infrared industries FGA, Руководство оператора

Страница: 29 / 58

28
H Y D R O C A R B O N S
Hydrocarbons (HC) are organic compounds made up of hydrogen and carbon atoms. The HC present in gasoline
engine exhaust is unburned gasoline vapor, and is measured in parts per million (PPM). HC levels in engine exhaust
vary with the air-fuel ratio. Figure 6 shows the relationship of HC concentration in exhaust to air-fuel mixture.
Figure 6: HC Vs. Lambda
As Figure 6 shows, the lowest HC emissions occur at an air-fuel
ratio of about 16.2:1. Since no engine combustion is perfect, some
vaporized HC in the combustion chamber is left unburned and
exits the engine with the exhaust gases. The amount of HC
emissions from the engine depends to a great extent on
combustion chamber design. Also, if the fuel mixture is too lean
or rich to support complete combustion, or if ignition does not
occur in the combustion chamber at all, HC emissions increase
dramatically.
Gasoline evaporating from the carburetor and fuel tank are also
sources of HC emissions, known as evaporative emissions.
Vehicles built since 1970 minimize the escape of evaporative
emissions.
C A R B O N M O N O X I D E
Carbon monoxide (CO) is an exhaust byproduct formed when combustion takes place in an engine with less than an
ideal volume of oxygen (rich fuel mixture), combining a carbon atom with only one oxygen atom. The carbon in the
combustion chamber comes from the HC fuel, and the oxygen from the inducted air.
Figure 7 CO Vs Lambda
The richer the fuel mixture in the combustion chamber (more HC, less
air), the higher the concentration of CO in the exhaust. Therefore,
anything that causes a rich air-fuel ratio results in a high CO content in
the exhaust shows the relationship between CO output and air-fuel
ratio.
Figure 7 shows that the CO level decreases as the air-fuel ratio
approaches about 15:1 (Lambda 1.05), and maintains this low level even
while the mixture is further leaned out. Because of this, CO is a good
indicator of fuel mixture richness, but a poor indicator of leanness.
This fact makes the HC and CO content in the exhaust good gauges of
engine performance, in addition to their importance for Clean Air Law
compliance. When HC and CO readings are compared with oxygen
and carbon dioxide readings, the results can be used to indicate catalytic
converter efficiency.