Understanding emission numbers and how they work

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.
Steve
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 fuel control.
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
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..

I'm curious how they can get a CO2 as high as 19% (and consider that "better"). Does the manual say? Complete stoichiometric combustion would result in a number close to 13% (15% on a "dry" basis).
Anyway, good luck on the test!
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Will excessive NO emission cut the life span of the catalytic converter???
I recently took my 98 Civic with 260,000km and original emission system for a test. CO is 0%, HC is 0% and NOX is about 300ppm (I'm not sure about those results and need to confirm them)
From those results (assuming they're correct), I'm assuming that the car is working properly, but the timing is advanced and the car is running lean. (Which would make sense since my 98 was noticeably more powerful then the 06 Civic coupe, I recently test drove).
When my car was newer and NOX was near to 0%, I was only able to get about 550km from a fill-up. Now, with the lean mixture, I'm able to get 650km per fill-up. The question is, which setup is more environmentally friendly, considering that I get an extra 100km per tank from a setup that's not standard.
Pars

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If you have a distributor-less ignition system, timing may not be adjustable on your car. If timing is correct, then the engine could be too lean (possibly due to clogged injectors), you could have carbon buildup, or an EGR problem. Since you say that performance is good, I'd lean towards the EGR system.

If the problem is due to a stuck EGR, you could fix it and have little or no effect on fuel economy.
--
Ray O
correct the return address punctuation to reply
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converter???
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Me too. Also, the old Catalytic converter could be also contributing to higher readings. I checked my results and the NOX is actually 220ppm (which is about 2/5 under the allowable limit).

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There's very little available power when you're rolling up a slight incline, while in 5th gear at about 1200rpm (without having to go down on the accelerator). If a properly working EGR is going to take 1/10th of the power out of the above scenario, it would mean that I'd need to cruise around in 4th gear instead of 5th and then my city mileage would go down the shithole.
Pars

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Pars wrote:

FWIW, on my 98 CX, all my levels were *lower* on my 2005 smog check as compared to my 2003 smog check.
main reason? the top (manifold) O2 sensor failed (MIL, and everything!)in 2004.
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Looking at the larger picture helps. The engine isn't likely to be running lean, since the mixture is controlled by feedback from the front O2 sensor. Your '98 should be OBDII controlled if you are in North America, so if the sensor were soft or failing the "check engine" light would be on. The timing could be advanced, but I wouldn't expect it... assuming you aren't getting "ping."
The two unknowns are EGR operation and the catalyst. EGR operation is loosely monitored and the catalyst is monitored for CO operation, but there is room for trouble in either of those. EGR is notorious for needing service after dealing with hundreds of thousands of km of exhaust. Most modern converters are "3 stage" - they have separate stages for breaking down CO, HC and NOx. Often the NOx stage is the first to go, for reasons I don't know.
If or when it reaches the point of having to do something about it (or is that now?), having a few years of history helps. Catalyst failures usually show up as a long, slow slide, while EGR failures tend to get worse more quickly. The catalyst can also be evaluated by a professional with a way of measuring temperature of each stage; if the stage is running cool and the emission output is high, it isn't doing the job any more.
Mike
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I do get 'pinged', unless I'm using premium gas. Since gas prices are very high, the extra cost for premium seems negligible compared to the overall gas prices.
Shell's premium gas returns comparable mileage when compared to the regular blend. Otherwise, switching to higher Octane gas (in my car) normally would reduce fuel consumption.

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Nope, I still have aways to go before it would fail. My last ready, for NOX, was about 220ppm and the limit was at about 550ppm.
having a few years of history helps. Catalyst failures usually

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In my last emission test (2 yrs ago), my NOX emission was similar to the current reading. If the EGR is quick to fail once it start to go bad, the poor reading would point toward Catalytic converter.

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Sounds like you are in pretty good shape. The converter could go quite a while before it gets out of limits, and it sounds like the timing is a little advanced to boot so you have a simple thing to try if it reaches the point where it fails... maybe squeeze out a couple more years ;-)
Mike
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In a couple of years, the car would have accumulated over 350,000km. If the original emission system can last that long. My hats off to Honda.
Otherwise, I'm just going have to turn it over to the niece so that she can go crazy with the performance add on and resign myself to something a bit more mundane.
Pars

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