What goes into the combustion chamber of an engine is fuel droplets and air. The fuel is hydrocarbon molecules which means they consist of Hydrogen atoms (H) and Carbon atoms (C). The air is about 4/5ths Nitrogen (N) which is fairly unreactive and 1/5th Oxygen (O) with a few other unreactive elements we don't need to worry about.
The hydrogen in the fuel reacts with oxygen atoms to form water (H2O). For every gallon of fuel burned, about a gallon of water is produced which exits the exhaust pipe in the form of steam.
The primary reaction we need to be concerned with is that between carbon atoms and oxygen atoms. This is the combustion that generates the power the engine produces. How this reaction proceeds depends heavily on the relative amounts of fuel and air in the mix (the A/F ratio) and how well combustion is initiated by the spark plug.
If the A/F mixture is weak (excess oxygen) then a lot of carbon dioxide (CO2) is formed. As the mixture gets richer there is less spare oxygen and so more carbon monoxide is formed (CO) and less CO2. Measuring the relative amounts of these two byproducts tells us most of what we need to know about the calibration of the fuel system. Under most combustion conditions there will be some CO formed and this is the primary measure of A/F ratio. Only when the mixture gets abnormally weak will there be no CO at all. Under these conditions there will be CO2 and also free unburned oxygen molecules (O2) which can be measured instead of CO to guide us as to what is happening inside the engine.
Rolling roads and engine dynos use gas analyzers to measure the byproducts of combustion and so help the operator calibrate the fuel system properly. A sophisticated analyzer will measure four gases - CO, CO2, O2 and hydrocarbons (HC). HC is a measure of the unburned fuel which passes straight through the engine without combining with oxygen atoms. HC will be high under two conditions. Firstly if the A/F mixture is very rich when there will be insufficient oxygen atoms to react with all the fuel present. Secondly if there is an ignition or other engine fault which prevents combustion taking place either properly or at all. CO and HC are the most important things to measure.
At idle a standard unmodified engine using a carburetor should show about 1% to 2% CO. Fuel injected cars will often have lower readings than this. Modified engines, especially those with long duration cams might need to run richer mixtures to idle smoothly.
At cruise the mixture is weakened further for best economy. CO readings should be almost zero.
At full power a standard engine will usually show about 3% CO which is a good compromise between power and economy / emissions. For best power this can be richened to 5% CO but fuel economy will suffer. Any richer than this and power starts to fall again.
Ideally this should be no more than a few hundred under most running conditions. Anything over 1000 means that a lot of fuel is not being burned. If both CO and HC readings are high then the mixture is too rich. If the CO reading is either low or correct but the HC is high then there is either an ignition fault which is preventing combustion happening properly, a basic engine fault like worn bores or valve seats, wrong cam timing etc, or a fuel atomization fault due to blocked or faulty injectors or carb jets. Engines like nice small well atomized fuel droplets which can mix properly with the air present. Big droplets tend to pass through the system without getting completely burned.
As a basic premise you can think of it as the CO value shows how much fuel is being burned and the HC value shows how much fuel is not being burned. The two together show how much fuel is being used in total. When both readings are correct you can be fairly certain there are no major faults with either the fuel or ignition systems.
The exact correlation between gas readings, air/fuel ratio and lambda values depends on how efficiently the engine is operating but the table below should be a good guide for average engines without catalytic converters.
|CO %||A/F Ratio||Lambda||Notes|
|0.00||15+||>1.00||Cruise mixture for good economy|
|0.50||14.5||0.99||Standard FI engine idle reading|
|1.50||14.05||0.96||Standard carb engine idle reading|
|2.50||13.50||0.92||Race engine idle reading|
|3.00||13.30||0.90||Standard engine at full throttle|
|3.50||13.10||0.89||Standard engine at full throttle|
|4.00||12.90||0.88||Good power with reasonable economy|
|5.00||12.60||0.86||Race engine at full throttle|
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