Snap-On Diagnostics DGA 1000 - Air-Fuel Ratio and Stoichiometric Point

Analyzer Screen Display Features
2-5
Air-Fuel Ratio and the Stoichiometric Point
For each fuel used in an internal combustion engine, there is an
air-fuel ratio that optimizes the combustion process. For common
gasoline, that ratio is 14.7 pounds of air to 1 pound of gasoline.
This ratio varies with each type of fuel. The point of optimal
combustion is called the stoichiometric point. The combustion
process burns fuel and oxygen. The following by-products result
from the combustion process:
• Mechanical force and heat.
• The pollutants HC (hydrocarbons), CO (carbon monoxide), NO
x
(oxides of nitrogen), and SO
2
(sulfur dioxide).
• The favorable gases CO
2
(carbon dioxide), O
2
(oxygen), and
H
2
O (water vapor).
The figure below shows the relationship between the air-fuel ratio
and certain exhaust gases. Note that optimal combustion is
identified by a low level of O
2
, a high level of CO
2
, and low levels of
the pollutants HC and CO.
Figure 2-4 : Relationship Between Air-Fuel Ratio and Certain Exhaust Gases