How does the bike computer calculate mpg (mileage)

[blackyam]

Hi, everyone.

 

How does the bike computer calculate mpg (mileage)? I have a 2010 R1200RT.

 

Last Monday, while riding home, the yellow triangle (!) warning light came on, my gas gauge dropped to empty, the low fuel icon was flashing, and the miles-to-fill-up dropped about 1 mile per second until it read ---mls. The tank was only half empty. I filled up before home, but, the next morning until now, the warning light was still on, the gas gauge was still reading empty, the low fuel warning was still flashing, and the miles-to-fill-up was still reading ---mls. I have apparently been struck by the notorious fuel-strip problem at 56k mi.

 

However, when I press the INFO button that cycles through average speed, average mpg, oil level, temp, and miles-to-fill-up, the average mpg still calculates a pretty good estimate compared with my fill-up-to-fill-up calculation using the trip odometer. How is the computer able to calculate the average mpg when the gas gauge reads empty constantly?

 

By the way, I have made an appointment to bring my bike into my dealer in a fortnight to have the fuel strip replaced if that is the problem.

 

Thanks.

 

---John.

[dirtrider]

Afternoon John

 

The computer extrapolates the MPG basically based on fuel injector pulse width (how long each injector squirts on each pulse, engine RPM's (how many times the fuel injectors squirt each minute) & miles traveled.

 

Minor inputs that effect the calculated MPG would be battery voltage (effects injection amount for each squirt).

 

Basically the computer knows how much fuel is injected, for how long, & how many miles the bike has traveled. (has nothing to do with the fuel strip or fuel gauge). But miles to empty does as well as low fuel light.

[mneblett]

My understanding: One of the bike's computers can count the fuel injection pulses to determine exactly how much fuel is being used -- totally independent from the fuel level sensor. It then takes the amount of fuel left in the tank and the historical consumption rate (i.e., an average of recent use using the injected amount and the odometer output) and provides an *estimate* of the remaining range. If your riding after the warning comes on is significantly more aggressive or at much higher speed, the estimate will not be accurate.

 

Possible reasons you did not see this change once your fuel level indication went south: (i) the fuel level sensor isn't fouled and is still providing good data to the computer but not the instrument display (i.e., a downstream problem -- not likely), or (ii) because the time constants in the range estimate algorithm are very long to minimize short term effects, if enough time had not yet passed (with the bike running) the effect of the bad level reading had not yet shown up in the range estimate.

[mneblett]

D.R. on the case yet again! I spent too long typing

[blackyam]

dirtrider and mneblett, Thank you very much for the information. It's very interesting.

 

So, if I understand correctly, the bike computer knows how much gas is used by tracking the fuel injector pulse width. Tangent: Could this be used as a fuel gauge? What if there were a button to reset the fuel gauge to FULL at fill-up, and then the gauge tells how much fuel is left based on the tank capacity and the amount of gas used by tracking the fuel injector pulse width? (Does that even make sense?) Forget about tank fuel strips, floats, &c.

 

---John.

[dirtrider]

dirtrider and mneblett, Thank you very much for the information. It's very interesting.

 

So, if I understand correctly, the bike computer knows how much gas is used by tracking the fuel injector pulse width. Tangent: Could this be used as a fuel gauge?

 

What if there were a button to reset the fuel gauge to FULL at fill-up, and then the gauge tells how much fuel is left based on the tank capacity and the amount of gas used by tracking the fuel injector pulse width? (Does that even make sense?) Forget about tank fuel strips, floats, &c.

 

Morning John

 

Yes, that might very well work, the BIG problem with that is tricking the fuel gauge to see (the correct) FULL. That isn't a simple task as the fuel strip output is not a simple analog signal but very complex output based on a heated fuel strip & computer interpretation of that output.

 

You can trick the fuel strip output to show something (enough to turn off the low fuel light) but to get it to show exactly full enough to get a trip computer true miles to empty would be very difficult, or at least very time consuming.

 

Your fuel strip should be under a special BMW fuel strip warranty so why not just get it replaced by BMW for free?

 

Added: probably the best way to travel with a dead fuel strip is to use the trip set odometer. I would imagine you have a good idea of how many miles you can go on a fill up. If not then start with 200 miles then see how much fuel it takes to fill the tank, then try 250 miles & see how much fuel it takes.

[duckbubbles]

So, if I understand correctly, the bike computer knows how much gas is used by tracking the fuel injector pulse width. Tangent: Could this be used as a fuel gauge? What if there were a button to reset the fuel gauge to FULL at fill-up, and then the gauge tells how much fuel is left based on the tank capacity and the amount of gas used by tracking the fuel injector pulse width? (Does that even make sense?) Forget about tank fuel strips, floats, &c.

If memory serves, there was such a device available in the aftermarket for the early K's before the fuel gauge was available. I believe it used the clock for a percent countdown of fuel remaining by using injector pulse rate and duration. I never tried it and don't know of anyone else who did.

 

Frank

 

Frank

[blackyam]

Your fuel strip should be under a special BMW fuel strip warranty so why not just get it replaced by BMW for free?

 

Oh, yes, I have an appt to have it replaced if needed next Friday. My question about an alternative means to measure the fuel level in the tank was academic. Thanks.

 

If memory serves, there was such a device available in the aftermarket for the early K's before the fuel gauge was available. I believe it used the clock for a percent countdown of fuel remaining by using injector pulse rate and duration.

 

That's very interesting. Thanks very much.

 

Thanks again to all. It's very nice to learn something new. :-)

 

---John.

[Hank R1200RT]

200 miles is probably fine, but I think 250 is pushing it. So much depends on riding style, rider/luggage weight, road (freeway, canyon, country roads), headwinds, and the all-important throttle hand. If you have the slightest doubt, gas up, especially on an unknown road.

 

Keep the rubber side down,

[RichM]

My personal record is 278 before stopping for fuel.

[dirtrider]

Afternoon Rich

 

Stopping is one thing but how much fuel it took is another.

 

I think my personal record is 50 some miles but it only took a little over a gallon.

 

250 miles to empty is about 37 mpg average (assuming a BMW fill & not a going-on-a-trip real-full fill) . I ride the living dickens out of my 1200Rt & usually get at least in the 40's.

[60av8tor]

Afternoon Rich

 

Stopping is one thing but how much fuel it took is another.

 

I think my personal record is 50 some miles but it only took a little over a gallon.

 

250 miles to empty is about 37 mpg average (assuming a BMW fill & not a going-on-a-trip real-full fill) . I ride the living dickens out of my 1200Rt & usually get at least in the 40's.

 

+2 - doubting 250 on an RT tank would seriously make me question the state of my RT, although I'm not sure how the tune would be bad enough to affect mileage that severely and not noticeably affect drivability. 7 gallons - 6.6 usable...something like that. In almost 10 years of RT ownership, 250 has been the time I begin thinking about fueling up - one of the many things I love about the RT.

[Paul In Australia]

Afternoon Rich

 

Stopping is one thing but how much fuel it took is another.

 

I think my personal record is 50 some miles but it only took a little over a gallon.

 

250 miles to empty is about 37 mpg average (assuming a BMW fill & not a going-on-a-trip real-full fill) . I ride the living dickens out of my 1200Rt & usually get at least in the 40's.

 

 

 

+2 - doubting 250 on an RT tank would seriously make me question the state of my RT, although I'm not sure how the tune would be bad enough to affect mileage that severely and not noticeably affect drivability. 7 gallons - 6.6 usable...something like that. In almost 10 years of RT ownership, 250 has been the time I begin thinking about fueling up - one of the many things I love about the RT.

 

+1

[blackyam]

Hi, again.

 

I do use the trip odometer to meter my fuel, and I usually try to top off by the time I get to about 200 mi. I have gone about 245 mi. One thing that discourages me from pushing it was an experience I had with our Ford Taurus car several years ago---the fuel pump failed, and I was told by the mechanic that a common reason for that is running the tank low too frequently. (I was told that the fuel pump sits in the tank and is cooled by the fuel, which makes little sense to me as a design.) Since I don't know if the fuel pump of my RT sits in the tank or not (shame on me!), I assumed that it does and, so, try not to let the tank get too low frequently.

 

What do y'all say? Thanks.

 

---John.

[dirtrider]

Morning John

 

Yes, your fuel pump is inside the tank & is both fuel cooled & fuel lubricated so not a good idea to run it completely out of fuel.

 

 

[blackyam]

Thanks, D.R.

 

I went into my shop last Friday to have the fuel strip replaced. (They also replaced my handlebar switches at the same time.) The fuel gauge is working again and, more importantly to me, the bright yellow warning light has been extinguished. Yea! The whole job, fuel strip and switches, took about five hours, but at no charge to me.

 

---John.