Groover wrote:
"I'll agree, many onboard mileage numbers tend to be optimistic. On my 2015.5 duramax, it is consistently about 0.5mpg high.
so, 2-5% high. Close enough for me. On our daughters car, similar experience, somewhere around 5% high."
My understanding is that on most gas engines the computer is not able to actually measure fuel usage but they do have sensors for measuring how much air is going into the engine. It then uses a simple calculation to determine how much fuel is required to be mixed with that amount of air and considers that to be the amount of fuel used. I am wondering if the calculations are based on using pure gasoline and lose accuracy when alcohol is mixed in. You lose about 3% of the energy per gallon when burning the standard 10% alcohol mix and most reports that I seen are that the computer calculations are off about that much. Has anyone checked their computers with different amounts of alcohol in the fuel?
on a gas engine ideal air/fuel ratio is around 14.5:1 don't recall the exact number and it is dependent on the fuel. with modern fuel injected engines other than WOT and maybe startup, the ECU is able to hold very close to the ideal number for the fuel in the tank. energy density of the fuel and what the exact fuel is will have an effect on the ideal ratio. In fact different blends of gasoline (summer vs. winter) vary in BTU/gallon, add the % alcohol just adds more variability. But for doing an onboard fuel calculation it is probably easier to assume 14.5 and calculate rather than measuring the precise amount of fuel consumed.
For a diesel, it's another story. you can't assume a static A/F ratio. It will vary between 200:1 or more down to 6:1 or less. so it means trying to calculate quantity of fuel used. For a DI diesel if they monitor fuel pressure, and injector duration time and number of injections/cycle you can get reasonably close. At least my Duramax gets with 5% or so.
