User's Manual

Gas Analysis
70
• CO
2
and O
2
are indicators of the integrity of the exhaust system and/or the sample
hose and probe
• CO
2
is an indicator of combustion efficiency that peaks at or near the stoichiometric
air/fuel ratio and decreases with rich or lean air/fuel ratios
• If CO
2
increases, O
2
decreases (inversely related)
If O
2
increase, CO
2
decreases (inversely related)
During a cold start, and no secondary air injection, CO will be above 1% and the
catalytic converter will be O
2
starved and will not ‘fire’ and the emissions will be the
same as pre-cat levels
Corrected CO
The Live Readings test offers the selection of Corrected CO (COK).
Corrected CO takes into account any dilution of the sampled gas, for example leaks
in the exhaust pipes and silencers. The formula to calculate COK is:
COK = CO x 15/CO
2
When stoichiometric combustion is achieved and there is no dilution, the CO
2
value
drops to approximately 15%, therefore, the correction factor is unity. If the sample is
diluted with air, the CO
2
value falls below 15% and the correction factor is <1.
Non-Catalyst Systems
Perfect combustion in an ICE has an AFR of 14.7:1. This is measured by weight, any
reading higher than 14.7 indicates a lean mixture and likewise any reading lower than
14.7 indicates a rich mixture. For diagnostics purposes the AFR must be settled at
14.7:1 before analysing any other gas reading. With AFR correct the following table
shows how the levels of other gases can indicate a problem with the engine with
relation to the time of combustion.
Non-Catalyst System Gases with AFR
These readings are typical and reference should be made to the manufacturers’
specifications, which usually only refer to CO and HC. Setting the CO level correctly
should bring the other gases to high CO
2
, low HC and low O
2
.
AFR 14.7
Gas Perfect Minimum Maximum Problem
CO In Specification
CO
2
Highest possible 11% Before combustion
HC Lowest possible 400ppm During combustion
O
2
Lowest possible 2% After combustion