ECM afm1000, Instruction Manual

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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. The analog output formulas and AFR
measurement range for commonly used fuels are given in the table below:
Fuel Type AFR vs. Vout AFR Measurement Range
Methanol (race car
“alcohol”)
AFR = 1.24 x Vout + 2.66 2.66 to 8.86
Ethanol (grain alcohol) AFR = 1.73 x Vout + 3.71 3.71 to 12.36
M85 AFR = 1.46 x Vout + 3.12 3.12 to 10.42
E85 AFR = 1.88 x Vout + 4.02 4.02 to 13.42
Natural Gas (CH
4
) AFR = 3.31 x Vout + 7.10 7.10 to 23.65
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
[Equation 2]
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.
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