Snap-On Diagnostics DGA 1000, Manual, Page: 84 / 116

5-4
Fundamentals
Some of the devices and factors that affect the formation of NO
x
are:
✓
The following descriptions should be considered
individually, that is with minimal effect from other dynamic
variables.
• Exhaust Gas Recirculation (EGR)
— EGR dilutes the air-fuel mixture, lowers combustion
chamber temperature, and reduces flame speed. When
operated at a steady state of about 50 km/h, five percent
EGR on a typical engine can reduce
NO
x
emissions by as
much as forty percent. Ten percent EGR can reduce NO
x
emissions about eighty percent. However, HC emissions
increase with higher EGR rates. An inoperative EGR system
does not decrease
NO
x
emissions.
• Valve Timing
— Intake and exhaust valve timing, (amount of overlap, lift and
duration) can affect inlet charge dilution and combustion
chamber temperature the same as EGR, but can not be
easily changed.
• Spark Timing
— Increasing spark advance at any load and speed increases
NO
x
emissions.
• Intake Manifold Vacuum
— A decrease in intake manifold vacuum increases load and
temperature, and decreases the mass of residual gases. As
a result, combustion time decreases. This increases
maximum cycle temperature which increases emissions.
— An increase in intake manifold vacuum decreases load and
temperature, and increases the mass of residual gases. As
a result, combustion time increases. This decreases
maximum cycle temperature which decrease
NO
x
emissions.
• Engine Speed
— An increase in engine speed increases flame speed due to
turbulence. This reduces heat losses per cycle and tends to
raise compression, combustion temperature and combustion
pressure. An increase in engine speed with rich mixtures
(which burn faster) increases
NO
x
formation because of
reduced heat losses at higher speeds. An increase in engine
speed with lean mixtures (which burn slower) decreases
NO
x
formation because of late burning.
• Compression Pressure
— An increase in compression pressure causes an increase in
combustion chamber temperature. This increases the
formation of
NO
x
. Engine design, camshaft design, valve
timing, and supercharging/turbocharging affect
compression.
• Intake Air Temperature
— High intake air temperatures can increase
NO
x
formation.
• Coolant Temperature
— Higher coolant temperature increases cylinder and gas
temperature. This increases
NO
x
formation. Improper
coolant temperature can cause cylinder and combustion
chamber deposit build-up, increasing compression ratio and,
consequently,
NO
x
concentration.