|
Combustion
and Smog
Before we get into the results, lets go over the basics behind combustion
and pollution. Combustion takes place when gasoline (Hydrocarbons, abbreviated
"HC") reacts with air, which is 20.7% oxygen and 78% Nitrogen.
This reaction obviously takes place in the combustion chamber, facilitated
by high heat and pressure. The resulting byproducts, in an ideal reaction
where all the components are consumed, are water (H20),carbon dioxide
(C02), and nitrogen (N2)... all harmless molecules.
However combustion is typically not so "clean". There are
always some amounts of unburnt fuel (raw HC) and partially burnt fuel
(Carbon Monoxide, CO) and nitrogen oxides (NOX) which are all very potent
byproducts that create smog, burn holes in the ozone layer, and cause
death as in the case of carbon monoxide.
 |
| Typical
catalytic converter. The honeycomb material contains the catalysts.
|
Catalytic
Converters
Catalytic converters (cats for short) were developed and installed
in the exhaust system of vehicles to help reduce the levels of dangerous
byproducts resulting from the combustion process.
A catalyst is something that helps a chemical reaction take place, but
it doesn't get used up in the chemical reaction. The catalysts that
do this are Platinum and/or Palladium, and Rhodium. These are 'noble
metals' that don't react much to anything, they just make reactions
happen around them.
The
Reduction Catalyst
Rhodium is a reduction catalyst, and is used to help reduce the NOx
emissions. When an NO or NO2 molecule contacts the catalyst, the catalyst
rips the nitrogen atom out of the molecule and holds on to it, freeing
the oxygen in the form of O2. The nitrogen atoms bond with other nitrogen
atoms that are also stuck to the catalyst, forming harmless N2. For
example:
2 NO => N2 + O2
2 NO2
=> N2 + 2O2
Since we remove oxygen, not add it, this is called a reduction reaction.
The Oxidization Catalyst
Platinum and Palladium are oxidation catalysts. They reduce the unburned
hydrocarbons and carbon monoxide by burning (oxidizing) them over a
platinum and palladium catalyst. This catalyst aids the reaction of
the CO and hydrocarbons with the remaining oxygen in the exhaust gas.
For example:
2CO + O2 => 2CO2
Notice that this reaction needs oxygen to burn up gasoline. The
chemical term is oxidation.
 |
| Image
from Scientific American |
With this
background, we can now describe the basic types of catalytic converters:
Conventional
converter: This is the original and most simple type of cat. It
is not used much on today's vehicles. This type of cat only cleans up
HC and CO. So it only has platinum or palladium in it. Because it only
cleans up two of our pollutants, it is called two-way catalytic converter.
HC, CO OXIDIZES TO H2O and CO2
Three-way cat: This converter type cleans up all three pollutants:
HC, CO and NOx. And so it contains either platinum and/or palladium,
and also rhodium. This is the basic kind of cat used on most modern
vehicles.
NO2 REDUCES TO N2 and O2
HC, CO OXIDIZES TO H2O and CO2
Dual
Bed Three-way cat: This has two sections, a front section and a
rear section. In the front there is a Three-way cat with the capability
of cleaning up all three pollutants. (Or there might be a section with
Rhodium for just NOx) Then in between the two sections, air is plumbed
in from an air injection system. In the back we have a Conventional
Two-way cat. In front we can concentrate on cleaning up NOx, or we can
clean up all three pollutants. Then in the back we can get real good
at cleaning up HC and CO. This is typical of what is under a late-model
Mustang.
NO2 REDUCES TO N2 and O2
HC, CO OXIDIZES TO H2O and CO2
|
Hi-flow
catalytic converters?
Contrary to popular belief, there is no such thing as a high-flow catalytic
converter, it's the entire H-pipe that is higher flowing than stock. Catalytic
converters are all made from essentially the same ceramic honeycomb "brick"
or "biscuit" material. The individual pores in the honeycomb
are around 1mm in diameter. What makes the H-pipe "high flowing"
is its larger 2.5" pipe diameter compared to the stock 2.25"
diameter. The inlet and outlets of the converter shell are also 2.5",
increasing the volume of air that can pass through the converter. Additionally
the fact that most aftermarket cat pipes have only two converters (rather
than four as on stock pipes) also reduces backpressure and increases flow.
 |
 |
| The
Car Sound (Magnaflow) 2.5" catalytic converter H-pipe, for
'87-'93 Mustangs. Features 2 three-way cats with the stock air injection
tube. |
Stock
2.25" four-converter "H" pipe. |
 An
2.5" off-road (not emissions legal) H-pipe for late model Mustang.
Expect a slight performance advantage over a high-flow 2.5"
two-cat pipe, but at the expense of increased sound and emissions. |
|
 |
 |
| The
Car Sound cat pipe retains the stock 02 sensor locations. Installation
of the H-pipe is no different than removing/replacing the stock
h-pipe. |
Because
the Car Sound cat-pipe uses dual bed three-way cats, the air tube
actually routes to the center of the two converters, rather than
before as with the stock cat pipe. |
 |
 |
| Fit
is nice and snug under the floor pan, and the Car Sound cat-pipe
bolts directly to stock or aftermarket "cat back" muffler
systems. |
The
two larger converters with 2.5" inlet and outlets provider
for greater surface area over the stock pipe. |
|
|
Emissions
Results
There are two types of smog machines, a four gas or five gas analyzer.
The difference being whether or not the machine tests for NOx. Surprisingly
however in California, and most other stringent smog test states, there
are limits for only two of the five gasses. Hydrocarbons, measured in
parts per million, and carbon monoxide, measured as a percentage of
the total exhaust, are the only two gasses for which the state has set
maximum allowable limits. The other gasses are important indicators
of engine function and can be used by the smog tech (or you) to determine
why a test failed. The following list indicates the five gasses which
are tested.
- HC:
Unburned Gasoline
- CO:
Partially Burned Gasoline
- CO2:
Completely Burned Gasoline
- O2:
Oxygen
- NOx:
Oxides of Nitrogen (This is only measured by a 5-gas smog machine)
We had
the '88 5.0L tested without cats (off-road pipe), and through the both
a stock cat pipe and 2.5" Car Sound cat pipe. The car was tested
on a four gas analyzer, thus there are no NOx numbers to report. Also,
we lost the CO,CO2,O2 numbers for the off-road pipe, but the HC shows
the real difference between having cats and not having cats. The numbers
shown are with the engine running at 2500 rpm.
The engine is a mild 5.0L with smog legal Edelbrock heads, Edelbrock
Performer RPM intake, and Lunati 51023 cam (215/222, .522 lift, 112LSA)
The engine is no means radical, but the slightly lopey cam may bring
up the HC's higher than stock.
| Smog
test results, '88 5.0L |
| 2500rpm |
MAX
ALLOWABLE (California) |
Off
road H-pipe |
Stock
2.25"
4 cats |
Car
Sound 2.5" cat pipe |
Car
Sound cats + 18deg timing |
| HC
PPM |
0140 |
400 |
0009 |
0001 |
1400 |
| CO% |
1.00 |
- |
0.01 |
0.00 |
- |
| CO2% |
no
limit |
- |
9.3 |
12.6 |
- |
| O2% |
no
limit |
- |
17.6 |
3.1 |
- |
| NOx |
no
limit |
- |
- |
- |
- |
Looking at
the numbers, it is clear that catalytic converters do their job. With
the off-road H-pipe the HC numbers alone would have resulted in a failed
test. The Car Sound catalytic pipe was actually cleaner than the stock
H-pipe with four cats, which is probably due to the fact the Car Sound
pipe was brand new.
The effects
of timing
Interestingly, when we first rolled into the smog station our timing was
set too high at 18 degrees. The sniffer indicated HC were at a whopping
1400ppm at idle. This was clearly due to a slight misfired that was occurring
due to the high advance. We could actually detect the misfiring cylinder
by watching the timing marks bounce around with a timing light pointed
at the harmonic balancer. With the timing down to 12 degrees, the misfire
was eliminated, and the idle HC numbers dropped to near zero.
Air
pump
Most
modern three-way catalytic converters require fresh air to be pumped
into the converter. On late model Mustangs this is accomplished with
an air pump and diverter valves that control when the air is directed
to the converter. We found our HC and CO numbers were above the limits.
Checking the diverter valves revealed that at some point we got the
hoses from the air pump mixed up and air was being dumped to the atmosphere
rather than down to the cats. Fixing this problem resulted in the cat
working properly and the HC's dropping well below the limit.
EGR
The Exhaust Gas Recirculation system has a direct influence on the
NOx numbers. The idea behind the EGR system is to allow a small portion
of the spent gasses to reenter the combustion chamber. This dilutes
the charge and lowers combustion chamber temps. High combustion chambers
temperature and pressures cause the formation of NOx.
|
| Analyzing
your results |
|
HC,
Hydrocarbons
HC is measured in parts per million (ppm), and a normal
good vehicle may put out about 50 or less. When we have HC coming
out the tailpipe, it is gasoline that didn't burn in the combustion
chamber or somehow escaped the flame of the combustion chamber.
Many conditions can cause this:
- Ignition
problems
Fouled plug, misfire, bad injector.) HC will
be near 2000ppm (highest the smog machine can read)
- Air/Fuel
ratio too rich or too lean
If
the Air/Fuel ratio is wrong, the conditions are not right to
burn up all the fuel, and some HC will end up coming out. If
the cylinder is too rich, there is not enough air too burn all
the fuel, and some is left over. If the mixture is too lean,
there is too much air which makes the fuel too far apart to
burn all of it effectively, so some escapes the flame.
- Mechanical
engine problems
Mechanical
problems cause excess HC. Worn piston rings which don't allow
good sealing of the combustion chamber can stop the high pressure
and temperature from developing, so good vaporization of the
fuel doesn't take place. Then it doesn't all burn and the HC's
are higher, just like in advanced timing. A burnt exhaust valve
will just let out raw gas into the exhaust. A burnt intake valve
may mess up the flow going into other chambers. If a connecting
rod is bent, like from an intake gasket leaking coolant into
the chamber that couldn't be compressed, you will have lower
compression pressures and poor vaporization. So HC's get out.
Lot's of carbon deposits in the chamber can absorb gasoline
so it escapes the flame, then gets released on the exhaust stroke
of the piston so, again, more HC gets out.
- Emission
control device not working properly
An
Emission Control Device that doesn't work right may be
an EGR valve where the spring has lost it's tension and the
valve opens too much under light load. So with too much exhaust
in the chamber, the flame front is cooled off as it tries to
spread out because the exhaust can't burn again. Again, too
much HC gets out. Or if air injection isn't working, there may
not be enough extra O2 to complete the burning of the leftover
HC's. If the catalytic converter isn't efficient, not all the
leftover HC will be burned up the way it should be.
|
CO,
Carbon Monoxide
Measured as a percentage (%) of the exhaust sample. Usually
0.5 % or less. Too much CO is always from a rich condition. There
wasn't enough oxygen to let the burn process finish to get to CO2.
The richer the condition, the more CO you will have. At the ideal
14.7:1 air/fuel ratio we get less than 1% CO. At 14:1 air/fuel ratio
we get about 1.4% CO, at 13:1 we get about 3.5% CO, at 12:1 we get
6% CO, at 11:1 we get a whopping 8% CO. You get the idea. And don't
forget, CO is very hungry to attach to another oxygen, (remember
that Oxygen atoms like to pair up) especially in a warm environment
like your lungs. It only takes 0.3% CO for about 30 minutes and
you are dead. |
CO2,
Carbon Dioxide
CO2 also gets measured as a percentage (%) of the exhaust
sample. Remember CO2 is one of the end products of the burning of
gasoline. And it is only created in the combustion chamber or the
catalytic converter. (Well, maybe a little in the exhaust passages
if it is really hot.) CO2 is an indicator of engine efficiency.
The more, the better. Usually 13 - 15 %, sometimes even more. Any
problem with the engine will bring CO2 down. Too rich, too lean,
misfire, these will all lower the engine efficiency and CO2 comes
down. Beware: proper air injection into the exhaust will also dilute
the CO2 and bring it down, but this is not a problem. But exhaust
leaks can be a problem for catalytic converter efficiency and they
will bring down CO2. (And the O2 will come up, but we'll get to
that next.) |
O2,
Oxygen
O2 is also measured as a percentage. Remember normal air has
about 20.7% oxygen, and air that has been burned has very little
oxygen left in it. (Maybe 1 - 3% depending on how leak free the
exhaust and muffler are.) So O2 is normally low, unless there is
a lot of air injection in the exhaust. This can bring the O2 up
to as much as about 8% O2 with an air pump, much less with pulse
air injection. So use O2 to tell you if the air pump is turned on
or if there are exhaust leaks. When there is a misfire, this will
also bring up the O2. Air got pumped in, it didn't burn, so you
will see it when it gets pumped out. O2 can also indicate a lean
condition. The leaner the engine runs, the more excess oxygen you
see coming out the exhaust. |
|
NOx,
Oxides of Nitrogen
Measured as parts per million (ppm). It is created under
high heat (over 2500 F) and pressure. Think stress. What
happens is that as everything is coming apart and recombining
in the restructuring of the combustion process, the high heat
and pressure of combustion just get to be too much. Nitrogen is
forced to combine with different amounts of Oxygen. And we get
NOx (NO and NO2). This happens only under a load, when the engine
is working hard. A lean air/fuel ratio can cause more heat than
normal so this will happen. Or if the engine is overheating. Maybe
the EGR valve isn't flowing enough, so the combustion isn't cooled
down the way it should be. Maybe carbon in the combustion chamber
is causing a higher compression ratio, this causes more pressure
and heat, and more NOx. Perhaps the ignition timing is too advanced.
This causes more pressure and heat because the spark is sooner,
and the piston does more upward travel, compressing more as the
flame front is already expanding. Last, but not least, the catalytic
converter may not be cleaning up all the NOx it should. High compression
engines produce more NOx than lower compression engines.
|
Want
to learn more about emissions? Download
a comprehensive report from Bosch!
|
|
Sources:
Car
Sound Exhaust System, Inc.
(Magnaflow Exhaust)
22961 Arroyo Vista
Rancho Santa Margarita
California 92688
The
Smog Site
|
|
|
