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Understanding 4 Gas Analysis

The following are a few general facts and tips to keep in mind when using a 4 gas analyser:

  1. Carbon Monoxide (CO) is, arguably, an indicator of combustion inefficiency as it represents partly burned fuel.
    • High CO readings usually indicate a fuel mixture richer than ideal.
    • (Rich mixture - air fuel ratio below 14.7:1 or below Lambda 1).
    • High CO levels result from insufficient oxygen supply required for complete combustion.
    • This indicates a rich mixture, (either too much fuel or not enough air).
    • Note: This is a gas that is treated in the catalyst, if fitted, therefore high readings may be attributed to catalyst efficiency.
  2. Carbon Dioxide (CO2) gives us a direct measure of combustion efficiency. The higher the value the more efficient the combustion, therefore the nearer to the ideal air-fuel ratio.
    • Low readings, (usually below approximately 12%), indicate a combustion fault, an exhaust system leak or a sample dilution.
    • Note: This gas is not treated by the catalyst, if fitted, but may be effected by the catalyst process.
  3. Hydrocarbon (HC) is, as CO, a measure of combustion inefficiency as it represents unburned fuel. High readings may indicate a lack of ignition or poor compression.
    Note: This is a substance which is treated by the catalyst, if fitted, therefore high readings may be attributed to catalyst efficiency.
  4. Oxygen (O2) readings can also show combustion efficiency, as it shows how much oxygen is left over after combustion has taken place. It starts with the ambient, which is around 20% and may be divided by the number of cylinders the engine runs and should result in as little as possible, although there should always be a little left over. High levels can indicate poor combustion or exhaust system and sample probe leaks. Low levels may indicate a rich mixture.
    Note: This is not a gas directly treated by the catalyst, if fitted, but will be effected by the cataylst process.

Reminder:

The emissions are dependant on the combustion process air-fuel ratio.
The list below shows what happens:

O2 combines with HC to form CO2 and H2O
O2 combines with CO to form CO2

Note: O2 and CO are equal at the stoichiometric air-fuel ratio

Last Reviewed: 27 March 2016Copyright © 2004...2016 Mark Stammers