HC vs CO

[Andrew S]

Hi All

To my classic Mini for its MOT today went straight through again without any advisories thank goodness. When reading the CO readings which were exactly 2.5% got me thinking about what's the difference between HC readings and Co readings. I have always thought of the Co as being the mixture or unburnt fuel. Is the HC the efficiency of the engine, the wear and tear so to speak affecting the emissions?

[DJT]

HC is unburnt HydroCarbons (ppm)
CO is percentage of Carbon monOxide.

[mike@thenook]

Congratulations to Mini on another years ticket!

[MandM]

Having just had my MOT, I know I have air leaks on my spindles (when I spray Brake cleaner around that area the rev's rise) my HC was 165ppm and the CO was 0.40%, think I need to do them this winter, car runs fine. any comments

[Andrew S]

Thanks Dave for clarification. Presume readings would work hand in hand as the revs go up being Co would drop at revs so should the HC’s as there is less unburnt fuel at higher speeds.

[Andrew S]

Congratulations to Mini on another years ticket!

Thanks Mike, great fun to drive and even better fun when it passed for some reason LOL

[Andrew S]

Hi Martin
Those readings are very very low but a good sign if engine is happy to run OK. Worth replacing those seals, Theres just three of them to do

[dasadrew]

.... CO was 0.40%...

sounds very low, unless there are different means of measuring in UK. Mine has a max allowable of 4%, and the last MoT was 3,85%

[Wheelz]

Emissions vary according to the air/fuel mixture (Lambda of 1 is 14.7:1). Various graphs on the internet show typical relations.

[trunt]

I learned a bit from nursing a tuned up TR through emissions tests for a number of years.. CO is easy, just set the carbs up correctly and it will be in, the issue I always had was HC, as previously noted it’s unburned fuel and at idle at least it helps to increase the airflow through the carb to flush the fuel. I found that if I retarded the ignition, which causes the engine to idle slower and open up the idle to bring the idle back up even a little higher if possible, I always got it through.. but it ran hot so normality was restored as soon as I got home!

This month I take the Stag in, first time as when I went last time covid had suspended emissions testing.. the TR had no Nox limits, the Stag being a later model does.. Word is that humid air helps on Nox so next week looks good.. we will see!

[Andrew S]

Mr Thickie here again. If you weaken or enrich carbs and looking at say a gas analyser, you can clearly see the percentage either going up or down, then I presume as you weaken the mixture, you automatically reduce the HC reading. They go hand in hand. I say this as rich fuel mixture generates unburnt fuel.

[trunt]

Mr Thickie here again. If you weaken or enrich carbs and looking at say a gas analyser, you can clearly see the percentage either going up or down, then I presume as you weaken the mixture, you automatically reduce the HC reading. They go hand in hand. I say this as rich fuel mixture generates unburnt fuel.

An overly lean mixture can raise HC as you get incomplete ignition. Even if you have the correct mixture there are usually other reasons that HC could be high.. the combustion could be incomplete due to say a bad plug lead, coil, timing etc causing raw fuel to get to the exhaust.

Heres another graph!
6wSVYt7.png