What do these emission numbers mean?

[mingmong]

Happy Xmas everyone!
Just a quickie. I've had my car on the road for a couple of years now (since it's restoration) and it's just passed the MOT for (hopefully) another years driving. Took a look at the emissions data from the test and compared it with previous years and here's what i found...

2006 CO 1.03% HC 34ppm @1200rpm
2007 CO 1.84% HC 31ppm @1200rpm
2008 CO 2.14% HC 23ppm @1200rpm

Seems my CO is rising and my HC falling. Is this normal?
Cars a '77 C3 and I average about 5K miles/yr.
Cheers
M

[rattlsnak]

This might help:


Emissions in a nutshell:

As a fuel is burned in an engine, various exhaust emissions are produced:
Hydrocarbons, HC's, from unburned fuel. HC's are formed from a rich fuel mixture (or a lean fuel mixture where there is excess air, leading to a lean misfire.
Carbon Monoxide, CO, from a rich fuel mixture, and never from a lean fuel mixture.
Carbon Dioxide, CO2, formed during any combustion process when oxygen and carbon are present in the primary combustion ingredients.
Oxygen, O2, from a lean fuel mixture.
Oxides of Nitrogen (Nitrogen Oxide) NOx. Nitrogen is present in the air we breath, and the engines air it consumes. Nitrogen displaces the air by approximately 75%. Nitrogen doesn't burn, but it can oxidize at temperatures over 2500 degrees F. NOx is a health hazard and one of the EPA's primary emission problems.
One method of controlling NOx is to reduce combustion temperatures. An EGR valve is the easiest device to use. It bleeds some inert exhaust gas into the incoming air stream, diluting the oxygen, and reducing combustion temperatures.
One other method of NOx reduction is to run a richer fuel mixture. By adding more fuel, the amount of air is displaced, reducing NOx. The leftover fuel is handled by the exhaust catalyst, converting the CO and HC into CO2. With a liquid fuel engine, the addition of more fuel also lowers the combustion temperature by the condensing effect. Here the fuel is evaporating and absorbing combustion heat. With a vapor fuel, the reverse if true. If the engine is running lean (over λ=1.2), the exhaust actually begins to cool down, thus reducing exhaust and combustion temperatures. Now we understand the rational of "Lean Burn"!

[mingmong]

Thanks for the reply. Am I right in thinking that these numbers are good. ie the mix is correct. Not too lean and not too rich?

[rattlsnak]

yeah, those are in the normal range. if you were to adjust your CO down a little, the HC would come back up. I wouldnt mess with it until your CO gets over 3.0, and then if your HCs didnt come down with it, then you might have an issue somewhere.

[mingmong]

Thanks very much...gives me peace of mind as I adjusted the mix myself and was never too sure if I'd got it right. Right...where are those car keys..I'm off for a burn...