Fundamentals
5-5
•
Carbon Build-up
— Carbon in the combustion chamber reduces combustion
chamber volume. This increases compression pressure and
increases the air and fuel mixture temperature which
contributes to NO
x
formation.
•
Fuel Octane
— Low octane fuel burns faster and less controlled than higher
octane fuel. This can cause NO
x
formation.
•
Air and Fuel Mixture
— Lean Engine Operation
•
Engine pinging indicates lean engine operation and the
formation of NO
x
. Any condition that causes the engine to
run lean can cause an increase in the formation of NO
x
. If
the mixture and exhaust are too lean, the converter can
convert HC and CO but can not control NO
x
.
•
Rich Engine Operation
— Rich mixtures and high CO can mask an NO
x
problem.
When an engine has carbon build-up from the rich condition
and the cause of excessive richness is corrected, the
resulting leaner condition can cause increased NO
x
formation. Also, the three-way catalytic converter requires
CO to convert NO
x
to N and CO
2
. So, high CO levels can
mask high NO
x
levels.
•
If the mixture and exhaust are too rich, the converter can
reduce NO
x
but can not control HC and CO.
•
Catalytic Converter
— A dual-bed or three-way type catalytic converter in good
working order can reduce NO
x
emitted from the engine as it
passes through the catalytic converter. If the mixture and
exhaust are rich, the converter can reduce NO
x
but can not
control HC and CO. If the mixture and exhaust are too lean,
the converter can convert HC and CO but can not control all
of the NO
x
.
•
Humidity
— An increase in mixture humidity can help reduce formation
of NO
x
due to the drop in maximum combustion chamber
flame temperature.
•
Air Pump Operation
— Air pump pressure can overcome the exhaust pressure in a
three-way catalytic converter if, during deceleration, air is
pumped into the converter instead of diverted into the
atmosphere. Under this condition, air can backflow into the
reducing portion of the catalyst, increasing NO
x
formation.
•
Fuel Injector Problems
— Restriction of one injector on an engine with port fuel
injection causes that cylinder to run lean and the cylinder
temperature to increase, forming NO
x
. NO
x
levels in other
cylinders remain normal, therefore, NO
x
levels at the tailpipe
are only slightly high.
— When an injector spray pattern is abnormal, hot spots can
form within the cylinder and increase NO
x
formation.
✓
Excessive NO
x
emissions may be caused by the effects of
several of these devices or factors at one time.
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