Emissions Theory

The Greek letter Lambda was chosen as the symbol for the optimum air-fuel ratio for an internal combustion engine. Lambda stands for:

Actual inducted air mass
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Theoretical inducted air requirement

for a given quantity of fuel. When the actual mixture is the same as the theoretical mixture, Lambda = 1.
The ratio is approximately 14.7:1 for gasoline engines. This means that 14.7 kilograms of air are required to completely burn 1 kilogram of fuel.
When gasoline is burned in a gasoline engine, the three main pollutants produced are:

Carbon monoxide (CO)
Hydrocarbons (HC)
Oxides of nitrogen (NOx)

Fig. 51 Carbon Monoxide Chart:

CARBON MONOXIDE
CO is the product of incomplete combustion. When insufficient oxygen is available, carbon monoxide is formed instead of carbon dioxide. The richer the fuel mixture, the higher the CO content in the exhaust. In theory, complete combustion should occur when the air/fuel ratio is between 0.9 - 1.1 , Fig. 51.

Fig. 49 Hydrocarbon Chart:

HYDROCARBONS
HC is simply unburned (or partially burned) fuel. HC emissions can be caused by a rich fuel mixture (insufficient oxygen for complete combustion), or a lean mixture (incomplete combustion because of misfiring), Fig. 49.
There are many other causes for high HC readings including:
Engine misfiring due to fuel injection malfunction
Improper ignition timing
Ignition system malfunction
Leaking exhaust valves

Fig. 50 Oxides of Nitrogen Chart:

OXIDES OF NITROGEN

NOx consists of nitrogen and oxygen. While these gasses will not combine normally, the high temperatures and pressures in the combustion chamber causes these gasses to combine, forming several oxides of nitrogen compounds. These compounds are referred to collectively as NOx.
NOx formation increases as combustion temperatures rise. The high temperatures produced by a slightly lean mixture, Fig. 50, cause the level of NOx to rise considerably. A rich fuel mixture, or introducing non combustable gasses (such as exhaust) into the mixture, reduce the reduce peak combustion temperatures, inhibiting the production of NOx.