| Air/Fuel Ratio Limits |
6.0:1
9.0:1
11.5:1
12.5:1
13.2:1
14.7:1
15.5:1
16.2:1
18-22:1
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Rich run limit
Low power, black smoke
Rich best torque at WOT
Safe best power at WOT
Lean best torque at WOT
Chemically ideal
Lean light load, part throttle
Best economy, part throttle
Lean run limit
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Making use of the Ideal Gas Law, a synthetic variable called IMAP can
be used to estimate the mass air-flow (MAF) of an internal combustion
vehicle, much like a MAF sensor. However, in order to make this work,
the engine displacement and volumetric efficiency of the engine must
be provided.
IMAP is integrated by the firmware using the following formula:
IMAP = RPM * MAP / IAT / 2
where RPM is engine speed in revolutions per minute, MAP (manifold
absolute pressure) is measured in KPa, and IAT (intake air temperature)
is measured in degrees Kelvin. This integrated value can be converted
into total air flow (grams) using the following formula:
(grams of air) = (IMAP/60)*(Vol Eff/100)*(Eng Disp)*(MM Air)/(R)
where R is 8.314 J/°K/mole and the average molecular mass of air (MM) is
28.97 g/mole. Note that in the above formula the volumetric efficiency
of the engine is measured in percent and the displacement is in liters.
Once we have MAF integrated over time, we can calculate fuel flow
using a specific air/fuel mixing ratio (A/F). A/F can range from 6 to 22.
The total mass of air which flows through the engine (as measured by
the MAF sensor or calculated using MAP, RPM and IAT) is divided by the A/F
ratio (e.g., 14.7) to derive the total mass of fuel that has been burned.
If the air mass is measured in grams, then the calculated amount of
fuel will also be in grams. Using an average density of gasoline of
6.17 pounds per gallon and knowing that there are 454 grams per pound,
then dividing the total fuel mass in grams by (454 * 6.17) yields the
total number of gallons of fuel burned for a given amount of air (as
measured by the MAF sensor).
Fuel consumed can be estimated from integrated MAF using the following
formula:
(gallons of fuel) = (grams of air) / (air/fuel ratio) / 6.17 / 454
Knowing the total fuel consumed and the distance traveled, the fuel
consumption rate in MPG (miles per gallon) then can be calculated.
Copyright ©2011 by Bruce D. Lightner. All rights reserved.
Last updated
Wed Oct 30 07:19:13 PDT 2013
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MAP- and MAF-Based Air/Fuel Flow Calculator