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Car's Classroom on the Web
by Fred
Bordoff
The need to control the emissions from automobiles gave rise to the
computerization of the automobile. Hydrocarbons, carbon monoxide and oxides of nitrogen
are created during the combustion process and are emitted into the atmosphere from the
tail pipe. There are also hydrocarbons emitted as a result of vaporization of gasoline and
from the crankcase of the automobile. The clean air act of 1977 set limits as to the
amount of each of these pollutants that could be emitted from an automobile. The
manufacturers answer was the addition of certain pollution control devices and the
creation of a self adjusting engine. 1981 saw the first of these self adjusting engines.
They were called feedback fuel control systems. An oxygen sensor was installed in the
exhaust system and would measure the fuel content of the exhaust stream. It then would
send a signal to a microprocessor, which would analyze the reading and operate a fuel
mixture or air mixture device to create the proper air/fuel ratio. As computer systems
progressed, they were able to adjust ignition spark timing as well as operate the other
emission controls that were installed on the vehicle. The computer is also capable of
monitoring and diagnosing itself. If a fault is seen, the computer will alert the vehicle
operator by illuminating a malfunction indicator lamp. The computer will at the same time
record the fault in it's memory, so that a technician can at a later date retrieve that
fault in the form of a code which will help them determine the proper repair. Some of the
more popular emission control devices installed on the automobile are: EGR VALVE, CATALYTIC
CONVERTER, AIR PUMP, PCV VALVE, CHARCOAL CANISTER.
Automotive emissions are controlled in three ways, one is to promote
more complete combustion so that there are less by products. The second is to reintroduce
excessive hydrocarbons back into the engine for combustion and the third is to provide an
additional area for oxidation or combustion to occur. This additional area is called a
catalytic converter. The catalytic converter looks like a muffler. It is located in the
exhaust system ahead of the muffler. Inside the converter are pellets or a honeycomb made
of platinum or palladium. The platinum or palladium are used as a catalyst ( a catalyst is
a substance used to speed up a chemical process). As hydrocarbons or carbon monoxide in
the exhaust are passed over the catalyst, it is chemically oxidized or converted to carbon
dioxide and water. As the converter works to clean the exhaust, it develops heat. The
dirtier the exhaust, the harder the converter works and the more heat that is developed.
In some cases the converter can be seen to glow from excessive heat. If the converter
works this hard to clean a dirty exhaust it will destroy itself. Also leaded fuel will put
a coating on the platinum or palladium and render the converter ineffective.
This is why, in the U.S.A., all fuels designed for automobile engines are
now unleaded.
The purpose of the positive crankcase ventilation (PCV) system, is to
take the vapors produced in the crankcase during the normal combustion process, and
redirecting them into the air/fuel intake system to be burned during combustion. These
vapors dilute the air/fuel mixture so they have to be carefully controlled and metered
in order to not affect the performance of the engine. This is the job of the positive crankcase
ventilation (PCV) valve. At idle, when the air/fuel mixture is very critical, just a
little of the vapors are allowed in to the intake system. At high speed when the mixture
is less critical and the pressures in the engine are greater, more of the vapors are
allowed in to the intake system. When the valve or the system is clogged, vapors will back
up into the air filter housing or at worst, the excess pressure will push past seals and
create engine oil leaks. If the wrong valve is used or the system has air leaks, the
engine will idle rough, or at worst, engine oil will be sucked out of the engine.
The purpose of the exhaust gas recirculation valve (EGR) valve is to
meter a small amount of exhaust gas into the intake system, this dilutes the air/fuel
mixture so as to lower the combustion chamber temperature. Excessive combustion chamber
temperature creates oxides of nitrogen, which is a major pollutant. While the EGR valve is
the most effective method of controlling oxides of nitrogen, in it's very design it
adversely affects engine performance. The engine was not designed to run on exhaust gas.
For this reason the amount of exhaust entering the intake system has to be carefully
monitored and controlled. This is accomplished through a series of electrical and vacuum
switches and the vehicle computer. Since EGR action reduces performance by diluting the
air /fuel mixture, the system does not allow EGR action when the engine is cold or when
the engine needs full power.
Gasoline evaporates quite easily. In the past, these evaporative
emissions were vented into the atmosphere. 20% of all HC emissions from the automobile are
from the gas tank. In 1970 legislation was passed, prohibiting venting of gas tank fumes
into the atmosphere. An evaporative control system was developed to eliminate this source
of pollution. The function of the fuel evaporative control system is to trap and store
evaporative emissions from the gas tank and carburetor. A charcoal canister is used to
trap the fuel vapors. The fuel vapors adhere to the charcoal, until the engine is started,
and engine vacuum can be used to draw the vapors into the engine, so that they can be
burned along with the fuel/air mixture. This system requires the use of a sealed gas tank
filler cap. This cap is so important to the operation of the system, that a test of the
cap is now being integrated into many state emission inspection programs. Pre-1970 cars
released fuel vapors into the atmosphere through the use of a vented gas cap. Today with
the use of sealed caps, redesigned gas tanks are used. The tank has to have the space for
the vapors to collect so that they can then be vented to the charcoal canister. A purge
valve is used to control the vapor flow into the engine. The purge valve is operated by
engine vacuum. One common problem with this system is that the purge valve goes bad and
engine vacuum draws fuel directly into the intake system. This enriches the fuel mixture
and will foul the spark plugs. Most charcoal canisters have a filter that should be
replaced periodically. This system should be checked when fuel mileage drops.
Since no internal combustion engine is 100% efficient, there will always
be some unburned fuel in the exhaust. This increases hydrocarbon emissions. To eliminate
this source of emissions an air injection system was created. Combustion requires fuel,
oxygen and heat. Without any one of the three, combustion cannot occur. Inside the exhaust
manifold there is sufficient heat to support combustion, if we introduce some oxygen than
any unburned fuel will ignite. This combustion will not produce any power, but it will
reduce excessive hydrocarbon emissions. Unlike in the combustion chamber, this combustion
is uncontrolled, so if the fuel content of the exhaust is excessive, explosions, that
sound like popping, will occur. There are times when under normal conditions, such as
deceleration, when the fuel content is excessive. Under these conditions we would want to
shut off the air injection system. This is accomplished through the use of a diverter
valve, which instead of shutting the air pump off, diverts the air away from the exhaust
manifold. Since all of this is done after the combustion process is complete, this is one
emission control that has no effect on engine performance. The only maintenance that is
required is a careful inspection of the air pump drive belt.
Copyright © 1983-2005, SmartTrac
Computer Systems, Inc.
All Rights Reserved.
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