Counter Pro
Course - Emission Controls 2
Exhaust Gas Recirculation
Controls (all cars '72-present)
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One emission controls system which technicians "love to hate" is
the exhaust gas recirculation or "EGR" system. There is
something about the concept of bleeding off some of the exhaust
gas and stuffing it into the intake side of the engine that just
runs against the grain of the typical technician. After all,
getting clean, fresh air into the engine is a major goal of the
technician and exhaust gas is anything but clean and fresh!
Nevertheless, the EGR system is important for emission control
and is used on virtually every vehicle engine sold today. Its
purpose is to lower the combustion temperatures within the
combustion chamber where the air/fuel mixture burns. If the
temperature is not kept below a certain level, then gases called
"NOx or "oxides of nitrogen" form. NOx is one of the emissions
that is legally restricted.
There is more than one way to keep the burning at reasonable
temperature levels. For instance, some aircraft engines and a
variety of racing and supercharged engines use water injection
sprays to cool off the heat of combustion. However, this has
been proven to be impractical for us on ordinary road-going
vehicles.
The method of exhaust gas recirculation used on most cars today
is based upon adding exhaust gas to the burble air/fuel mixture.
This dilutes the air/fuel mass, causing it to burn at lower peak
temperatures (see Fig. 1).
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However, exhaust gas can't be bled into the intake manifold at
just any time or in any quantity. To achieve complete fuel and
air combustion, the stoichiometric ratio of 14.7:1 (pounds of
air to pounds of fuel) must be maintained as accurately as
possible.
Too much exhaust gas in the cylinder, or at the wrong time,, will
spoil this ratio and cause severe starting and driveability
problems. Too little exhaust gas at the wrong time will cause
very high burn temperatures, produce NOx emissions and create
combustion chamber conditions severe enough to cause pinging.
It is important to remember this somewhat obscure fact: EGR
systems are not only emission controls; they are also a means of
lowering peak combustion temperatures to protect the engine from
preignition or detonation when using today's low octane fuels.
Thus, if you "defeat" the EGR system, you can also seriously
decrease engine performance and life.
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Evolution of the
EGR system
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The EGR system can be simple or quite complex. Early versions
were simple. They used EGR bleed ports inside the intake
manifold, supplying a fairly constant volume of exhaust gas to
the cylinders (see Fig. 2). These early versions resulted in an
engine that ran horribly, particularly at idle. As the systems
improved, they also became more complicated. Some of the EGR
system's controls are computer driven while others are purely
mechanical in operation. All are critical to good engine
performance and satisfactory driveability. Since the EGR system
protects the engine from destructive pinging or detonation, in
addition to lowering emissions, it should never be disconnected.
EGR valves can be operated in a variety of ways. They can also
be controlled in their operation by a wide variety of modifier
components. While certain failure modes are relatively easy to
detect, others are more difficult. The manufacturer's diagnostic
literature should be used to determine whether the EGR valve or
some other system component has failed to work correctly. |
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EGR Valve
(virtually all cars, early '70s to present)
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The EGR valve (Fig. 3) is a vacuum-operated valve with a base
usually made of cast iron. Figure 4 illustrates its operation.
The valve plunger and seat are subjected to extremely erosive
conditions. Very hot exhaust gas, as well as the variety of
chemicals in the exhaust, can create a number of problems. They
may act to eat away at the seat, put deposits on the seat and
prevent full closing of the valve, or "freeze" the mechanism in
an open or closed position. All of the above may damage the
car's starting ability and driveability, in addition to
increasing the emissions of NOx.
The EGR valve is supposed to meter a specific amount of exhaust
gas, depending upon operating conditions. The size of the
metering orifice is critical: the bigger the orifice, the
greater the amount of gas entering the system. |
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While there are hundreds of original equipment part numbers, the
number of valves a jobber must stock has been reduced. This
reduction is due to the development of a valve with replaceable
metering orifices (see Fig. 5). In addition, aftermarket valves
can be taken apart and cleaned up if a dirty valve seat and
plunger are at fault. In removing the valve for service, the EGR
gasket is destroyed and must be replaced. However, a limited
amount of this cleanup service may be done. When the mechanism
is badly damaged or the actuating diaphragm ruptures, the
complete valve must usually be replaced.
The valves we sell have another important feature called
"clocking", which is not found on OE valves. On an OE valve, the
diaphragm vacuum motor is fixed in position. Therefore, if the
vacuum line running to the valve points in the same direction as
the original equipment valve, it will only reach the valve's
vacuum nipple. Since our clocking valve also has a vacuum motor
(besides having replacement orifice inserts and disassembly
capabilities for cleaning), it can be turned a full 360 degrees
(see Fig. 6). Therefore, the technician who replaces the valve
can always point the vacuum nipple toward the vacuum line. This
"clocking" feature consolidates EGR valves for easier stocking
and selling.
Directions on orifice replacement, as well as the handling of the
disassembly and clocking features, are included in the EGR valve
box.
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Service interval
and diagnosis
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Whenever any fault conditions occur, the EGR valve and system
should be checked for proper operation. The valve should also be
checked every 12,000 miles. Sometimes, the action of the valve
is visible as the diaphragm moves the actuating rod. To
determine whether the opening and closing of the valve changes
the engine rpms, it is sometimes necessary to speed the engine to
high idle (1500 rpm) and then replace the EGR vacuum hose. A
more detailed diagnostic procedure can be found in car
manufacturer literature, as well as in the buyer's guide of your
emission controls catalog. |
EGR
Controls
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The distance the EGR valve opens, as well as when it opens, are
as important as the size of the orifice. There are conditions
when you want no exhaust at all in the system, such as during hot
start-up. And, there are times when you want more exhaust gas,
such as when NOx would be most likely to form or when pinging
might occur. Each condition of engine load, speed, throttle
position, etc., creates a different need for exhaust gas
recirculation. Or, it may create the need to eliminate exhaust
gas recirculation altogether. Because of these varying needs, a
great number of controls have been added to numerous EGR systems
on various car makes, models and engine types.
The devices used to control the EGR act upon the vacuum signal to
the vacuum motor diaphragm. By starting or stopping the vacuum
signal, and applying more or less of the vacuum available, you
can vary the amount of exhaust gas delivered, as well as the
exact time of delivery. |
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