PURPOSE AND FUNCTION
A catalytic converter is an After treatment
device used to reduce exhaust emissions outside of the engine. The catalytic
converter uses a catalyst:
·
A catalyst is a chemical that
helps start a chemical reaction but does not enter into the chemical reaction.
·
The catalyst materials on the
surface of the material inside the converter help create a chemical reaction.
·
The chemical reaction changes
harmful exhaust emissions into nonharmful exhaust emissions.
·
The converter therefore
converts harmful exhaust gases into water vapor (H2O) and carbon dioxide (CO2).
This device is installed in the
exhaust system between the exhaust manifold and the muffler and usually is
positioned beneath the passenger compartment.
The location of the converter is important
since as much of the exhaust heat as possible must be retained for effective
operation. The nearer it is to the engine, the better.
CATALYTIC CONVERTER
CONSTRUCTION
Most catalytic converters are
constructed of a ceramic material in a honeycomb shape with square openings for
the exhaust gases:
·
There are approximately 400
openings per square inch (62 per sq. cm), and the wall thickness is about
0.006 inch (1.5 mm).
·
The substrate is then coated
with a porous aluminum material called the wash coat, which makes the surface rough.
·
The catalytic materials are
then applied on top of the wash coat. The substrate is contained within a round
or oval shell made by welding together two stamped pieces of aluminum or stainless
steel.
The ceramic substrate in
monolithic converters is not restrictive; however, the converter can be
physically broken if exposed to shock or severe jolts. Monolithic converters
can be serviced only as a unit.
An exhaust pipe is connected
to the manifold or header to carry gases through a catalytic converter and then
to the muffler or silencer.
V-type engines can use dual
converters or route the exhaust into one catalytic converter by using a
Y-exhaust pipe.
CATALYTIC CONVERTER OPERATION
The converter substrate
contains small amounts of rhodium, palladium, and
platinum. These elements act
as catalysts. As mentioned, a catalyst is an element that starts a chemical
reaction without becoming a part of, or being consumed in, the process.
In a three-way (catalytic)
converter (TWC) all three exhaust emissions (NO x , HC, and CO) are converted
to carbon dioxide)
CO
2 ) and water (H 2 O).
As the exhaust gas passes
through the catalyst, oxides of nitrogen (NO x ) are chemically reduced (that is,
nitrogen and oxygen are separated) in the first section of the catalytic
converter. In the second section of the catalytic converter, most of the
hydrocarbons and carbon monoxide remaining in the exhaust gas are oxidized to
form harmless carbon dioxide (CO 2 ) and water vapor (H 2 O).
Since the early 1990s, many
converters also contain cerium, an element that can store oxygen. The purpose
of the cerium is to provide oxygen to the oxidation bed of the converter when
the exhaust is rich and lacks enough oxygen for proper oxidation. When the
exhaust is lean, the cerium absorbs
the extra oxygen. For the most
efficient operation, the converter should have a 14.7:1 air-fuel ratio but can
use a mixture that varies slightly:
·
A rich exhaust is required for
reduction—stripping the oxygen (O 2 ) from the nitrogen in NO x .
·
A lean exhaust is required to
provide the oxygen necessary to oxidize HC and CO (combining oxygen with HC and
CO to form H 2 O and CO 2).
If the catalytic converter is
not functioning correctly, ensure that the air-fuel mixture being supplied to
the engine is correct and that the ignition system is free of defects.
CONVERTER LIGHT-OFF
TEMPERATURE
The catalytic converter does
not work when cold, so it must be heated to its light-off temperature of close
to 500°F (260°C) before it starts working at 50% effectiveness. When fully
effective, the converter reaches a temperature range of 900°F to 1,600°F (482°C to 871°C). In spite of the intense
heat, however, catalytic reactions do not generate a flame associated with a
simple burning reaction. Because of the extreme heat (almost as hot as
combustion chamber temperatures), a converter remains hot long after the engine
is shut off. Most vehicles use a series of heat shields to protect the
passenger compartment and other parts of the chassis from excessive heat.
Vehicles have been known to start
fires because of the hot converter causing tall grass or dry leaves beneath the
just-parked vehicle to ignite, especially if the engine is idling.
This is most likely to occur if the heat shields have been removed from the
converter.
CONVERTER USAGE
A catalytic converter must be located
as close as possible to the exhaust manifold to work effectively. The farther
back the converter is positioned in the
exhaust system, the more the
exhaust gases cool before they reach the converter. Since positioning in the
exhaust system affects the oxidation process, vehicle manufacturers that use only
an oxidation converter generally locate it underneath the front of the
passenger compartment.
Some vehicles have used a
small, quick heating oxidation converter called
a pre converter or a pup (mini) converter that
connects directly to the exhaust manifold outlet. These have a small catalyst
surface area close to the engine that heats up rapidly to start the oxidation
process more quickly during cold engine warm-up. For this reason, they were
often called light-off converters (LOCs).
The larger main converter,
under the passenger compartment, completes the oxidation reaction started in
the LOC.
Most older vehicles used a
catalytic converter that had the reduction section first to separate the oxygen
from the nitrogen in NOx. The oxygen released during this action help provide extra
oxygen to help oxidize the HC and CO into harmless water)H2O) and
carbon dioxide (CO2). However, since 2004, emission
standards for oxides of
nitrogen are stricter, and therefore a larger reduction section is often
needed. Therefore, the reduction section is now often after the oxidation
section, which is the opposite of the way it was in converters for older
models.
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