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How Does it Work /
Catalytic
Converter
How a Catalytic Converter Works
The catalytic converter was developed in the early 70's when severe air
pollution was prevalent, but wasn't made mandatory by
the US government until 1976. The catalytic converter or "cat" for short is one
of your vehicles main emission control devices. The
name was derived from the operation the catalytic converter performs; it
utilizes a catalyst and converts the unburned gases to burned gases, "catalytic
converter". The catalyst is heated to excite the chemical (the catalyst is made
of), this burns the unburned fuel making the catalyst even hotter furthering the
effect (thermo reactor). Because of the extreme temperatures the catalytic
converter produces the catalyst material can crack and come apart clogging the
outlet port of the converter causing low power and stalling.
Catalytic Converter Location
Typical Vehicle Underside
The catalytic converter location can vary, on some vehicles the catalytic
converter is built in to the exhaust manifold while others may have multiple
catalytic converters. This car has one main catalytic converter and is located
just behind the exhaust flex tube.
Plugged Catalytic Converter
In this example you can see how when the catalyst breaks apart it can clog
the outlet of the catalytic converter causing the exhaust to back up into the
engine forcing the engine to have low power output and sometimes stall. The
exhaust system in your vehicle is designed to release exhaust gases from the
engine to the rear of the car. This system is designed to have about 3 pounds of
exhaust backpressure under full throttle. This means
the engine should not have to push more than 3 pounds of pressure to release the
exhaust at any given time. If a catalytic converter plugs or breaks apart it
will plug the exhaust system causing engine surging, low power and stalling.
Catalytic Converter Starting to Fail
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 vehicles emissions system.
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.
Over time, 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 then placed 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 to keep our air clean.
Common Problems
- The catalyst in the catalytic converter can break apart causing low power
and stalling.
- The catalytic converter can get very hot causing a fire hazard when
vehicle is parked over dry brush.
How Does it Work
/ Catalytic
Converter
Check Manufacturer Specific Catalytic
Converter Questions and Answers
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