5-6
Fundamentals
Hydrocarbons (HC)
HC is an organic compound composed of one hydrogen (H) and
one carbon (C) atom. The HC in gasoline engine exhaust is
unburned gasoline vapor that is measured in parts per million
(ppm). HC levels in engine exhaust vary with the air-fuel ratio.
The lowest HC emissions occur at an air-fuel ratio of about 16:1.
Since no engine combustion is perfect, some vaporized HC in the
combustion chamber remains unburned and leaves the engine
with exhaust gases. The amount of HC depends largely on
combustion chamber design.
Also, HC increases dramatically when the fuel mixture is too lean
or rich to support complete combustion, or when ignition does not
occur in the combustion chamber at all.
When compared along with O
2
and CO
2
readings, HC can also
indicate catalytic converter efficiency.
Gasoline evaporating from the carburetor and the fuel tank are
also sources of HC, known as evaporative emissions.
Figure 5-4
: Hydrocarbons Versus Air-Fuel Ratio
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