Catalytic Converters

W. Addy Majewski

Abstract: Ceramic catalyst cores are typically wrapped in mounting mats made of ceramic fibers and packaged into steel housings. A number of converter canning technologies were developed, including clamshell, tourniquet, and stuffing. In each of the technologies, the converter shell geometry has to provide the required mounting density for the mat. The design of converter inlet and outlet headers or cones affects the gas flow distribution and pressure drop. In applications with space limitations, catalysts can be placed inside catalytic mufflers.

Introduction
Packaging Materials
Mounting System Design
Canning Methods
Header Design
Converter Testing
Integrated Aftertreatment Systems

Introduction

Catalyst substrates coated with the active catalyst washcoat are packaged in steel housings to form catalytic converters. The major catalytic converter design considerations include:

mechanical durability
heat losses
flow distribution
pressure drop.

Emission performance durability and mechanical durability are the two key aspects of the overall durability of an emission control system. The emission durability depends on the quality of the catalyst coating and on the operating conditions such as temperature or levels of catalyst poisons in the exhaust gas. The catalytic converter design has to provide the required mechanical durability.

Catalytic converters must provide adequate protection for the substrate under the operating conditions in the vehicle’s exhaust system—including exposure to high temperatures and thermal shock, moisture and corrosive environments, as well as mechanical vibration, Table 1 [910]. The final mechanical durability of the emission control system is a combination of the substrate durability, durability of packaging materials and packaging technology.

**Table 1**
Operating conditions of catalytic converters
	Gasoline	Diesel
Temperature range, °C	300-1100	100-650
Temperature gradient, °C	100-300	100-200
Space velocity, 1/hr	30,000-100,000*	60,000-150,000*
Vibration acceleration, g	28	10-20
* - higher S.V. may be used in aftermarket applications

The required catalytic converter life expectancy is on the order of hundreds of thousands of kilometers, depending on the type of application. For example, since 2004, the US EPA durability requirement for emission control systems on heavy-duty diesel engines is 10 years/22,000 hours/435,000 miles (700,000 km), whichever occurs first.

If thermal losses from the converter are important, they have to be modeled during the converter design. Double walled designs with either air gaps or ceramic fiber insulation are commonly used on gasoline converters in the close-coupled location, which are optimized for cold start hydrocarbon performance. Cold start and low temperature performance has also been increasingly important for diesel catalytic converters, in both light- and heavy duty applications. Due to the low temperature of diesel exhaust gases, diesel converters should be placed close to the exhaust manifold or exhaust system insulation should be applied to assure satisfactory catalyst performance.

The geometry of converter headers, especially that of the inlet header, can influence the exhaust gas flow distribution in the catalyst. Flow maldistribution may negatively affect catalyst performance and/or durability. Even if emission performance is not affected, a skillful design of the converter headers can decrease the catalytic converter pressure loss.

Catalyst canning technologies have evolved since the 1990s, driven by the demands of California LEV, ULEV and SULEV gasoline applications and, later, by the emission requirements for various categories of diesel engines. Important factors that have driven the evolution in catalytic converter technology include:

In gasoline applications, the converter was either moved to a close-coupled location, or a second close-coupled pre-converter was introduced in addition to the underfloor converter. The close-coupled converter, installed very close to the exhaust port of the engine, is exposed to high temperatures, high thermal shock conditions, and increased vibration, thus calling for more robust canning.
Ultra-thin wall ceramic substrates for gasoline engines have wall thickness on the order of 0.002-0.003" (0.050-0.075 mm). Example commercial configurations introduced in the late 1990s include 600/3 and 900/2 substrates. These parts are weaker than the older, thicker wall substrates (e.g., 400/6.5, of 0.0065" or 0.17 mm wall thickness). Thinner wall substrates have also been used for diesel applications. New materials and/or packaging methods were necessary to accommodate these parts.
Exhaust systems on heavy-duty engines, especially those incorporating diesel particulate filters (DPF), require very robust packaging. While monolithic DPF substrates are packaged using essentially the same methods as catalytic converters, they are much heavier and larger, thus creating new challenges.
Aftertreatment systems in modern diesel engines can be complex and include a number of catalysts as well as a DPF. These catalysts can be installed as independent units or as integrated aftertreatment systems. In heavy-duty engines, there is a trend to package all aftertreatment components into a single module aftertreatment unit or muffler.

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