Most modern cars are equipped with three-way catalytic converters. "Three-way" refers to the three regulated emissions it helps to reduce -- carbon monoxide, VOCs and NOx molecules. The converter uses two different types of catalysts, a reduction catalyst and an oxidization catalyst. Both types consist of a ceramic structure coated with a metal catalyst, usually platinum, rhodium and/or palladium. The idea is to create a structure that exposes the maximum surface area of catalyst to the exhaust stream, while also minimizing the amount of catalyst required (they are very expensive).
The Reduction Catalyst
The reduction catalyst is the first stage of the catalytic converter.
It uses platinum and rhodium to help reduce the NOx emissions. When an
NO or NO2 molecule contacts the catalyst, the catalyst rips the nitrogen
atom out of the molecule and holds on to it, freeing the oxygen in the
form of O2. The nitrogen atoms bond with other nitrogen atoms that are
also stuck to the catalyst, forming N2. For example: 2NO
=> N2 + O2 or 2NO2 => N2 + 2O2
The Oxidization Catalyst
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. This catalyst aids the reaction
of the CO and hydrocarbons with the remaining oxygen in the exhaust gas.
For example: 2CO + O2 => 2CO2
But where did this oxygen come from?
The Control System
The third stage 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 is. 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.
Other Ways to Reduce Pollution
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.
The three harmful compounds are:
Hydrocarbons (in the form of unburned gasoline)
Carbon monoxide (formed by the combustion of gasoline)
Nitrogen oxides (created when the heat in the engine forces nitrogen
in the air to combine with oxygen)
Carbon monoxide is a poison for any air-breathing animal. Nitrogen oxides lead to smog and acid rain, and hydrocarbons produce smog.
In a catalytic converter, the catalyst (in the form of platinum and
palladium) is coated onto a ceramic honeycomb or ceramic beads that are
housed in a muffler-like package attached to the exhaust pipe. The catalyst
helps to convert carbon monoxide into carbon dioxide. It converts the hydrocarbons
into carbon dioxide and water. It also converts the nitrogen oxides back
into nitrogen and oxygen.
There are two ways a converter can fail:
It can become clogged.
It can become poisoned.
A catalytic converter relies on receiving the proper mix of exhaust gases at the proper temperature. Any additives or malfunctions that cause the mixture or the temperature of the exhaust gases to change reduce the effectiveness and life of the catalytic converter. Leaded gasoline and the over-use of certain fuel additives can shorten the life of a catalytic converter.
A catalytic converter can also fail because of:
Bad exhaust valves on the engine
Fouled plugs causing unburned fuel to overheat the converter
Sometimes you can tell that a converter is clogged because you don't go any faster when you push the gas pedal. Also, there usually is a noticeable drop in gas mileage associated with a clogged catalytic converter. A partially clogged converter often acts like an engine governor, limiting the actual RPMs to a fast idle. A totally clogged converter causes the engine to quit after a few minutes because of all the increased exhaust back pressure. (Plus you may hear "hisses" through any leaks or clamps in the echaust system)
The catalytic converter, like the rest of the emissions system, typically has a warranty length that exceeds the term of the warranty for the rest of a typical U.S. automobile.
Here is a safety reminder: Do not park your car over tall grass or piles of dry leaves. Your car's perfectly running catalytic converter gets very hot…enough to start fires! You can keep it running well by keeping the ignition system in top shape, to prevent any unburnt fuel from entering the catalytic converter.
How can I tell If it's Clogged?
You can do a vacuum check at idle and then at 1500 RPM. If the vacuum
is say 21 inches at idle and like 15 inches at 1500 then the cat is likely
clogged. In a carbureted car you can remove the air cleaner and have someone
hold a strong light near the intake of the carb. Then look across the top
of the carb into the light and rev it up. If you see a cloud of fuel vapor
pumping up and down sort of hovering over the carb then the cat is likely
plugged.
Next, remove the oxygen sensor and start it up. The hole where the oxygen
sensor was will relieve the back pressure and it should run a tad better.
If so, then disconnect the exhaust pipe at the exhaust manifold and try
it again. If it is a clogged cat it will scream like a banchee and rev
up to its normal 4000 rpm.