Catalytic Converter Technical Details

The catalytic converter consists of several components:

1. The core, or substrate. In modern catalytic converters, this is most often a ceramic honeycomb, however stainless steel foil honeycombs are also used. The purpose of the core is to "support the catalyst" and therefore it is often called a "catalyst support". The ceramic substrate was invented by Rodney Bagley, Irwin Lachman and Ronald Lewis at Corning Glass for which they were inducted into the National Inventors Hall of Fame in 2002.

2. The washcoat. In an effort to make converters more efficient, a washcoat is utilized, most often a mixture of silicon and aluminium. The washcoat, when added to the core, forms a rough, irregular surface which has a far greater surface area than the flat core surfaces, which is desirable to give the converter core a larger surface area, and therefore more places for active precious metal sites. The catalyst is added to the washcoat (in suspension) before application to the core.

3. The catalyst itself is most often a precious metal. Platinum is the most active catalyst and is widely used. However, it is not suitable for all applications because of unwanted additional reactions and/or cost. Palladium and rhodium are two other precious metals that are used. Platinum and rhodium are used as a reduction catalyst, while platinum and palladium are used as an oxidization catalyst. Cerium, iron, manganese and nickel are also used, though each has its own limitations. Nickel is not legal for use in the European Union (due to nickel hydrate formation). While copper can be used, its use is illegal in North America due to the formation of dioxin.


Rich Burn Spark Ignition Engines

Catalytic converters are used on spark ignition (gasoline; liquified petroleum gas (LPG); flexible fuel vehicles burning varying blends of E85 and gasoline; compressed natural gas (CNG)) engines; and compression ignition (diesel) engines.

For spark ignition engines the most commonly used catalytic converter is the three-way converter, which works best used on engines equipped with closed-loop feedback fuel mixture control employing an oxygen (lambda) sensor. While a 3-way catalyst can be used in a open-loop system (and has been for years in the non-road engine market), NOx conversions tend to be less than stellar - and since World emissions regulations are primarily aimed at NOx reduction, open loop fuel systems are now obsolete. To keep the air fuel ratio at stoichiometric (14.7:1 for gasoline), closed loop fuel systems are either fuel injection or a carburetor equipped for feedback mixture control. Within that band, conversions are very high, sometimes approaching 100%. However, outside of that band, conversions tend to fall off very rapidly (see bell curve). Two-way converters have been abandoned on spark ignition engines, due to an inability to control NOx.

A three-way catalyst reduces emissions of CO (carbon monoxide), HC (hydrocarbons), and NOx (nitrogen oxides) simultaneously when the oxygen level of the exhaust gas stream is below 1.0%, though performance is best at below 0.5% O2. Unwanted reactions, such as the formation of H2S (hydrogen sulfide) and NH3 (ammonia), can occur in the three-way catalyst. Formation of each can be limited by modifications to the washcoat and precious metals used. It is, however, difficult to eliminate these side products entirely.

For example, when control of H2S (hydrogen sulfide) emissions is desired, nickel or manganese is added to the washcoat - both substances act to block the adsorption of sulfur by the washcoat. H2S is formed when the washcoat has adsorbed sulfur during a low temperature part of the operating cycle, which is then released during the high temperature part of the cycle and the sulfur combines with HC. For "lean burn" spark ignition engines (e.g. compressed natural gas, or compressed natural gas with diesel fuel pilot injection), an oxidation catalyst is used in the same manner as in a compression ignition engine.

Recently, systems have used a separate early catalytic converter in the system to reduce startup emissions and burn off the hydrocarbons from the extra-rich mixture used in a cold engine. Also, upstream and downstream parts are now often separated in the system to provide an optimum temperature and space for extra oxygen sensors. The converter needs to be placed close enough to the engine to quickly reach operating temperature but far enough away to avoid heat damage.

Early three-way catalytic converters utilized an air tube between the first part of the converter (the NOx part) and the second part, which is virtually unchanged from earlier two-way catalytic converters. This tube was fed by either an air pump (derived from the earlier A.I.R. systems) or by a Pulse Air system. The extra oxygen was used to offset the less precise control of earlier systems by providing the oxygen for the catalyst's oxidizing reaction. The first section was still prone to difficulties on lean conditions with too much oxygen for the NOx reduction to be complete, but the second section always had oxygen available. These systems also commonly included an upstream air injector, either a modified A.I.R. system or another opening in the manifold, to add oxygen into the system to burn the extra-rich mixture used in a cold engine and to allow the additional burning to happen as close to the converter as possible to heat it up to operating temperature quickly.

Newer systems use several techniques to avoid the air tubes. They provide a constantly varying mixture that quickly cycles lean and rich mixtures to keep the first catalyst (NOx reduction) from becoming oxygen loaded and the second catalyst (CO oxidization) sufficiently oxidized, which is less of a concern due to the oxygen created in the first section. They also utilize several oxygen sensors to monitor the exhaust, at least one before the catalytic converter for each bank of cylinders, and one after the converter. Newer systems also often have several units mounted along the pipe to provide different functions rather than one monolithic system.


Diesel Engines

For compression ignition (i.e., Diesel) engines, the most commonly used catalytic converter is the diesel oxidation catalyst. The catalyst uses excess O2 (oxygen) in the exhaust gas stream to oxidize CO (Carbon Monoxide) to CO2 (Carbon Dioxide) and HC (hydrocarbons) to H2O (water) and CO2. These converters often reach 90% effectiveness, virtually eliminating diesel odor and helping to reduce visible particulates (soot), however they are incapable of reducing NOx as chemical reactions always occur in the simplest possible way, and the existing O2 in the exhaust gas stream would react first.

To reduce NOx on a compression ignition engine it is necessary to change the exhaust gas - two main technologies are used for this - selective catalytic reduction (SCR) and NOx (NOx) traps (or NOx Adsorbers).

Another issue for diesel engines is particulate (soot). This can be controlled by a soot trap or diesel particulate filter (DPF), as catalytic converters are unable to affect elemental carbon (however they will remove up to 90% of the soluble organic fraction). A clogging soot filter creates a lot of back pressure decreasing engine performance. However, once clogged, the filter goes through a regeneration cycle where diesel fuel is injected directly into the exhaust stream and the soot is burned off. After the soot has been burned off the regeneration cycle stops and injection of diesel fuel stops. This regeneration cycle will not affect performance of the engine.

All major diesel engine manufacturers in the USA (Ford, Caterpillar, Cummins, Volvo, MMC) starting January 1, 2007 are required to have a catalytic converter and a soot filter inline, as per a new DoT legislation.


Oxygen storage

In order to oxidize CO and HC, the catalytic converter also has the capability of storing the oxygen from the exhaust gas stream, usually when the air fuel ratio goes lean. When insufficient oxygen is available from the exhaust stream the stored oxygen is released and consumed. This happens either when oxygen derived from NOx reduction is unavailable or certain maneuvers such as hard acceleration enrich the mixture beyond the ability of the converter to compensate.