Theoretical Air-fuel Ratio
The theoretical air-fuel ratio is the ratio of the minimum amount of fuel and air (which contains oxygen) needed to fully burn the fuel. Gasoline is a mixture of several types of hydrocarbons of which is the most predominant octane (C8H18). 2C8H18 + 25O2 !16CO2 + 18H2O In order for 1g of octane to produce water (H2O) and carbon dioxide (CO2) when burned, 15g of air is necessary. The actual fuel is not pure octane but octane and various HCs. Therefore the theoretical air-fuel ratio is about 14.7.
Graph Showing the Generation of CO/HC/NOx
The graph on the left, shows the air-fuel ratio and theamounts of CO/HC/NOx generated.
1. Richer
CO/HCIncrease
NOxDecrease
2. Leaner
CODecrease
HCDecrease
However it increases because of the misfire when the air-fuel ratio is too lean. NOxThe amount generated is greatest when the actual air-fuel ratio is slightly leaner than the theoretical air-fuel ratio. When the ratio becomes even leaner, the amount decreases because the combustion temperature decreases. The generated amounts of CO/HC/NOx increase under the following conditions, except for the graph on the left.
3. While engine is cold
The generated amounts of CO/HC increase because the rich air-fuel mixture is supplied.
4. At heavy load
The emission gas is increased because both the fuel and the air are increased.
The generated amounts of CO/HC increase because the rich air-fuel mixture is supplied.
The generated amounts of NOx increase becausethe combustion temperature rises.
HINT:
PPMAbbreviation for Parts Per Million. Used as a unit to indicate concentration or content.