Borrowed this from a training manual for an advanced ASE diagnostics test.
Thought it would be good for others to read and get a basic understanding of
what happens in a engine and how emissions are formed. I learned a lot
typing it, and I hope it helps me pass the test.
Exhaust Gas Analyzer
One of the more useful Diagnostic Devices available to a technician is the 4
or 5 gas analyzer. Not only are they readily available, but many of them can
be taken out for a test drive. This feature makes them an ideal tool for
base lining a vehicle that has failed an inspection maintenance test. Most
state inspections are in grams per mile (GPM) of pollutants while most shop
exhaust analyzers measure in Parts Per Million (PPM) or in percentage (%).
The difference does make the readings difficult to compare, but does not
eliminate the use of the shop analyzer for diagnosis purposes. Most
analyzers available today will measure HC in PPM, C0 in %, CO2 on %, and O2
in %. An option to measure NOX in PPM can be added to most units. Lets look
at the individual gases and analyze why they are produced.
Gasoline is comprised of mostly hydrocarbons (HC). When HC is burned it
combines with oxygen (O2) and under ideal conditions produces water (H20)
and carbon dioxide (CO2). Notice the emphasis on the Ideal. To be practical,
ideal conditions really do not exist in engines that have to operate under
varying loads and speeds. As a result, we not only get water and carbon
dioxide but we also see some raw gas come out of the tailpipe (HC) and some
carbon monoxide (CO). Under extreme heat conditions we also see some varying
conditions for excessive HC and CO. Some states additionally check for NOx
You will remember that gasoline was composed of HC and yet after the burn in
the cylinders we still see some HC coming out of the tailpipe, or going into
the catalytic converters. How can this be? The burn in the cylinder is never
totally complete; some gasoline will always be left over. The amount left
over will be relatively small, sometimes as low as 100 PPM. This 100-PPM
will enter the converter and be converted to CO2 and H2O reducing the HC
emissions to near zero. However, lets add another element, a complete
misfire from an open spark plug wire. What will this do?
Think about what should be occurring in a cylinder: we should ignite the HC
with ignition. If ignition never takes place, all of the HC that the fuel
system put into the cylinder will come out the engine and possible the
tailpipe. This may be in excess of 5000 PPM. Quite a difference between 100
PPM and 5000-PPM HC! Actually the amount of HC that will come out of the
cylinder is imposable to predict because of the variables of load, speed,
engine size, temperature, etc., must be calculated in. Put HC into a
cylinder, don't ignite it, and it will exit the cylinder in the same form it
went in - raw gas. HC emissions from dead cylinders are generally easy to
diagnose. Dead cylinders that produce excessive HC are generally the result
of ignition or compression problems isolated to the cylinder in question.
Run a compression test and an ignition scope test and you will probably
identify the problem without much difficulty. Partial burns are much more
difficult to track down. By partial, we mean some of the fuel has been
burned, but not all Again ignition or compression may be at fault. A plug
that tends toward fouling or has a badly eroded gap, or an ignition coil
that has lost some of its capacity might cause part, but not all of the fuel
charge to burn.
Also, weak or lower compression might cause the cylinder to only partially
burn the fuel. A partial burn will result in increased HC emissions. Another
factor to consider besides compression or ignition is fuel control. Fuel
control is a simple way of saying that the air fuel ratio is held within
design parameters of the engine. Generally this is 1407:1 (14.7 pounds of
air to 1 lb of fuel). If the fuel system cannot hold or control this ratio
then the engine is not in fuel control
Lets look at the two ends of not being in fuel control, too rich or too
lean. Adding much more fuel than the engine needs or is designed to handle
(running rich) can produce an inefficient burn in the cylinder. This usually
results in excessive C0 production, as you will se in the next section,
however it can increase HC emissions. If the cylinder has way too much fuel,
some of it might not be ignited. You know what happens when we do not ignite
the fuel - it comes out as HC, just like it went into the engine. An
excessively rich engine will increase the HC emissions. At the other end of
not being in fuel control is the too lean condition. Being "too lean" is
another way of saying an excessive amount of air has entered the cylinder:
excessive for the amount of fuel.
Another way of saying this would be to say that a smaller amount of fuel
than specified is in the cylinder. A partially plugged fuel injector, or a
vacuum leak are both examples of a lean condition. Cut back on the fuel and
add the air and you are creating a lean condition. If the cylinder is
running lean, than the burn within the cylinder will result in excessive HC,
because the amount of fuel present does not support combustion. This is
generally referred to as a "lean misfire" There was fuel in the cylinder,
but not enough to burn and produce power. This smaller than specified fuel
charge will exit the engine as HC.
Make no mistake about it; a vehicle that is not in fuel control can and
frequently does produce excessive HC. Go back to the basics, for cylinders
to produce equal power they must have equal compression, ignition and be in
CO Emissions are generally related to whether the vehicle is in fuel control
or not. The "O" in CO stands for oxygen. If we burn fuel with sufficient
oxygen then we will produce CO2. It takes two O's to produce CO2 and only
one to produce CO. This is why a rich running vehicle produces more CO than
it should. More fuel in the cylinder will normally require more oxygen.
Because of the rich condition, there is insufficient oxygen for the amount
of fuel - this will produce CO because it is easier to produce. It will take
more air (oxygen), which means the system must run leaner, to lower the CO
production and increase the CO2 production. A correctly functioning engine
will generally produce very low levels of CO (1-2%), which will be converted
into more CO2 in the catalytic converter.
A vehicle that is in fuel control and has a functioning catalytic converter
will generally have near zero CO out the tailpipe.
We have repeatedly referred to CO2 production and hinted that it will go up
if everything if functioning OK. Specifically, CO2 Levels for a vehicle with
good ignition, compression, fuel control and a functioning converter will
generally be in the teens (13.0% - 19.0%). View the CO2 Levels as an
indication of efficiency - the higher the better!
Another byproduct of incorrect combustion is Nitric Oxide NOx. NOx is
produced in excess if the internal temperature of the cylinder gets around
2500 degrees F. At this temperature the nitrogen in the air becomes unstable
and reacts with the oxygen (o). This forms NOx. NOx is measured in parts per
million and is generally is less than 50 - 100 PPM. Think about the
conditions within the engine that will contribute to higher temperature.
They include lean mixture, Carbon in combustion chambers, EGR problems,
preheaters stuck in the heat position, advanced timing etc.
Anything that will add heat to the chamber will generally allow for
increased production of NOx. To be practical, NOx production is almost
always the result of one of three items; advanced timing, carbon or an EGR
problem. The exception to this statement is if the vehicle is not in fuel
control on the lean side. A leaner cylinder will run hotter and therefore
produce more NOx than desired. Notice that we are back looking at being in
fuel control as an issue again. There is probably no single item as
important as being in fuel control for reducing emissions. If the air fuel
ratio can be held tightly to the 14.7:1 ideal, emissions will be reduced,
except CO2 which will increase.
If the vehicle has been running rich and has failed for CO, your repair will
probably increase the production of NOx. A rich running engine will tend to
become carboned up, which increases the compression ratio. Once you have
leaned the engine to the correct ratio, the carbon will increase the
temperature of the cylinder and there goes the NOX