GxT Ferret 54 ENGINE DIAGNOSTIC CENTER - Manual (Page 75)

THE EMISSIONS COMPONENTS
HYDROCARBONS (HC)
Petroleum based products are made up of
hydrocarbons (hydrogen and carbon). Any
fuel that is not burned in the combustion
chamber will be pumped into the exhaust,
where it may be converted to water and
carbon dioxide in the catalytic converter. Any
remaining fuel will show up as HC at the
tailpipe. Hydrocarbons are measured in parts
per million (ppm).
Hydrocarbons are present in exhaust gas as
the result of incomplete combustion, caused
by:
• Incorrectly timed spark
• Insufficient spark
• Misfire due to rich or lean mixture.
HC is an excellent indicator of problems in
the ignition system.
CARBON MONOXIDE (CO)
Carbon monoxide is formed when there is
not enough oxygen to support combustion.
Fuel system problems are the usual cause
of high CO.
In carbureted engines high CO may be
caused by:
• Dirty air filter
• Sticking choke
• Float not set properly
• Idle mixture set too rich
In fuel injected engines high CO readings
may be caused by:
• Dirty air filter
• Leaky injectors
• High fuel rail pressure
• Engine computer control malfunction.
Carbon monoxide is an excellent indicator of
rich air/fuel ratio because it responds quickly
and dramatically to changes in mixture.
CARBON DIOXIDE (CO2)
This is the only gas for which a high reading
is desirable. Carbon Dioxide peaks when
combustion is most efficient, so the higher
the reading, the better. The usual range is
from 12-15%, but the reading will fall off
quickly when the mixture is either too rich
or to lean.
Carbon dioxide is an excellent indicator of
overall combustion efficiency.
OXYGEN (O2)
Ideally, all available oxygen will combine with
all the fuel during combustion. If there is not
enough fuel to combine with all the oxygen,
excess O2 will be present at the tailpipe.
For that reason, oxygen is an excellent
leanness indicator.
It is possible to have a normal or a rich
mixture and still get a high O2 reading. A leak
in either the engine exhaust or the gas
detector's sample path may allow air to enter
and dilute the sample. To find leaks in the
gas detector ’s sample path, follow the
instructions in the MAINTENANCE section
for performing a Leak Check.
OXIDES OF NITROGEN (NOx)
Since air is mostly nitrogen, the combustion
taking place in the engine will be in the
presence of surplus nitrogen. The high
temperatures of combustion will cause
oxidation of some of that nitrogen. This
process also consumes some of the oxygen
that would otherwise be available for
combination with fuel components. In a
properly functioning system the resulting
oxides of nitrogen (NOx) are reduced back
to nitrogen in the catalytic converter.
F
OR MORE DETAIL
,
REFER
TO
GAS THEORY
7-2