Air/Fuel Ratio

The air/fuel ratio (AFR) mixture induced into the cylinder will ignite properly and com­bust only if the air/fuel ratio lies within a certain range. The normal operating range for a naturally aspirated spark ignition engine is between 12 and 18:1 AFR.

Note that combustion limits for gasoline/air mixtures theoretically are 3:1 to 40:1 but practically are nearer to 9:1 to 25:1.

The stoichiometric air/fuel ratio is defined as the mass of air necessary to completely combust a mass of fuel (i. e., there is just enough oxygen for conversion of all the fuel into completely oxidized products).

The stoichiometric AFR is dependent on the composition of the fuel. (Typically for gasoline, it is 14.5:1.) For this reason, two additional parameters are used for defining mixture composition

. (A/P) actual

Excess air factor A, =

(A/F) stoichiometric (F/A) actual

Fuel-to-air equivalence ratio 0

(F/A) stoichiometric

Table 13.1 shows some typical air/fuel ratios from rich to lean. (Rich mixtures are less than XL)

TABLE 13.1 AIR/FUEL RATIO VERSUS LAMBDA

Rich

——— ►

Lean

AFR

12:1

14:1

16:1

18:1

X

0.83

0.97

1.10

1.24

E

1.21

1.04

0.91

0.81

The air/fuel ratio can be calculated by measuring the air and fuel mass flow rates. How­ever, in most instances, the air/fuel ratio is measured by exhaust gas analysis. From the relative concentrations of exhaust pollutants, the air/fuel ratio can be calculated from the SPINDT equation.

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