What variables doe the computer consider when calculating range?

[shertex]

Oops..."DOES the computer..."

Just curious what factors, other than amount of fuel remaining, are taken into account when the computer (in this case, W211) calculates range. It's often skewed way low, so I'm gathering that's due to extrapolating from fuel consumption at that moment. Just wondering what other factors might come into play.

[ah-kay]

In an ideal world, there would be an array of laser mounted on top of the fuel tank to bounce off the fuel surface to measure the air gap. This would be an accurate way to measure the volume of fuel.

In the real world, it would be the dumb fuel gauge measuring the resistance of the float to calculate the volume. The range would be guesstimated as volume x average mpg. So put up with the number. Enjoy the car, it would not be perfect on mpg, range etc.

Electric car would have a more accurate range but it would also on the conservative side as you don't want to run out of juice in the middle of the desert.

[GregMN]

I do not know for sure how our w211 systems calculate it.

My 1980's Audi 5000's and 200's calculated the remaining fuel range by using the average mpg of the last 20 miles driven.

My observations on the w211 have been that it might calculated the remaining fuel range on a shorter history, maybe as short as 5 miles. So when you accelerate from a stop, or onto the freeway, it takes a few miles for that event to pass out of the fuel range calculation. I also notice this in hilly terrain where the uphill accelerations and down hill coasting enter and leave the range calculation.

I can tell if I am currently getting better or worst mileage then the mpg display by observing whether the range is going up or down.
If the range is rising, I know that I am currently getting more mpg then the displayed mpg and it will eventually go up.
If the range is falling, I know that I am currently getting less mpg then the displayed mpg and it will eventually go down.
If the range is steady, then I know that I am no longer getting better mpg than the displayed mpg.

On my car, the mpg display is less than the actual mpg.
On my wife's daily commute of 60 miles round trip, it is short 2.5 to 3.5 mpg.
On a recent 3800 mile trip it was short zero mpg (once) to 2 mpg (twice) on 10 fill-ups.

[shertex]

Originally Posted by GregMN

I do not know for sure how our w211 systems calculate it.

My 1980's Audi 5000's and 200's calculated the remaining fuel range by using the average mpg of the last 20 miles driven.

My observations on the w211 have been that it might calculated the remaining fuel range on a shorter history, maybe as short as 5 miles. So when you accelerate from a stop, or onto the freeway, it takes a few miles for that event to pass out of the fuel range calculation. I also notice this in hilly terrain where the uphill accelerations and down hill coasting enter and leave the range calculation.

I can tell if I am currently getting better or worst mileage then the mpg display by observing whether the range is going up or down.
If the range is rising, I know that I am currently getting more mpg then the displayed mpg and it will eventually go up.
If the range is falling, I know that I am currently getting less mpg then the displayed mpg and it will eventually go down.
If the range is steady, then I know that I am no longer getting better mpg than the displayed mpg.

On my car, the mpg display is less than the actual mpg.
On my wife's daily commute of 60 miles round trip, it is short 2.5 to 3.5 mpg.
On a recent 3800 mile trip is was short zero mpg (once) to 2 mpg (twice) on 10 fill-ups.

So far, on mine, the mpg display is skewed high by about 3 mpg. Over about 4,000 miles my Fuelly average is 33.0 but the computer says 36.3.

[ah-kay]

Originally Posted by GregMN

I can tell if I am currently getting better or worst mileage then the mpg display by observing whether the range is going up or down.
If the range is rising, I know that I am currently getting more mpg then the displayed mpg and it will eventually go up.
If the range is falling, I know that I am currently getting less mpg then the displayed mpg and it will eventually go down.
If the range is steady, then I know that I am no longer getting better mpg than the displayed mpg.

Isn't this obvious to anyone? Range = fuel x mpg.

[jcyuhn]

It's certainly based on a very short term fuel economy calculation. Simply driving up a 1 mile long hill will cause range to fall significantly, and coasting down the other side will bring it back up. I would guess it's calculated not over a fixed distance, but rather a periodic time. For example, it samples the "instantaneous" fuel economy every 15 seconds, and then averages the last 20 samples to determine the fuel economy number used in the MPGxFuel calculation to determine range.

[GregMN]

Originally Posted by ah-kay

Isn't this obvious to anyone? Range = fuel x mpg.

I don't see in your sig. that you own one of these cars. So, I will give you the benefit of the doubt that you do not know what we are talking about because you have never seen it yourself.

The "range" displayed is not the mpg displayed X the gallons in the tank. It is the current, short history, mpg x the gallons in the tank.

Also: Since the W210, (which I again do not see that you own) the fuel sender information is not display directly on the fuel gauge. The sender information is sent to the computer. The computer displays what it calculates as the fuel level in the tank. When the car is moving, the gauge display is buffered by the computer to smooth out variations from sloshing fuel. If you add fuel to the tank while moving, the gauge display will take about one minute to adjust for one gallon of additional fuel. If you stop the car, turn it off and back on again, it will instantly display the correct fuel level. When you fill the car at a stationary fuel station, you can watch the gauge go up instantly as the fuel is added.

And, yes, I have added fuel to a tank with a MB sending unit connected to the car's MB gauge while driving down the freeway. W210 fuel gauge puzzle

[GregMN]

Originally Posted by shertex

So far, on mine, the mpg display is skewed high by about 3 mpg. Over about 4,000 miles my Fuelly average is 33.0 but the computer says 36.3.

It might be the length of trips, or the type of driving on those trips that explains the difference between your display discrepancy and mine. In the ML we have the same under reporting of the actual mpg.

[Stretch]

SRJ was asking about early morning MPG figures on his computer - someone chimed in to say the outside air temperature was used in the estimation (sorry "someone" I forget who you were)

[TimFreeh]

Originally Posted by jcyuhn

It's certainly based on a very short term fuel economy calculation. Simply driving up a 1 mile long hill will cause range to fall significantly, and coasting down the other side will bring it back up. I would guess it's calculated not over a fixed distance, but rather a periodic time. For example, it samples the "instantaneous" fuel economy every 15 seconds, and then averages the last 20 samples to determine the fuel economy number used in the MPGxFuel calculation to determine range.

That would be my guess as well for approximately how it works, I've passed on my CDI to a relative but another trivia fact I seem to remember is that the fuel gauge on the W211 was generated with a bar graph comprised of approximately 20 individual bars, each bar on the graph was a pretty close approximation to 1 gallon in the tank. Don't know if that was by design but when I was calculating range it was a good rule-of-thumb for how many gallons I had in the tank.

The fuel gauge and range indicator in my W211 was extremely accurate - one time I was trying to get a 900 mile round trip done and concluded back at my home fuel pump. I purposely ran the tank down to the point where the range indicator said 5 miles to empty, the reserve light had been on for a good 80 miles at that point.

Can't remember the specific number but I filled the tank to the brim and got within 1/10th of a gallon of the published fuel tank capacity. At least on my car if the range indicator said it was empty it was indeed empty.

[BillGrissom]

Don't know about modern diesels, but I think modern gas cars estimate fuel flow from the fuel injector duty cycle. Of course this assumes you have the factory injectors, all flow the same as original, and the fuel rail pressure is correct (engine computer doesn't know that or even if the fuel pump is working). Vehicle speed comes from the speedometer, which is electronic today and some might even use the ABS wheel rpm sensors. Since modern diesels have electronic fuel controls, that would be similar to a gas fuel injector.

My 1984 & 1985 300D's have a "rack position sensor" on the injection pump which could be used as a poor-man's fuel flow sensor, when combined w/ engine speed (existing electronic tachometer). Fairly simple today, but microprocessors were not so easy and cheap in 1985.

The instantaneous mileage display doesn't need to be super accurate, since they are best used to judge changes. For example, in my 2002 T&C I see the instant mileage change from 23 mpg to 31 mpg when I follow a large truck at 65 mph and there is no cross-wind. I saw the MythBuster's episode and I don't need to follow very close, indeed other cars sometimes cut in between me and the truck. For absolute mileage, you can't beat using the state calibrated fuel pumps and your odometer (once verified to mileage markers or a GPS). But I have found my T&C instant mileage display to agree very closely on long trips.

[ah-kay]

Originally Posted by GregMN

If you add fuel to the tank while moving, the gauge display will take about one minute to adjust for one gallon of additional fuel. If you stop the car, turn it off and back on again, it will instantly display the correct fuel level. When you fill the car at a stationary fuel station, you can watch the gauge go up instantly as the fuel is added.

I have NEVER done what you described. Who in the world would add fuel into the tank when car is moving?

I don't own any of this modern car. Not because I cannot afford it just don't want to smog them in CA. I work in software and all this is common sense. Range = avg mpg x fuel ( however simple or complicated it may be for the car to figure it out ). Enough said.