Fuel Mileage Caluculation

[DGD]

Just purchased a used 2007 Subaru Legacy 2.5i wagon. The display on
the dash has the ability to display "instantaneous fuel consumption"
and average fuel consumption, both in liters/100 km. The average fuel
consumption reading is tied to the trip odometer, so that is where the
calculation gets its total distance travelled. However, how does the
calculation get total fuel consumed, given that the fuel consumption
rate is a continuously changing variable? How accurate would these
fuel consumption algorithm be?

Thx.

 

[JD]

Just purchased a used 2007 Subaru Legacy 2.5i wagon. The display on
the dash has the ability to display "instantaneous fuel consumption"
and average fuel consumption, both in liters/100 km. The average fuel
consumption reading is tied to the trip odometer, so that is where the
calculation gets its total distance travelled. However, how does the
calculation get total fuel consumed, given that the fuel consumption
rate is a continuously changing variable? How accurate would these
fuel consumption algorithm be?

Thx.

It uses the MAF sensor and the VSS (vehicle speed) and it is very accurate
as long as you know the desired A/F ratio. The car's ECU has a target for
Air/Fuel (A/F) Ratio (usually, a little less that 14.7:1 Air to Fuel by
mass). Based on how much air is coming in, the calculation is relatively
simple.

Read the MAF which gives grams of air/second
Divide that by the A/F ratio which gives grams of fuel/second
multiply that by 3600 gives grams of fuel/hour
divide that by 786 (the mass of a litre of fuel in grams) which gives
litres/hour
then take the vehicle speed (from the VSS) and divide that by the
litres/hour which gives litres/km
multiply that by 100 which gives litres/100km.

The ECU has to calculate grams of fuel anyway based on the MAF and
corrections applied based on the O2 sensors to tell the ECU whether it got
it right or not, so it is pretty accurate.

 

[DGD]

It uses the MAF sensor and the VSS (vehicle speed) and it is very accurate
as long as you know the desired A/F ratio. The car's ECU has a target for
Air/Fuel (A/F) Ratio (usually, a little less that 14.7:1 Air to Fuel by
mass). Based on how much air is coming in, the calculation is relatively
simple.

Read the MAF which gives grams of air/second
Divide that by the A/F ratio which gives grams of fuel/second
multiply that by 3600 gives grams of fuel/hour
divide that by 786 (the mass of a litre of fuel in grams) which gives
litres/hour
then take the vehicle speed (from the VSS) and divide that by the
litres/hour which gives litres/km
multiply that by 100 which gives litres/100km.

The ECU has to calculate grams of fuel anyway based on the MAF and
corrections applied based on the O2 sensors to tell the ECU whether it got
it right or not, so it is pretty accurate.

This would provide the instantaneous fuel consumption reading that is
constantly varying, unless one is driving at a constant speed on a
level road. To get the average fuel consumption, one must conduct
some sort of an integration of all the instantaneous calculations to
derive the fuel consumed over the same period of time the trip
odometer is accumulating km travelled.

 

[JD]

This would provide the instantaneous fuel consumption reading that is
constantly varying, unless one is driving at a constant speed on a
level road. To get the average fuel consumption, one must conduct
some sort of an integration of all the instantaneous calculations to
derive the fuel consumed over the same period of time the trip
odometer is accumulating km travelled.

That's right. It is probably just an averaging function of some set amount
of time or distance.