What is a Catalytic Converter?

The catalytic converter or 'Cat-Con' is a mechanical device. It reduces the

harmful emissions created in the exhaust system of an engine. It is an
important device
as it works with
the harmful
gases, which the
engine creates
during
combustion
of fuel. The main
purpose of a
catalytic
converter is to
reduce exhaust
emissions.

Engine Exhaust System

The Cat-Con reduces the

harmful exhaust gases
through chemical
reactions. It reacts with
the harmful pollutants in
the exhaust gases and
turns them into lesser
harmful gases. The
catalytic converter
consists of a special
catalyst. It is made of platinum and palladium which carries out the chemical
reactions.

For an automobile’s internal combustion engine to operate, a controlled

combustion reaction needs to occur inside the vehicle’s engine. But this
reaction also produces harmful burnt gases that contribute significantly to air
pollution. And good air quality is very important for an individual’s overall
health.

In order to reduce air pollution, modern automobiles are equipped with a

device called a catalytic converter that reduces emissions of three harmful
compounds found in car exhaust:

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  Carbon monoxide (a poisonous gas)
 Nitrogen oxides (a cause of smog and acid rain)
 Hydrocarbons (a cause of smog)

These are converted into less harmful compounds before leaving the car’s
exhaust system. This is accomplished using a catalyst, which gives the device
its name

How does it work?

Before catalytic converters were

developed, waste gases made by a car
engine blew straight down the exhaust
tailpipe and into the atmosphere. The
catalytic converter sits between the
engine and the tailpipe, but it doesn't
work like a simple filter: it changes the
chemical composition of the exhaust
gases by rearranging the atoms from
which they're made:

1. Molecules of polluting gases are

pumped from the engine past the
honeycomb catalyst, made from
platinum, palladium, or rhodium.
2. The catalyst splits up the
molecules into their atoms.
3. The atoms then recombine into
molecules of relatively harmless
substances such as carbon
dioxide, nitrogen, and water, which
blow out safely through the
exhaust.

Construction of Catalytic Converter:

The catalytic converter housing consists of a honeycomb core from inside. It is
coated with precious metals such as platinum and rhodium. These metals react
with the engine’s exhaust gases. They reduce the toxic contents of the gases
and turn them into carbon dioxide and water.

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BASIC COMPONENTS:

(1) Substrate: is ceramic honeycomb like

structure with thousands of parallel channels
that provide a large surface area for the
engine exhaust.

(2) Wash Coat: A coating that increases the

effective surface area of the substrates
& facilitates the application of precious metal
catalyst onto the surface of the ceramic surface of the ceramic substrate.

(3) Catalyst: Precious metal catalyst-the heart of catalytic converter, applied to

wash coated ceramic substrate

(4) Mat: It provides thermal insulation & protects against mechanical shock
&chassis vibration.

(5) Can: A metal package that encases all the above components.

(6) Heat Shields: They are used to protect various parts surrounding the
catalytic converter, form thermal shocks

Different Variations

The key types of catalytic

converters are listed below
with a brief introduction:

 Two-way
oxidation - The
two-way oxidation
instruments
performs two
simultaneous tasks
of oxidation of
carbon monoxide to carbon dioxide and oxidation of hydrocarbons to
carbon dioxide and water. This converter is widely used on diesel
engines to reduce hydrocarbon and carbon monoxide emissions.

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 However, this is no longer used in the U.S. and Canada due to their
inability to control oxides of nitrogen.

 Three-way oxidation-reduction - The three-way oxidation-reduction

devices are used in vehicle emission control systems in most parts of
the world including the U.S. and Canada. The strict vehicle emission
regulations have almost made it mandatory to use the three-way
converters on gasoline-powered vehicles. These converters have come
to be recognized as one of the most important inventions in the history
of the automobiles. These perform three simultaneous tasks, namely,
reduction of nitrogen oxides to nitrogen and oxygen, oxidation of
carbon monoxide to carbon dioxide, and oxidation of unburnt
hydrocarbons to carbon dioxide and water.

 Diesel Oxidation Catalyst (DOC) - DOC's are most commonly used for
compression-ignition such as diesel engines. This device uses oxygen in
the exhaust gas stream to convert carbon monoxide to carbon dioxide and
hydrocarbons to water and carbon dioxide. These converters are known
to perform at 90% efficiency, wherein they manage to remove diesel odor
and reduce visible particulates and nitrous emissions.

Symptoms of a bad catalytic converter:

1. “Check Engine Light” is ON:
 If your vehicle’s catalytic converter is failing or has gone bad, the “Check
Engine Light” will illuminate on the dashboard. Modern vehicles contain
air fuel/oxygen ratio sensors which are able to monitor the catalytic
converters efficiency by checking the exhaust’s gas levels. If these
sensors detect the exhaust gases are not being catalyzed properly for
whatever reason, the “Check Engine Light” will come on.

2. Engine Poor Power:

 The most common symptom of a bad catalytic converter is a lack of power
in the engine, particularly when you try to accelerate the vehicle. The
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 exhaust system of the vehicle contains the catalytic converter, which is
why the engine’s performance is affected when the converter experiences
problems.

3. Fuel Economy is Low:

 A failing catalytic converter means that fuel use will be impacted by the exhaust
process. It is hard to detect small drops in your miles-per-gallon but if the
highway MPG drops more than 10%, then there is a problem. For example, if a
vehicle gets 35 highway MPG and it drops to under 32 MPG, then the catalytic
converter is likely to blame.

4. Rattling Noise:
 A bad catalytic converter could create a rattling noise, especially if the
converter is internally damaged or just old. Often times, years of fuel
mixing will cause the honeycomb meshes of the converter’s interior to
break apart, which is why the rattling noise occurs. You’ll hear the noise
when you start the car, but then the rattling will get louder as you continue
to drive your car.

5. Emissions Increase:
 The catalytic converter may not perform properly if its internal chemical
mechanisms are contaminated with substances like antifreeze or motor
oil. If something like this occurs, the carbon emission levels of your
exhaust will be a lot higher. This will require you to replace the catalytic
converter and then repair the main area where the problem started. If you
don’t fix this source, your whole exhaust system will get damaged and
then emit large amounts of emissions that will rise into the atmosphere.
There are laws against emitting too many emissions, so you could get in
legal trouble if you let this problem continue.

Environmental Impact
The catalytic converter was specifically invented to decrease harmful pollution
caused by the combustion of hydrocarbon-based fossil fuels in cars. Studies
reveal that these devices can decrease hydrocarbon emissions by about

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almost 87%, carbon monoxide by 85%, and nitrous oxide by 62% during the
expected life of a vehicle.

Exhaust gas recirculation

In internal combustion engines, exhaust gas recirculation (EGR) is a nitrogen
oxide (NOx) emissions
reduction technique used
in petrol/gasoline and diesel
engines. EGR works by
recirculating a portion of
an engine's exhaust
gas back to the
engine cylinders. This
dilutes the O2 in the
incoming air stream and
provides gases inert to
combustion to act as absorbents of combustion heat to reduce peak in-
cylinder temperatures. NOx is produced in a narrow band of high cylinder
temperatures and pressures.

In a gasoline engine, this inert exhaust displaces the amount of combustible

matter in the cylinder. In a diesel engine, the exhaust gas replaces some of
the excess oxygen in the pre-combustion mixture. Because NOx forms
primarily when a mixture of nitrogen and oxygen is subjected to high
temperature, the lower combustion chamber temperatures caused by EGR
reduces the amount of NOx the combustion generates (though at some loss
of engine efficiency( .Gases re-introduced from EGR systems will also contain
near equilibrium concentrations of NOx and CO; the small fraction initially
within the combustion chamber inhibits the total net production of these and
other pollutants when sampled on a time average. Most modern engines now
require exhaust gas recirculation to meet emissions standards. Chemical
properties of different fuels limit how much EGR may be used. For
example methanol is more tolerant to EGR than gasoline.

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The exhaust gas, added to the fuel, oxygen, and combustion products,
increases the specific heat capacity of the cylinder contents, which lowers
the adiabatic flame temperature.

In a typical automotive spark-ignited (SI) engine, 5% to 15% of the exhaust

gas is routed back to the intake as EGR. The maximum quantity is limited by
the need of the mixture to sustain a continuous flame front during the
combustion event; excessive EGR in poorly set up applications can cause
misfires and partial burns. Although EGR does measurably slow combustion,
this can largely be compensated for by advancing spark timing. The impact of
EGR on engine efficiency largely depends on the specific engine design, and
sometimes leads to a compromise between efficiency and NOx emissions. A
properly operating EGR can theoretically increase the efficiency of gasoline
engines via several mechanisms:

 Reduced throttling losses. The addition of inert exhaust gas into the
intake system means that for a given power output, the throttle
plate must be opened further, resulting in increased inlet manifold
pressure and reduced throttling losses.

 Reduced heat rejection. Lowered peak combustion temperatures not

only reduces NOx formation, it also reduces the loss of thermal energy
to combustion chamber surfaces, leaving more available for conversion
to mechanical work during the expansion stroke.

 Reduced chemical dissociation. The lower peak temperatures result

in more of the released energy remaining as sensible energy near TDC
(Top Dead-Center), rather than being bound up (early in the expansion
stroke) in the dissociation of combustion products. This effect is minor
compared to the first two.

EGR is typically not employed at high loads because it would reduce peak
power output. This is because it reduces the intake charge density. EGR is
also omitted at idle (low-speed, zero load) because it would cause unstable
combustion, resulting in rough idle.

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Since the EGR system recirculates a portion of exhaust gases, over time the
valve can become clogged with carbon deposits that prevent it from operating
properly. Clogged EGR valves can sometimes be cleaned, but replacement is
necessary if the valve is faulty.

In diesel engines
In modern diesel engines, the EGR gas is cooled with a heat exchanger to
allow the introduction of a greater mass of recirculated gas. Unlike spark-
ignition engines, diesels are not limited by the need for a contiguous flame
front; furthermore, since diesels always operate with excess air, they benefit
from EGR rates as high as 50% (at idle, when there is otherwise a large
excess of air) in controlling NOx emissions. Exhaust recirculated back into the
cylinder can increase engine wear as carbon particulates wash past the rings
and into the oil.

Since diesel engines are unthrottled, EGR does not lower throttling losses in
the way that it does for SI engines. Exhaust gas—largely nitrogen, carbon
dioxide, and water vapor—has a higher specific heat than air, so it still serves
to lower peak combustion temperatures. However, adding EGR to a diesel
reduces the specific heat ratio of the combustion gases in the power stroke.
This reduces the amount of power that can be extracted by the piston. EGR
also tends to reduce the amount of fuel burned in the power stroke. This is
evident by the increase in particulate emissions that corresponds to an
increase in EGR.

Particulate matter (mainly carbon) that is not burned in the power stroke is
wasted energy. Stricter regulations on particulate matter (PM) call for further
emission controls to be introduced to compensate for the PM emission
increases caused by EGR. The most common is a diesel particulate filter in
the exhaust system which cleans the exhaust but causes a constant minor
reduction in fuel efficiency due to the back pressure created. The nitrogen
dioxide component of NOx emissions is the primary oxidizer of the soot
caught in the diesel particulate filter (DPF) at normal operating temperatures.
This process is known as passive regeneration. Increasing EGR rates cause
passive regeneration to be less effective at managing the PM loading in the
DPF. This necessitates periodic active regeneration of the DPF by burning

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diesel fuel in the oxidation catalyst in order to significantly increase exhaust
gas temperatures through the DPF to the point where PM is quickly burned by
the residual oxygen in the exhaust.

By feeding the lower oxygen exhaust gas into the intake, diesel EGR systems
lower combustion temperature, reducing emissions of NOx. This makes
combustion less efficient, compromising economy and power. The normally
"dry" intake system of a diesel engine is now subject to fouling from soot,
unburned fuel and oil in the EGR bleed, which has little effect on airflow.
However, when combined with oil vapor from a PCV system, can cause
buildup of sticky tar in the intake manifold and valves. It can also cause
problems with components such as swirl flaps, where fitted. Diesel EGR also
increases soot production, though this was masked in the US by the
simultaneous introduction of diesel particulate filters.[11] EGR systems can
also add abrasive contaminants and increase engine oil acidity, which in turn
can reduce engine longevity.

Though engine manufacturers have refused to release details of the effect of

EGR on fuel economy, the EPA regulations of 2002 that led to the introduction
of cooled EGR were associated with a 3% drop in engine efficiency, bucking a
trend of a .5% a year increase.

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