address the entire spectrum of harmful emissions. This led innovators to develop more advanced converter options.

Three-Way Catalytic Converters (TWCs)

Three-way catalytic converters (TWCs) were a monumental leap in capabilities. They can convert simultaneously:

– Carbon monoxide (CO)

– Hydrocarbons (HC) 

– Nitrogen oxides (NOx)

TWCs utilize platinum, palladium, and rhodium to provide diverse catalytic sites for reduction and oxidation reactions. On-board oxygen sensors detect the exhaust composition and feed data to the engine computer. This allows real-time fuel adjustments to maintain optimal converter conditions for three-way conversion.

Introduced in the 1980s, TWCs remain the standard for gasoline engines today, capable of reducing pollutants by over 90% when operating as designed. Their integrated approach set a new bar for emissions control.

Read More: How Long Do Catalytic Converters Last?

Diesel Oxidation Catalysts (DOCs)

Diesel engines have distinct emissions challenges that require specialized converters like the diesel oxidation catalyst (DOC):

DOCs leverage platinum and palladium catalysts that serve to oxidize:

  • Carbon monoxide (CO) into carbon dioxide
  • Hydrocarbons (HC) in water and carbon dioxide

They provide targeted control of the excess CO and HC emitted by diesel combustion. DOCs are durable, affordable oxidation converters on diesel vehicles, from light trucks to heavy equipment.

While limited in reducing NOx, combining DOCs with downstream catalyst technologies provides a comprehensive solution for mitigating diesel emissions. DOCs deliver specialized catalytic functions where needed.

Selective Catalytic Reduction (SCR) Converters

Selective catalytic reduction (SCR) converters focus on reducing challenging nitrogen oxide (NOx) emissions. They work by:

  • Using a zeolite-based wash coat containing vanadium or iron catalysts optimized to reduce NOx through chemical reactions.
  • Metering a urea-based solution, Diesel Exhaust Fluid (DEF) or AdBlue, into the exhaust stream.
  • DEF vaporizes and decomposes to ammonia, which reacts with NOx on the SCR catalyst surface to form nitrogen and water vapor.

Introduced in the 2010s, SCR systems provide up to 90% NOx conversion for diesel vehicles. Combining an SCR catalyst with a DOC converter enables comprehensive diesel emissions solutions.

Diesel Particulate Filters (DPFs)

Diesel engines suffer from high particulate matter (PM) emissions that must be controlled via filters. Enter the diesel particulate filter (DPF):

  • DPFs are flow-through devices that physically capture PM in the exhaust stream through porous walls that let gases pass.
  • Trapped particulates are eventually oxidized into CO2 by reaction with NO2 or active DPF regeneration, where heat burns off the soot.
  • Advanced engine controls raise exhaust temperatures periodically to clean the DPF through regeneration, avoiding buildup and clogging issues.

First appearing in the 2000s, DPFs now provide a critical reduction of health-hazardous PM from diesel vehicle classes from light-duty to heavy-duty. They complement DOC and SCR systems for comprehensive diesel emissions control.

Lean NOx Traps (LNT)

Lean NOx traps (LNTs) provide an alternate approach to controlling NOx emissions:

  • LNTs utilize alkali or alkaline earth catalysts to adsorb NOx under oxygen-rich exhaust.
  • The NOx is temporarily trapped until sensors trigger active LNT regeneration through rich engine operations.
  • Reductants like CO, HCs, and H2 release the NOx and reduce it to N2 with platinum group metal catalysts.

First appearing in 2001, LNT systems deliver efficient NOx reduction for lean-burn and diesel engines. While requiring complex cycling between lean and rich modes, LNTs exemplify cutting-edge innovations in catalytic converters.

Dual-Bed Catalytic Converter

A dual-bed catalytic converter, or a dual-bed catalytic converter system, is an advanced emissions control device in some vehicles to reduce harmful pollutants in exhaust gases. It consists of two distinct catalytic beds or catalyst substrates arranged in a series within a single converter housing.

A dual-bed catalytic converter is designed to further improve the reduction of specific pollutants, mainly nitrogen oxides (NOx) and carbon monoxide (CO), beyond what can be achieved with a single catalytic bed.

Close-Coupled Catalytic Converter

A specific types of catalytic converter configuration used in the exhaust system of internal combustion engines, primarily in automobiles, is a close-coupled catalytic converter. They position this type of catalytic converter close to the engine’s exhaust manifold, which is the part of the engine where it first expels exhaust gases from the combustion chambers.

Underfloor Catalytic Converter

An underfloor catalytic converter is a specific type of catalytic converter configuration used in the exhaust system of internal combustion engines, especially in many modern vehicles. Unlike close-coupled catalytic converters positioned close to the engine’s exhaust manifold, underfloor catalytic converters are typically located downstream in the exhaust system, often beneath the vehicle’s chassis or floor.

Conclusion on Types of Catalytic Converter

The world of catalytic converters is far from one-size-fits-all. As vehicles and engines evolve to meet stringent emission standards, various converter types have emerged to address distinct challenges. Understanding these different types is essential for making informed choices in vehicle design, maintenance, and compliance with environmental regulations. Whether it’s the versatility of TWCs, the efficacy of SCR converters, or the specialized roles of DPFs and LNTs, each type contributes to a cleaner, more sustainable future on our roads.

I am Nicolas, an automobile engineer with over 5 years of experience in exhaust systems and catalytic converters. I am passionate about learning and understanding how things work, and I am always looking for new ways to improve the performance and efficiency of automotive exhaust systems.

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