Question:What kinds of emissions should I be concerned
about?
Answer:All emissions if you're serious about clean air
-- or at least worried you won't pass an emissions test. Most
emission testing programs to date only check for only two pollutants:
carbon monoxide (CO) and hydrocarbons (HC). In areas that have
the new "enhanced" I/M 240 emissions testing program,
they also check for oxides of nitrogen (NOX) and the operation
of your "evaporative emissions" control system (the
system that captures and holds vapors from your fuel tank).
CARBON MONOXIDE (CO)
Of the three main pollutants, carbon monoxide is the deadliest
because you can't see it or smell it. A concentration of only
half a percent (0.5%) CO in the air can render a person unconscious
-- and kill within 10 to 15 minutes! Even concentrations as small
as four hundredths of a percent (0.04%) can cause headaches and
be life threatening after several hours exposure.
WARNING: Never run an engine inside an enclosed garage, not
even for a few minutes. The fumes can build up quickly and overcome
you before you realize what's happening. Carbon monoxide is invisible
and odorless so you can't really tell when it's around.
Carbon monoxide is formed when the fuel mixture is rich and
there is insufficient oxygen to completely burn all the fuel.
The richer the fuel mixture, the greater the quantity of CO produced.
So high CO emissions indicate incomplete combustion typically
caused by carburetor maladjustment, a clogged air filter, sticking
choke, defective heated air intake system, plugged PCV valve,
faulty oxygen sensor, excessive fuel pressure or a fuel injection
metering problem.
Carbon monoxide production is highest when the engine is first
started because the fuel mixture is richer than normal during
this time and the catalytic converter has not yet reached operating
temperature.
Carbon monoxide emissions are minimized by maintaining a balanced
to slightly lean fuel mixture. This requires careful adjustment
of the carburetor idle mixture screws (which may have "limiter
caps" to limit the amount of adjustment or are covered with
plugs to prevent tampering). On some fuel injected engines, there
is also an adjustment for the idle mixture (but it is usually
factory sealed to prevent tampering). The fuel mixture is further
balanced by the oxygen sensor and computer system. Most of the
carbon monoxide that is produced by the engine is converted into
carbon dioxide (CO2) by the catalytic converter.
HYDROCARBONS (HC)
Hydrocarbon emissions are unburned gasoline and oil vapors.
Though not directly harmful, they are a major contributor to
smog and ozone pollution (which are toxic). Hydrocarbons in the
atmosphere react with sunlight and break down to form other chemical
compounds that irritate the eyes, nasal passages, throat and lungs.
HC emissions, which are usually measured in parts per million
(PPM), can go up as a result of ignition misfiring (a fouled plug
or bad plug wire), "lean" misfiring (incorrect carburetor
idle adjustment or vacuum leaks that creates a lean mixture that
misfires), loss of compression (such as a burned or leaky exhaust
valve), or engine wear that causes the engine to burn oil (worn
valve guides, rings and/or cylinders).
Hydrocarbon emissions are controlled by maintaining the fuel
mixture so it is neither too lean nor too rich to ignite, by keeping
the combustion chamber tightly sealed (good rings and valves),
and by maintaining the ignition system (changing the plugs periodically).
HC that is produced in the engine is reburned in the catalytic
converter and changed into water vapor and carbon dioxide.
OXIDES OF NITROGEN (NOX)
Nitrogen makes up about 78% of the air we breathe. Though
normally inert and not directly involved in the combustion process,
combustion temperatures above 2500 degrees F cause nitrogen and
oxygen to combine and form various compounds called "oxides
of nitrogen," which is abbreviated NOX. This mostly occurs
when the engine is under load and the throttle is open wide.
NOX is a nasty pollutant both directly and indirectly. In
concentrations as small as a few parts per million, it can cause
eye, nose and lung irritations, headaches and irritability. Higher
concentrations can cause bronchitis and aggravate other lung disorders.
Once in the atmosphere, it reacts with oxygen to form ozone (which
is also toxic to breathe) and smog.
To reduce the formation of NOX, Exhaust Gas Recirculation
(EGR) is used. By recirculating a small amount of exhaust gas
back into the intake manifold to dilute the air/fuel mixture,
EGR has a "cooling" effect on combustion, thus keeping
temperatures below the NOX formation threshold.
On 1981 and later engines with computerized engine controls,
a special "three-way" catalytic converter is used to
reduce NOX in the exhaust. The first chamber of the converter
contains a special "reduction" catalyst that breaks
NOX down into oxygen and nitrogen. The second chamber contains
the "oxidation" catalyst that reburns CO and HC.
High NOX emissions are almost always due to a defective EGR
valve (or some component that controls the operation of the EGR
valve). A related symptom that usually occurs when EGR is lost
is spark knock (detonation) during acceleration.
EVAPORATIVE EMISSIONS
The fuel vapors that evaporate from your fuel tank can be
another source of smog and ozone pollution. So fuel systems for
the past twenty years have been sealed to prevent the loss of vapors.
Some venting of the tank must be provided so it can "breathe"
during temperature changes and when the engine is running, so
this is provided by hoses connected to a charcoal filled canister
usually located in the engine compartment. The charcoal particles
in the canister soak up and store fuel vapors when the engine
is not running. Then, when the engine is started, a "purge
valve" opens to siphon the vapors into the engine where they
are burned.
If the canister or any of its hose connections leak (or the
gas cap does not seal tightly), fuel vapors can escape into the
atmosphere around the clock. The amount of pollution can really
add up, especially during hot weather, so it's important to make
sure the system is functioning properly. The new OBD II test
program includes a pressure check of the fuel tank system as well
as a flow test of the purge valve.
