| Good
combustion | Bad
combustion | Smog
Machine measurements |
HC
| CO
| CO2
| O2
| NOx
| Review
|
When we do exhaust analysis, we are being a detective.
We look at what came out of the exhaust and figure out what could have happened
before to create those emissions.
What happened in the combustion chamber, or before the combustion chamber, to
create these results?
We can use clues and patterns of exhaust readings
to figure out if we have a problem in one of the following areas:
Then we know where to start our diagnosis
with visual and functional tests.
Let's start by reviewing good combustion. The idea
is to properly burn up all the gasoline
and not have any "leftovers". Into the combustion chamber we put
gasoline, symbolized by 'HC' for hydrocarbons. These are combinations of
hydrogen and carbon atoms, organic matter from old dinosaurs maybe? We also add
lots of air, which contains oxygen, symbolized by 'O2'. (Oxygen atoms feel more
comfortable going around in pairs.) Normal air is about 20.7% oxygen, and if
your shop smog machine doesn't show about this when reading the air inside your
shop, you could have a bad oxygen sensor in your smog machine, or a serious
problem with the air in your shop, or the planet has a problem... Back to
combustion. The air we add to the combustion chamber is mainly nitrogen, about
78%. (No, that's not nitrous, but related.) This doesn't burn, it just goes
along for the ride and expands with the heat, helping to push down the piston.
Coming out of the combustion chamber we have
carbon dioxide, water and nitrogen. The carbon dioxide is symbolized CO2. (One
carbon atom combined with two oxygen atoms) It's good, in that plants like it
and it doesn't hurt us, but too much is blamed for global warming. The water is
symbolized by H2O, two hydrogen atoms combined with one oxygen atom. Did you
realize that for every gallon of gas we burn, the tailpipe puts out about about
a gallon of water? And then good combustion also puts out all the nitrogen that
came in.
Good combustion is simply put this way: HC
+ O2 + N2 = H2O + CO2 + N2.
I leave out the numbers which show
proportions. Most of you know we want an ideal mixture of 14.7 pounds of air to
one pound of gasoline for the cleanest burning. (Stoichiometric ratio, a term
used in chemistry where the right amount of ingredients are present so
everybody has a dance partner and nobody is left out.)
Now for Bad Combustion. This is where the wrong things happen, and the
byproducts of combustion produce gases which contribute to air pollution or
other problems. One example is raw gasoline (HC) which goes in, then comes out,
and isn't burnt up in the process. Another example is carbon monoxide (CO). It
doesn't create smog, but it's deadly, so you don't want it around. A third
example is NOx. It helps create out brown smog. These are all a problem, and we
are soon going to talk about them in more detail. But first, look at what it
takes to create photochemical smog:
HC + NOx + Still air + Sunlight = Smog. Get the idea? The HC and NOx
are what it takes to create smog, so if we prevent them from coming out of the
tailpipe, we cut down on the smog.
Next we need to know what the Smog Machine measures. These are the gases that the 4 or 5-gas smog machine
sees:
When the tailpipe emissions are bad, what kind of problem do we
look for? Here is a summary of what we are going to talk about:
Now, let's talk about these gases in more
detail, and see what causes each of them to be out of normal range.
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. So when we see excess HC, think Misfire or Bad Burn. Many conditions can cause this:
An Ignition example: if a spark plug
is fouled, gas in that chamber doesn't burn, and lots of raw HC is pumped right
out the tail pipe. You will often see close to 2000 ppm in this case. (2000 is
the highest many machines will read.) Or, if the catalytic converter is working
pretty well, you may see a lot less. Many high levels of HC come from a variety
of ignition problems: open or grounded spark plug wires, switched plug wires,
weak coils, etc.... Don't forget that higher HC's can come from ignition timing
that is too far advanced. Why? The spark occurs before the air/fuel mixture was
compressed enough for best vaporization, so it doesn't burn as completely. And
some HC are left over. Or if the spark plug gap is too small, not enough gas
molecules are ignited to start the flame, and the flame may not eat up all the
HC.
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 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.
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: We measure
CO as a percentage (%) of the exhaust sample. And less is better, 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 history and won't ever
have to renew your smog license again.
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.) So the more
CO2 we create, the better our engine and cat were working. So we use CO2 as an
indicator of engine efficiency. The more, the better. Usually we see 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: We measure
NOx 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. 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. (It
should do a lot, that's what we pay it for.)
To review: when these gases are abnormal, look for this kind of
problem:
Now I have a confession to make, I've been
holding out on you. You need to see this in chart form. We call it the Five Gas Chart. It helps
you think about relationships. (Not with your girlfriend,
but among the gases--how they relate to each other.) The chart shows how much
each of the gases we normally see coming out the tailpipe, depending on how
rich or lean the air fuel ratio is. The colored curved lines represent the
gases. And rich or lean is represented by how far left or right you are on the
chart. So, as you go to the left, you get richer and the CO line goes up. And
as you go to the right you get leaner, the oxygen increases and the O2 line
gets higher. And you notice the lowest emissions are in the center at stoichiometry, where we have
the highest efficiency. And HC's get worse as you go too much to the left or
right, either too rich or too lean.