Here's the scoop on NOX, your truck is running to lean, or the egr valve isnt opening, or the egr port is clogged with carbon. Read this:
OXIDES OF NITROGEN (NOX)
Nitrogen makes up almost 80% of the atmosphere. Though
normally inert and not directly involved in the combustion
process itself, flame temperatures above 2500 degrees F cause
nitrogen and oxygen to combine and form various compounds called
"oxides of nitrogen" or NOX. This typically occurs when the
engine is under load and combustion temperatures soar. NOX formation
is greatest when the fuel mixture is lean (about 16:1).
Most of the NOX that comes out the tailpipe is in the form of
nitric oxide (NO), a colorless poisonous gas. It then combines
with oxygen in the atmosphere to form nitrogen dioxide (NO2),
which creates a brownish haze in badly polluted areas.
NOX is a nasty pollutant both directly and indirectly. In
concentrations as small as a few parts per million, it can cause
eye, nose and lung irritations, headaches and irritability. It
has an odor that becomes noticeable in concentrations as small as
1 to 3 ppm. When levels reach 5 to 10 ppm, NOX causes eye and
nose irritation in some people. Higher concentrations can cause
bronchitis and aggravate other lung disorders. Prolonged
exposure to 10 to 40 ppm can have serious health consequences.
Once in the atmosphere, it reacts with oxygen to form ozone
(which is also toxic to breathe) and smog.
EGR REDUCES FORMATION OF NOX
To minimize the formation of NOX in the engine, exhaust gas
recirculation (EGR) is used. Recirculating a small amount of
exhaust gas back into the intake manifold to dilute the air/fuel
mixture has a "cooling" effect on combustion, thus keeping
temperatures below the NOX formation threshold.
3-WAY CONVERTERS REDUCE NOX IN EXHAUST
On many 1981 and later engines with computerized engine
controls, a special "three-way" catalytic converter is also used
to further reduce NOX in the exhaust. The first chamber of the
converter contains a special "reduction" catalyst that breaks NOX
down into oxygen and nitrogen. The second chamber in the
converter contains the "oxidation" catalyst that reburns CO and
HC.
CAUSES OF INCREASED NOX EMISSIONS
The most common cause of elevated NOX emissions is a defective or
inoperative EGR system. Loss of EGR allows combustion temperatures to
rise about the 2500 degree F NOX formation threshold, allowing NOX
emissions to rise.
Other causes include:
* Defective 3-way catalytic converter. The converter should reduce
NOX into oxygen and nitrogen. But if the catalyst is contaminated or
worn out, this will not occur.
* Incorrect operation of AIR system. Too much air injected upstream
of the converter interferes with the reduction of NOX.
* Incoming air/fuel mixture too hot. Underlying causes may include an
air cleaner preheat door stuck in the "hot" (closed to outside air)
position. A heat riser valve on a V6 or V8 engine exhaust manifold that
is stuck shut or does not open fully can overheat the intake manifold.
Engine overheating due to a cooling problem (low coolant, thermostat stuck
shut or not opening fully, clogged radiator, defective water pump, etc.),
or an exhaust restriction can also increase engine heat and combustion
temperatures.
* Lean fuel mixture. NOX formation peaks when fuel mixtures are lean
(about 16:1). A lean fuel mixture also increases the amount of unburned
oxygen in the exhaust which reduces the conversion efficiency of the
catalytic converter for breaking down NOX.
* Advanced ignition timing (possibly due to a computer incorrectly
advancing timing because of incorrect inputs from a faulty BARO sensor,
MAP sensor or EGR position sensor).
* Very retarded ignition timing. Slightly retarded timing reduces NOX
and engine temperatures. But severely retarded timing reduces engine
efficiency, makes the engine work harder and raises temperatures and NOX
levels.
* Carbon deposits in the engine that raise compression.
DETECTING NOX
Until the arrival of 5-gas exhaust analyzers and "enhanced"
emissions testing programs, the only way to measure NOX was in a
laboratory. Ordinary 4-gas exhaust analyzers cannot measure NOX.
So the only way to tell if an engine is producing excessive NOX
emissions without 5-gas is to visually inspect the EGR system to
see that it is working (not an easy task on many engines), or to
listen for a change in idle quality when vacuum was applied to
the valve. Another symptom would be detonation (spark knock) when
accelerating under load (which indicates loss of EGR).
With 5-gas, NOX can be measured directly. NOX emissions are
measured in either parts per million (ppm) like hydrocarbons, or
in grams per mile (gpm)
NOX LEVELS
Acceptable NOX levels vary according to the model year. Typically,
NOX emissions are between 500 to 1,000 ppm with EGR functioning
under cruise conditions at approximately 2,000 rpm. Without EGR,
levels can rise to 1700 to 2500 ppm at cruise, and even higher when
the engine is under load.
The applicable NOX emission limits for new vehicles are as follows:
Uncontrolled: up to 4.0 grams per mile(g/mi) or 1200 ppm*
Up through 1980: 2.0 g/mi or 600 ppm
1980: 2.0 g/mi or 600 ppm
1981 to 1988: 1.0 g/mi or 300 ppm
1989 to 1993: 1.0 g/mi or 300 ppm
1994 to 2000: 0.4 g/mi or 120 ppm
2001 to 2004: 0.2 g/mi or 60 ppm
*NOTE: The parts per million equivalent of grams per mile will vary somewhat
depending on engine displacement, rpm and vehicle weight. The ppm values
are approximate only.
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Tuesday, December 28th, 2010 AT 12:29 AM
