What Does the Catalytic Converter Do?
Catalytic converters convert some of the harmful and toxic constituents of exhaust gas, such as carbon monoxide, various sulfur-based compounds, NOx (oxides of nitrogen), various aromatics, as well as various volatile organic compounds into oxygen, nitrogen, carbon dioxide, and water vapor.
Why is the Catalytic Converter Needed?
If automotive fuels were made of only hydrocarbons, and these fuels were combusted as part of stoichiometric air/fuel mixtures*, the only exhaust emissions we’d have would be water and carbon dioxide.
* A stoichiometric air/fuel mixture is one in which all of the available air is used to combust all of the available fuel. As a point of interest, if the fuel were gasoline, a stoichiometric air/fuel ratio would be 14.7 parts of air to 1 part of fuel. If the fuel were diesel, the stoichiometric ratio would be 14.5 parts of air to 1 part of fuel. In practice though, some diesel engines can run on mixtures that contain as many as 60 parts of air to 1 part of fuel under some operating conditions.
However, while gasoline and diesel are both based on hydrocarbons, both fuels each contain more than 150 chemicals that include, among many others, lubricants, detergents, combustion enhancers, combustion stabilizers, and various others that determine a fuel’s resistance to ignition, and especially its resistance to premature ignition, also known as “knocking”, which can cause fatal engine damage.
We need not delve into the finer details of fuel chemistry here, but suffice it to say that there exists a direct correlation between the steady increase in the dynamic compression ratio of engines over the past six or seven decades and the steady change in the composition of exhaust gas over the same period. It turns out that as engineers progressively increased the compression ratio of each new generation of engines to improve their performance, the bigger the contribution that fuel additives played in the creation of harmful oxides of nitrogen and sulfur-based compounds in exhaust gas, became.
Put differently, this means that the more engineers improved combustion processes to improve engines’ power output, the more exhaust emissions the improved engines created. This was particularly true in the case of oxides of nitrogen, which is among the chief components of urban smog, the toxic brown haze that began to smother cities by the late 1960s.
Of course, internal combustion engines were not the only emitters of oxides of nitrogen. However, the vast numbers of vehicles that were concentrated in large urban areas at the time played a major part in creating the vast clouds of toxic pollution that caused symptoms in humans that ranged from minor allergic reactions to respiratory distress, to respiratory failure, to pollution-induced cancer and premature death.
By the early 1970s, this grim situation prompted regulatory authorities in the US automotive market to issue directives to car manufacturers to develop car-mounted devices and methods to clean up the exhaust emissions their products produced. While the history of how these directives affected car design decisions falls outside the scope of this article, we can say that at the time, carmakers generally thought that it would not be possible to develop such devices and methods for at least another twenty years or more.
Nonetheless, regulatory authorities insisted that carmakers find a solution much sooner than that, and so by 1973, the Engelhard Corporation had developed a catalytic converter that was suitable for use on motor vehicles. We can skip over the rest of this history lesson, except for saying that as a result of this development, regulatory bodies in the US mandated that all new cars must be fitted with the newly developed catalytic converters from the beginning of 1975.
We can conclude this section by saying that from 1975 onwards, the new two-way catalytic converters produced consistent improvements in the air quality in large metropolitan areas, although smog remained a major health concern because two-way catalytic converters could not eliminate nitric oxide and nitrogen dioxide from vehicle exhaust gas. These two substances are known collectively as NOx and are the among main ingredients of urban smog.
To address the limitations of two-way catalytic converters, more effective three-way catalytic converters that were designed expressly to eliminate NOx from gasoline exhaust were introduced in 1981. The intervening years have seen some refinements in catalytic converter design, such as the development of specific catalysts and selective catalytic reduction processes for use in diesel vehicles. As a result, three-way catalytic converters remain the most effective way of removing harmful substances from both diesel and gasoline exhaust.
How Does the Catalytic Converter Work?
NOTE: The chemical actions/reactions that occur within catalytic converters are hugely technical subjects that require a solid grounding in both inorganic chemistry and metallurgy to fully understand. Therefore, the finer details of the chemical actions/reactions that make catalytic converters work fall outside the scope of this article, but we can do the next best thing, which is to explain some of the technical terms that pertain to catalytic converters, which should help to explain how catalytic converters work. Let us start with-
Also known as, a “core”, the substrate in modern catalytic converters is a block of ceramic material with a honeycomb-like structure, such as the example shown below-
This greatly enlarged view shows the channels that extend the length of the block of a substrate, and through which the exhaust gas flows as it passes through the catalytic converter. This type of design greatly increases the surface area of the catalyst-coated substrate that is in direct contact with the exhaust gas as it flows through the converter.
Catalysts are substances whose presence in a chemical action or reaction initiates and/or sustains the process of changing one substance into another, without the catalyst being consumed in the process.
In the context of automotive catalytic converters, typical catalysts are precious metals that include platinum, palladium, and rhodium, as well as trace amounts of cerium, iron, manganese, and nickel. Note that these metals are applied to all the surfaces in the substrate’s structure that are exposed to exhaust gas in varying amounts and relative concentrations, depending on the vehicle and the fuel used.
The wash coat can be thought of as a sort of primer that helps the catalysts to “stick” to the smooth surfaces of the substrate. Typical wash coat materials include aluminum oxide, titanium dioxide, silicon dioxide, and/or a mixture of silica and alumina, all of which leave a rough, uneven surface after application, which further increases the area that is exposed to the exhaust gas. In almost all catalytic converter designs, the catalytic metals are mixed with the wash coat before the wash coat is applied to ensure even distribution of the catalysts throughout the substrate.
Also known as, a catalytic converter’s operating temperature, this temperature is critical for the operation of any catalytic converter, because the process of converting toxic substances into harmless substances can only begin and become self-sustaining at high temperatures.
Depending on both the vehicle and design of a catalytic converter, this temperature can be as low as 400 degrees (F), or as high as 750 degrees (F), assuming that a) the vehicle’s engine is in good mechanical condition, and b) that there are no excessive hydrocarbon loads (un-combusted fuel and oil) in the exhaust stream.
Two-way catalytic converter
Two-way catalytic converters are called “two-way converters” because they could only perform two functions. These functions are-
1) Converting toxic carbon monoxide into carbon dioxide (which occurs naturally in the atmosphere) through a process of oxidation, in which process oxygen is added to the carbon monoxide, which then turns into carbon dioxide.
2) Converting uncombusted and partially combusted fuel molecules (hydrocarbons) through a process of oxidation, but in this reaction, the excess oxygen causes the hydrocarbons to combust fully, thus producing carbon dioxide and water vapor.
As stated elsewhere, two-way catalytic converters could not reduce nitric oxide, so these converters were replaced by-
Three-way catalytic converters
Three-way catalytic converters perform the same function as two-way converters, but with the added advantage that they also convert nitric oxide and nitrogen dioxide into nitrogen and oxygen through a process of reduction, which process involves removing oxygen from the nitric oxide.
As a practical matter, this process requires a different combination of catalytic materials. To provide this, most three-way catalytic converters contain two separate substrate blocks, each with a different combination of catalytic materials, but note that in some cases, the reduction catalysts are housed in a separate enclosure known as the “pre-catalytic converter” that is located immediately after the exhaust manifold, which brings us to this question-
How efficient are modern catalytic converters?
As a general rule, new catalytic converters on new vehicles remove between 95 and 98 percent of harmful substances from vehicle exhaust gas, but only for their first 4000 miles (or so) of service as a result of new engines typically burning more oil than an engine that has been “run-in”.
In practice, though, after about 4000miles of use, the efficiency of catalytic converters generally reduces to about 92 percent, and provided the engine remains in good mechanical condition, fuel trims remain within specifications, and the engine oil is replaced regularly, most modern catalytic converters will remain between about 92 and 95 percent efficient for several years.
However, it is important to note that as a result of revised Euro 6 emissions regulations, the threshold at which a modern engine management system will deem a catalytic converter to be inefficient can be as high as 90 percent (or higher on some vehicles) based on inputs not only from the oxygen sensors but also from several other sources.
Depending on the vehicle, the engine management system is programmed to set one or more catalytic converter efficiency trouble codes when a catalytic converter’s efficiency reduces to between 90 and 92 percent, at which point the vehicle will not pass an emissions test.
Note also that since catalytic converter failures within 60 000 to 80 000 miles of use very seldom involve catalytic converters themselves, the underlying cause of such a failure must be found and resolved before any catalytic converter is replaced to avoid a recurrence of premature catalytic converter failure(s).
Where is the Catalytic Converter Located on the Engine?
This image shows an example of a catalytic converter forming an integral part of the exhaust manifold on a 2005 Honda Civic Hybrid. In these kinds of designs, the converter and exhaust manifold must be replaced as an assembly.
In cases where the catalytic converter is integrated into the exhaust system, it is always placed as close to the engine as possible, as shown above. Theoretically, it could be placed anywhere in the exhaust system, but if it is located close to the engine, the hot exhaust gas passing through it after a cold start greatly reduces the time it takes for the converter’s core to reach its “lighting off”, aka it’s operating temperature. Note that on some vehicles, the catalytic converters’ warm-up time is further reduced by computer-controlled heating elements that are built into the converters’ core.
What Does the Catalytic Converter Look Like?
This image shows an example of a typical three-way catalytic converter. Note, though, that although many catalytic converters often resemble mufflers, the actual appearance and location of converters vary greatly between vehicle makes and models.
Nonetheless, in all cases, catalytic converters are always located upstream of mufflers, meaning that converters will always be the first muffler-like object in the exhaust system based on the direction of flow through the exhaust system.
What are the Symptoms that the Catalytic Converter is Bad?
Possible symptoms of defective catalytic converters are largely similar across all vehicle makes and models, depending on the nature of the failure, could include one or more of the following-
- Stored trouble codes and illuminated warning lights will be present
- The idling quality may be poor as a result of excessive exhaust back pressure upstream of the converter
- Varying degrees of power loss may be present as a result of poor exhaust gas scavenging
- In some cases, the engine may overheat, or run hotter than usual as a result of poor exhaust gas scavenging
- Most engines will exhibit misfire-like symptoms at most, if not all engine speeds and loads if the catalytic converter is blocked or restricted for whatever reason
- Fuel consumption may increase noticeably
- Depending on the severity of a catalytic converter failure, such as when the core had melted, a hard starting or even no-start condition may be present
- The vehicle will not pass a mandatory emissions test if any fault codes relating to a catalytic converter failure are present
Note though, that in some cases, there may be no drivability symptoms present.
How do you test the Catalytic Converter?
There was a time (up to about 2015) when it was possible to infer the efficiency of a catalytic converter by comparing the temperatures of the converter’s inlet with the outlet. If the engine was in closed-loop operation, the temperature difference typically exceeded about 100 degrees (F) if the converter worked as expected, because the catalytic materials in the converter were actively converting harmful exhaust gases into harmless substances such as water vapor.
This is no longer a valid test method because advanced engine designs and refinements in the design of catalytic converter cores now combine to produce insignificant temperature differences between the inlet and outlet of a modern catalytic converter. In fact, in many cases, the temperature difference might be only a few degrees, or there may be no temperature difference at all. So, as a general rule, temperature differences, or more to the point, the lack of temperature differences in catalytic converters on post-2015 vehicles do not have much diagnostic value.
However, it is important to note that there are exceptions to this rule, because some types of engine management, exhaust system, and catalytic converter failures sometimes create conditions that affect the operation, efficiency, and durability of catalytic converters, but we need not delve into the causes of all possible such failures and malfunctions here.
Suffice it to say that most of these kinds of issues typically manifest in excessive exhaust backpressures upstream of an affected converter, excessively high exhaust gas temperatures both before and after an affected converter, and sometimes, a melted catalytic converter core as a result of both excessively high exhaust backpressure and exhaust gas temperatures.
In terms of practicalities, however, it is very seldom, if ever, possible to test and/or verify the efficiency of a catalytic converter on a DIY basis, because the process requires special tools, equipment, and a high level of technical knowledge. Therefore, we strongly recommend that you seek professional assistance with diagnosing or testing a suspected failed catalytic converter.
How do you replace the Catalytic Converter?
With some exceptions, most catalytic converters are welded into the exhaust system, meaning that removing such a catalytic converter from a vehicle on a DIY basis could require removing much, if not most of the exhaust system from the vehicle. In practice, this is not a practical proposition if you do not have access to a vehicle lift or specialized cutting and welding equipment.
Note that removing a bolted-in catalytic converter can sometimes be challenging even for professional mechanics that do have the required skill, equipment, and tools to undertake this kind of work, which could include removing and/or disassembling unrelated components to access the suspect catalytic converter.
Based on the above, we do not recommend that non-professional mechanics attempt to remove a catalytic converter from any vehicle, due to the very real possibility that the attempt could result in serious personal injury or significant damage to the vehicle.