There are millions of cars on the road in the United States, and
each one is a source of air pollution. Especially in large cities, the
amount of pollution that all the cars produce together can create big
problems.

To solve those
problems, cities, states and the federal government create clean-air
laws that restrict the amount of pollution that cars can 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.

Catalytic Converter Image Gallery

The location of a catalytic converter in a car.See more catalytic converter pictures.

Catalytic
converters are amazingly simple devices, so it is incredible to see how
big an impact they have. In this article, you will learn which
pollutants are produced by an engine and how a catalytic converter
deals with each of these pollutants to help reduce vehicle emissions.

There are millions of cars on the road in the United States, and
each one is a source of air pollution. Especially in large cities, the
amount of pollution that all the cars produce together can create big
problems.

To solve those
problems, cities, states and the federal government create clean-air
laws that restrict the amount of pollution that cars can 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.

Catalytic Converter Image Gallery

The location of a catalytic converter in a car.See more catalytic converter pictures.

The third stage of conversion is a control system that monitors the exhaust stream, and uses this information to control the fuel injection system. There is an oxygen sensor
mounted upstream of the catalytic converter, meaning it is closer to
the engine than the converter. This sensor tells the engine computer
how much oxygen is in the exhaust. The engine computer can increase or
decrease the amount of oxygen in the exhaust by adjusting the
air-to-fuel ratio. This control scheme allows the engine computer to
make sure that the engine is running at close to the stoichiometric
point, and also to make sure that there is enough oxygen in the exhaust
to allow the oxidization catalyst to burn the unburned hydrocarbons and
CO.

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.

One
simple solution to this problem is to move the catalytic converter
closer to the engine. This means that hotter exhaust gases reach the
converter and it heats up faster, but this may also reduce the life of
the converter by exposing it to extremely high temperatures. Most
carmakers position the converter under the front passenger seat, far
enough from the engine to keep the temperature down to levels that will
not harm it.

Preheating the catalytic
converter is a good way to reduce emissions. The easiest way to preheat
the converter is to use electric resistance heaters. Unfortunately, the
12-volt electrical systems on most cars don't provide enough energy or
power to heat the catalytic converter fast enough. Most people would
not wait several minutes for the catalytic converter to heat up before
starting their car. Hybrid cars that have big, high-voltage battery packs can provide enough power to heat up the catalytic converter very quickly.

Catalytic converters in diesel engines
do not work as well in reducing NOx. One reason is that diesel engines
run cooler than standard engines, and the converters work better as
they heat up. Some of the leading environmental auto experts have come
up with a new system that helps to combat this. They inject a urea
solution 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.

source:www.HowThingsWork.com