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Car Forum / Subaru Cars / February 2008Fuel Mileage Caluculation
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. > Just purchased a used 2007 Subaru Legacy 2.5i wagon. The display on > the dash has the ability to display "instantaneous fuel consumption" [quoted text clipped - 6 lines] > > 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. > > Just purchased a used 2007 Subaru Legacy 2.5i wagon. The display on > > the dash has the ability to display "instantaneous fuel consumption" [quoted text clipped - 25 lines] > 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. >> > Just purchased a used 2007 Subaru Legacy 2.5i wagon. The display on >> > the dash has the ability to display "instantaneous fuel consumption" [quoted text clipped - 35 lines] > 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. |
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