Catalytic converters are amazingly simple devices that have proven to be highly effective at reducing harmful emissions produced by car engines. How much do you know about catalytic converters?
Cities, states and the federal government have created clean-air laws that restrict the amount of pollution that cars may produce. Over the years, automakers have made many refinements to car engines and fuel systems to keep up with these laws. One of these changes came about in 1975 with an interesting device called a catalytic converter. The job of the catalytic converter is to convert harmful pollutants into less harmful emissions before they ever leave the car's exhaust system.
The stoichiometric point is the ideal ratio of air to fuel. Theoretically, at this ratio, all of the fuel will be burned using all of the oxygen in the air. For gasoline, the stoichiometric ratio is about 14.7:1, meaning that for each pound of gasoline, 14.7 pounds of air will be burned. The fuel mixture actually varies from the ideal ratio quite a bit during driving. Sometimes the mixture can be lean (an air-to-fuel ratio higher than 14.7), and other times the mixture can be rich (an air-to-fuel ratio lower than 14.7).
The main emissions of a car engine are nitrogen gas, carbon dioxide and water vapor. These emissions are mostly benign, although carbon dioxide emissions are believed to contribute to global warming.
Gallon for gallon, new lawn mower engines contribute 93 times more smog-forming emissions than new cars. It's no wonder that the EPA and the state regulators of California are trying hard to get golf ball-sized catalytic converters into lawn mower and other small engines. Pending regulations proposed in California could reduce emissions by the equivalent of 800,000 cars per day.
In chemistry, a catalyst is a substance that causes or accelerates a chemical reaction without itself being affected. Catalysts participate in the reactions, but are neither reactants nor products of the reaction they catalyze.
The catalytic converter does a great job at reducing the pollution, but it can still be improved substantially. One of its biggest shortcomings is that it only works at a fairly high temperature. When you start your car cold, the catalytic converter does almost nothing to reduce the pollution in your exhaust.
A urea solution is injected in the exhaust pipe, before it gets to the converter, to evaporate and mix with the exhaust and create a chemical reaction that will reduce NOx. Urea, also known as carbamide, is an organic compound made of carbon, nitrogen, oxygen and hydrogen. It's found in the urine of mammals and amphibians. Urea reacts with NOx to produce nitrogen and water vapor, disposing more than 90 percent of the nitrogen oxides in exhaust gases.
A standard catalytic converter contains several hundred dollars worth of platinum, palladium and rhodium.
The oxidation catalyst is the second stage of the catalytic converter. It reduces the unburned hydrocarbons and carbon monoxide by burning (oxidizing) them over a platinum and palladium catalyst.
There are two main types of structures used in catalytic converters -- honeycomb and ceramic beads. Most cars today use a honeycomb structure.