Table of Contents
Advertisement
Fuel Type Compensation
The standard AFM output is for gasoline (measurement range of 6.0 to 20.0 AFR) with a fuel
H:C ratio of 1.85 and an O:C ratio of zero. If a fuel of a different composition is used, the
AFR measurement range will be different.
measurement range for commonly used fuels are given in the table below:
Fuel Type
Methanol (race car
"alcohol")
Ethanol (grain alcohol)
M85
E85
Natural Gas (CH
For fuel types not given in the table above, and for AFR read via the SMB, Equation 2 can be
used to compensate (i.e. correct) the AFM's output.
AFRcorr = [(2.368 x (m - 2p + 4)) / (m + 16p + 12)] x AFRmeas
where: AFRcorr is the AFR for the fuel of H:C=m and O:C=p composition.
AFRmeas is the AFR output by the AFM (i.e. 6.0 to 20.0 AFR).
m > (2p - 4).
Pressure Compensation
All wide-range exhaust sensors have a pressure sensitivity. Errors occur when the sensor is
operated at pressures different from the pressure at which it was calibrated. Changes in
pressure come about from changes in weather, altitude changes, and engine backpressure.
There is no sensitivity to pressure at stoichiometric (Lambda = 1) conditions. The sensitivity
gets greater the further from stoichiometric the engine is operated. Increases in pressure
make the sensor read further from stoichiometric (i.e. if lean, reads leaner, if rich, reads
richer). For example, an increase in exhaust pressure of 127 mmHg above the calibration
pressure (which would result from calibrating at 1 mile above sea level and then driving
down to sea level) would make a 12.65 AFR engine read 12.50 or a 16.77 AFR engine read
17.00.
If Equation 2 (fuel type compensation) and Equation 3 (pressure compensation) are to both to
be used, apply Equation 3 first, then Equation 2.
AFR vs. Vout
AFR = 1.24 x Vout + 2.66
AFR = 1.73 x Vout + 3.71
AFR = 1.46 x Vout + 3.12
AFR = 1.88 x Vout + 4.02
)
AFR = 3.31 x Vout + 7.10
4
The analog output formulas and AFR
AFR Measurement Range
2.66 to 8.86
3.71 to 12.36
3.12 to 10.42
4.02 to 13.42
7.10 to 23.65
[Equation 2]
9
Advertisement
Table of Contents
