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Exhaust/Emissions Systems Overview
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| Exhaust/Emissions Systems Overview |
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| Updated |
Jul 7, 2004 20:29:32 |
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 411 ( -14 -3.4% ) |
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Description:
Below is an overview of this system's operation
Emissions Testing
Many states require emissions tests on vehicles. This
means that you drive to a facility where the test people take a sample of
your emissions and run it through some analysis. The results are
printed,and you pass or fail depending on the percentage of toxic
emissions that turn up in your car's sample.
The only way to "study" or prepare for this test is to
take good care of your car, including its emission system. If you use
preventative maintenance, and keep your car tuned properly, you will pass.
If you tamper with your emissions system, you will not pass. You can have
your car checked independently before your emissions test if you want to
resolve problems before going to the emissions test station.
One other good thing to do is to save your printouts from
the test from year to year. If you compare them, you will be able to
monitor your car (if its score is getting worse) and catch any problems
before the emissions people catch you.
The Emission Control System
The purpose of the emission control system is just that;
it controls the emissions and exhaust from your vehicle. The idea is to
turn the harmful gases your car manufactures into harmless ones that don't
ruin the environment, or us. Some of the problem gases are:
hydrocarbons (unburned)
carbon monoxide
carbon dioxide
nitrogen oxides
sulfur dioxide
phosphorus
lead and other metals To
help control these substances, we (along with federal regulations) have
made changes in our gasoline to eliminate them. Also, with a push from
federal regulations, we have developed ways, varying from state to state,
to test emissions, that have caused automotive manufacturers to develop
better, safer emission systems.
Although emissions control systems vary between
manufacturers and vehicles, they all have the same goal and use many of
the same methods. The addition of computers to ignition systems allows the
engine to monitor and adjust itself continuously, so it just isn't true
that emission controls lower the amount of mileage we get fromfuel.
The best news is that emission controls have reduced
carbon monoxide and hydrocarbon emissions by about ninety-six percent from
pre-control vehicles. That's almost a hundred percent!
The Tailpipe
The tailpipe is a long metal tube attached to the
muffler. It sticks out from under the body of your car, at the rear, in
order to discharge the exhaust gases from the muffler of your engine into
the air outside the car.
The Muffler
Exhaust gases leave the engine under extremely high
pressure. If these gases escaped directly from the engine the noise would
be tremendous. For this reason, the exhaust manifold sends the gases to a
muffler where they go through metal plates, or tubes, with a series of
holes. The pressure of the gases is reduced when they pass through the
muffler, so they go out of the tail pipe quietly.
The muffler is made of metal and is located underneath
the body ofthe car. It's connected between the tail pipe and the catalytic
converter.
There are two types of muffler design. One type uses
several baffled chambers to reduce noise. The other type sends the gases
straight through perforated pipe wrapped in metal or fiberglass. This type
of muffler is designed for the purpose of reducing back pressure and,
consequently, makes slightly more noise.
Since a muffler cannot reduce the noise of the engine by
itself, some exhaust systems also have a resonator. Resonators are like
little mufflers, and are usually the "straight through" type. They are
added at the end of the exhaust system to take care of any noise that has
made it through the muffler.
The muffler quiets the noise of the exhaust by "muffling"
the sound waves created by the opening and closing of the exhaust valves.
When an exhaust valve opens, it discharges the burned gases at high
pressures into the exhaust pipe, which is at low pressure. This type of
action creates sound waves that travel through the flowing gas, moving
much faster than the gas itself (up to 1400 m.p.h.), that the muffler must
silence. It generally does this by converting the sound wave energy into
heat by passing the exhaust gas and its accompanying wave pattern, through
perforated chambers of varied sizes. Passing into the perforations and
reflectors within the chamber forces the sound waves to dissipate their
energy.
The Catalytic Converter
When your engine burns fuel, it produces gases that are
bad for the environment. These noxious gases are hydrocarbons, carbon
monoxide and nitrogen oxides. To prevent the engine from polluting the
environment with these gases, we include a catalytic converter in our
emission systems.
The catalytic converter is installed in the exhaust line,
between the exhaust manifold and the muffler, and makes use of chemicals
that act as a catalyst. A catalyst is a chemical that causes a reaction
between other chemicals without being affected itself. In the case of the
catalytic converter, the chemicals it contains cause a reaction in the
pollutants in the exhaust. The pollutants are changed from harmful gases
to harmless ones before they are let into the environment through the tail
pipe.
Basically, the harmful gases enter the catalytic
converter, a kind of stainless steel container. The converter is lined
with chemicals such as aluminum oxide, platinum and palladium. These
chemicals cause the carbon monoxide and hydrocarbons to change into water
vapor and carbon dioxide. Some converters have a third lining of
chemicals, platinum and rhodium, that reduce nitrogen oxides (three-way,
dualbed converter).
The reason that leaded gas cannot be used in an engine
with a catalytic converter is that the lead coats the chemicals in the
converter. This makes them unable to do the job anymore, since the
chemical lining can't come in contact with the pollutants. At first,this
was a big disappointment, because lead acted as a lubricant and helped to
reduce wear on some of the engine parts. Luckily for our engines and the
environment (not to mention us), car manufacturers soon got around the
problem by making tougher parts and coating them with special
metal.
The EGR Valve
The Exhaust Gas Recirculation (EGR) valve is used to send
some of the exhaust gas back into the cylinders to reduce combustion
temperature. Why would we want to do this?
Nitrous oxides (nasty pollutants) form when the
combustion temperature gets above 2,500 degrees F. This happens, because
at such temperatures, the nitrogen in the air mixes with the oxygen to
create nitrous oxides. Did you ever have two friends that were fine by
themselves but just awful when they got together? Well, our good friend,
the sun, is just like that. When it's sunny, the nitrous oxides from the
exhaust get together with the hydrocarbons in the air to form our
not-so-good friend, smog. That's when the EGR valve comesin handy.
By recirculating some of the exhaust gas back through the
intake manifold to the cylinders, we can lower the combustion temperature.
Lowering the combustion temperature lowers the amount of nitrous oxide
produced. Consequently, less of it comes out the tail pipe.
There are two types of EGR valves. One operates through
the use of a vacuum, and the other operated through the use of pressure.
Both types allow the exhaust gas in to lower the combustion temperature
when it gets too high.
PCV Valve
The process of combustion forms several gases and vapors;
many of them quite corrosive. Some of these gases get past the piston
rings and into the crankcase. If left in the crankcase, these substances
would cause all kinds of bad things (rust, corrosion, and formation of
sludge), so they have to be removed. Back in the old days, they used to be
dumped out into the atmosphere through a tube. Once we realized what a
problem pollution was in the sixties, the PCV (Positive Crankcase
Ventilation) system was developed to take the place of the old "dump
tube."
The PCV system uses a hose connected between the engine
and the intake manifold to draw these gases out of the engine's crankcase
and back into the cylinders to burn with the regular fuel. The only
problem to solve is how to keep these gases from going willy-nilly into
the manifold and upsetting the required air-fuel ratio. The solution to
this problem is the PCV valve.
The PCV valve controls the release of crankcase gases and
vapors to the intake manifold. The valve is kept closed by spring action
when the engine is at rest. When the engine is running normally, the low
vacuum it creates allows the valve to open and release crankcase vapors
and gases into the intake manifold for burning. If the engine is idling or
you are slowing down, the vacuum level rises and pulls the valve plunger
into the valve opening. This partially blocks off the opening so that only
a small amount of vapors and gases can be drawn into the intake
manifold.
One really comforting feature of the PCV valve is its
behavior in the event of a backfire. If your car backfires in the
manifold, the pressure makes the spring close the valve completely. With
the valve closed, there is no chance that the flame can move into the
crankcase and cause an explosion.
The Air Pump
The air pump sends (or pumps) compressed air into the
exhaust manifold and in some cases to the catalytic converter. The oxygen
in the pressurized air helps to burn quite a bit of any unburned
hydrocarbons (fuel) and therby converts the poisonous carbon monoxide into
good old carbon dioxide.
A belt from the engine drives the air pump. It has little
vanes (thin, flat, curved fins) that draw the air into the compression
chamber. Here, the air is compressed and sent off to the exhaust manifold
where it speeds up the emissions burning process. Stainless steel nozzles
are used to shoot the air into the exhaust manifold, because they will not
burn.
Some engines use a pulse air injection system. This
system uses pulses of exhaust gas to operate an air pump that delivers air
into the exhaust system.
The Exhaust Manifold and Header
The exhaust manifold, usually constructed of cast iron,
is a pipe that conducts the exhaust gases from the combustion chambers to
the exhaust pipe. It has smooth curves in it for improving the flow of
exhaust.
The exhaust manifold is bolted to the cylinder head, and
has entrances for the air that is injected into it. It is usually located
under the intake manifold.
A header is a different type of manifold; it is made of
separate equal-length tubes.
Manifold to Exhaust Pipe Gaskets
There are several types of gaskets that connect the
exhaust pipe to the manifold.
One is a flat surface gasket. Another type uses a ball
and socket with springs to maintain pressure. This type allows some
flexibility without breakage of the seal or the manifold. A third type is
the full ball connector type, which also allows a little
flexibility.
Exhaust Pipe Hangers
Hangers hold the exhaust system in place. They give the
system flexibility and reduce the noise level. The hanger system consists
of rubber rings, tubes and clamps.
Catalysts
The materials within a catalytic converter vary between
cars. Catalytic converters are designed to do different things, depending
on the design of the converter.
Some catalytic converters use what is called an
"oxidation" catalyst; this usually consists of ceramic beads coated with
platinum to reduce hydrocarbons and carbon monoxide. Through the catalytic
action, the hydrocarbons and carbon monoxide are "burned" to creat ewater
vapor and carbon dioxide. This type of catalytic converter needs an input
of oxygen, so oxygen is usually injected into the cylinder head, or
directly into the exhaust header or manifold.
Newer catalytic converters have a two part design. The
front half is a "three-way" catalyst, which burns various pollutants, and
reduces hydrocarbons, carbon monoxide, and oxides of nitrogen into water,
carbon dioxide and nitrogen. These converters require exact fuel air
mixtures in order to maintain efficient exhaust reduction. The rear
section of these converters is the normal oxidation catalyst that further
reduces hydrocarbons and carbon monoxide. Air from the air pump is
injected into the center of these converters. Here the air is allowed to
mix with the exhaust before it passes into the oxidation catalyst, where
it burns off its toxic chemicals and reduces emissions.
Exhaust Pipe
The exhaust pipe is the bent-up or convoluted pipes you
will notice underneath your car. Some are shaped to go over the rear axle,
allowing the rear axle to move up and down without bumping into the
exhaust pipe; some are shaped to bend around under the floor of the car,
connecting the catalytic converter with the muffler. Exhaust pipes are
usually made out of stainless steel, since the high heat conditions
involved with the muffler system will cause rust.
Reverse-flow Muffler
The reverse-flow muffler is oval-shaped and has multiple
pipes. Four chambers and a double jacket are used to accomplish muffling
of the exhaust noise. Exhaust gases are directed to the third chamber,
forced forward to the first chamber, from where they travel the length of
the muffler and are exhausted into the tail pipe.
Straight Through Muffler
The straight through muffler has a central tube,
perforated with several openings which lead into an outside chamber packed
with a sound absorbing (or insulating) material. As the exhaust gases
expand from the perforated inner pipe into the outer chamber, they come in
contact with the insulator and escape to the atmosphere under constant
pressure. Because of this, the expanding chamber tends to equalize or
spread the pressure peaks throughout the exhaust from each individual
cylinder of the engine.
A V-8 engine requires two exhaust manifolds and either
one or two mufflers and often accompanying resonators. If one muffler is
used, the exhaust pipe from one manifold meets the other one in the form
of a "Y".
Dual Exhaust System
The advantage of a dual exhaust system is that the engine
exhausts more freely, thereby lowering the back pressure which is inherent
in an exhaust system. With a dual exhaust system, a sizable increase in
engine horsepower can be obtained because the "breathing" capacity of the
engine is improved, leaving less exhaust gases in the engine at the end of
each exhaust stroke. This, in turn, leaves more room for an extra intake
of the air-fuel
mixture. |
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