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AUTO THEORY

How Do Catalytic Converters Work?

The first cars that first started using catalytic converters are now 30 years old, making them easy candidates for "antique" or "collector car" status. Since some of us are now - or will be shortly - collecting these vehicles it's high time we explain how the converters work.

Back in the pre-emissions regulation days cars put out a lot of pollutants. To solve this problem a number of states and municipalities started mandating emissions levels, the first of which appeared in 1968 in California. By the early '70s the federal government stepped in and started a long phase-in of ever-more-restrictive emissions standards that gradually resulted in today's clean cars (99% cleaner, that is.) The first standards (for 1975 model year cars) were strict enough that conventional technology being used in internal combustion engines was too dirty. Since it was too costly to chuck engine manufacturing designs and components and start over, engineers required most manufacturers to treat the exhaust before it left the car through the use of catalytic converters. They worked so well that they are still used in virtually all vehicles today.

To explain how converters work we must explain the proper stoichiometric point of the fuel. That is, the closer the ratio of fuel vapor to oxygen is kept to "perfect," the more clean the exhaust is. Since the "perfect" ratio is 14.7:1, a car's fuel/ignition system would have to work very hard to maintain that ratio over the entire range of operation. For carbureted cars this was impossible and today's cars are so sophisticated that it would take pages of text to explain the differences. Suffice to say that it takes massive computer power to maintain the proper ratio.

Anyway, if a car's fuel burning was too lean or too rich the emissions left over from combustion would leave through the exhaust. These emissions - nitrogen, carbon dioxide and water vapor - chemically combine and become carbon monoxide, hydrocarbons and volatile organic compounds. Sunlight reacts with some of these to form ozone and nitrogen oxides, just to make matters worse.

Back to the catalytic converter. These muffler-type devices were installed in cars to chemically change the leftover emissions into less-destructive compounds. They do so by means of a reduction catalyst. That is, a very large surface area on the inside of the catalytic converter is coated with a catalyst, usually platinum or rhodium. The surface area is made as large as possible through the use of layered, wound, honeycombed or other metallic structures to "treat" the largest volume of exhaust gas in the shortest time.

When the exhaust gas contacts the catalyst, the catalyst rips a nitrogen atom out of the pollution molecule and holds on to it, freeing the oxygen. The nitrogen atoms bond with other nitrogen atoms that are also stuck to the catalyst, forming N2. Both are then let out of the exhaust and into the air.

This reaction only works at high temperatures, so catalytic converters tend to run very, very hot. Therefore, they require a heat shield to protect the bottom of the car and any flammable materials underneath on the road surface.

Where Is My Catalytic Converter?

The converter is most efficient when it is placed as close to the exiting exhaust from the cylinders to utilize maximum heat. Therefore, great efforts were (and are) made to locate the device as far forward in the exhaust piping as possible, usually within a foot or two of the manifold(s). A heat shield is usually placed above and below the converter to protect the materials inside the car and to prevent fires from parking over leaves, etc.

Things Could Go Wrong

Early catalytic converters consisted of an outer shell (looking much like a muffler) either filled with catalyst "pellets" or layered sheets of metal coated with platinum. Within a relatively short time moisture and other compounds in the exhaust created corrosive acids that eroded away the materials inside the converter. This resulted in rust, which in turn collapsed inside and partially clogged the exhaust path along with failing to clean the exhaust of pollutants.

The resultant back-pressure in the exhaust caused engine performance problems, sometimes total failure. Many people chose to remove the converters from the exhaust system entirely.

The other problem was the gasoline itself. For decades the oil industry used tetraethyl lead as the chief anti-knock compound in gasoline. The lead was the one material that could ruin the catalyst in a converter very quickly, typically within one tank of gas. Unleaded gas was introduced in the early 1970s but it was some years before the general population settled on its use. Meanwhile, catalytic converters were rendered useless by leaded gas.

The other problem associated with earlier catalytic converters was the heat shield. These shields would rust and fall loose from their mountings, creating noise. Owners would rip them off and not replace them, often causing leaf or brush fires.





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