Auto Questions: How Catalytic Converters Work

The catalytic converter plays a major role in automobile emission reduction. A guide to function, repair and parts.

If you own a car that's 10 years old or older and you live in an area that requires annual testing for automotive emissions, you may have already heard the dreaded words "You need a new catalytic converter". If you haven't, you soon will. If you're like most drivers, you may be vaguely aware that your car has a catalytic converter but you have only a vague notion of its function. Hopefully this article will help you understand a bit more about this key component of your vehicle.

In the 1970's when we first became concerned about the effect of automobile exhaust on air quality, a great deal of research was done to define the problem. If cars were perfect and burned their fuel completely, the exhaust would consist of carbon dioxide (CO2) and water (H2O). Since nothing is perfect, least of all cars, the analysis of actual exhaust gasses revealed the presence of several troublesome components. First, it was found that a portion of the gasoline that entered the engine was not completely burned and escaped into the exhaust. This component was labeled as "hydrocarbons (HC)". Another result of this incomplete burning was "carbon monoxide (CO)". The third major component was labeled "nitrogen oxides (NOx)". Air is actually composed of over 70% nitrogen which when subjected to the high temperatures and pressures inside an automotive engine combines with some of the oxygen in the air to form these compounds. The exhaust has other components but these are the main elements that the automotive industry focused on.

Because most of these components were created as a result of incomplete burning of the gasoline, a lot of the early work to reduce them was centered on improving the mixing and burning of the air and fuel. Manufacturers experimented with engine and fuel system designs and modified the proportions of fuel to air. Fuel injections systems began to replace carburetors in order to more precisely control the flow of fuel and electronics became a factor in engine control. All of these efforts had some degree of success but as air quality standards became ever more restrictive it was evident that a new approach was needed. It was at this point that the concept of the catalytic converter began to be seriously considered.



What is a catalyst? Many years ago, chemists discovered that there were materials present in certain chemical reactions that were not changed by the reaction. In other words, these materials were the same after the reaction as they were before it. Even more surprising, the reaction would not occur if these materials were not present. Somehow they enabled the reaction to occur without taking part in it. They called these materials catalysts and they are widely used in industry. The materials are often rare earth metals like palladium and platinum which are very expensive. But luckily the amount needed for use as a catalyst is small and since the catalyst is not changed by the reaction it can be used over and over.

As it turns out, experiments showed that passing exhaust gasses over a catalyst of a certain composition will cause the pollutants in the exhaust to be completely burned and the result is water (H2O) and carbon dioxide (CO2). The challenge was to create a catalyst that could be attached to the car and survive for 50,000 or so miles of driving. The catalytic converter is the answer to that challenge. A catalytic converter starts with a ceramic honeycomb about 3 inches by 5 inches by 8 inches. Some are larger and some are smaller. This honeycomb is coated inside and out with a very thin layer of catalyst material. It is them place inside a metal container with a hole where the exhaust gasses enter and one where they exit. The unit is then placed in the exhaust system near the engine. In order for the converter to operate properly, several conditions must be met.

First, the exhaust gasses must be at or above a certain temperature. This is why the converter is placed close to the engine. Second, there must be a certain minimum surface area of catalyst for the gasses to come in contact with. This is the reason for the honeycomb design. It provides a large surface area in a small space. Third, the ratio of exhaust gas to air must be maintained within very rigid limits. If these limits are not observed, the converter will not function and could even be damaged. These limits are maintained by placing a special sensor in the exhaust just before the converter. This sensor detects variations in the ratio and signals the fuel supply system to increase or decrease the amount of fuel being supplied to the engine. Lastly, the converter has to survive its own operating temperature of 1500 degrees F or more, exposure to water, mud, snow and road debris and continue operating for a government mandated period of 50,000 miles or more. The catalytic converter is an amazing device that allows us to enjoy the freedom of our automobiles while helping keep our air clean.

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