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.
CATALYTIC
CONVERTER
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.
PCV VALVE
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.
EGR VALVE
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.
EVAPORATIVE
CONTROLS
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.
AIR INJECTION
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.
