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      03-05-2011, 03:04 PM   #1
Brutus1
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How does the vehicle calculate mpg?

I know how to calculate mpg but how does a vehicle determine this? Obviously it is some kind of average but is it over some set time,gallons, or what? I have done a search but found no answers. TIA.
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      03-05-2011, 03:46 PM   #2
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It keeps calculating over whatever consumption you allow between resets. I'm not certain about the calculation, but I've been thinking that the computer knows how much fuel it sends to the engine, so I'm guessing it uses that and the actual distance the vehicle moves.
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      03-05-2011, 05:50 PM   #3
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If you sit in the car with the engine idling for about 5-10 minutes, you can see the mpg dropping down 0.1 at a time (depending on how long since you last reset the computer) which suggests it's pretty accurate or at least that it wants you to believe that it is!

If you reset the computer and the work out your own mpg between two fill ups, you can compare and get an idea of how good a job it's doing.

Lotus will know the technical answer to how it actually measures mpg
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      03-06-2011, 02:44 AM   #4
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Quote:
Originally Posted by sfax View Post
If you sit in the car with the engine idling for about 5-10 minutes, you can see the mpg dropping down 0.1 at a time (depending on how long since you last reset the computer) which suggests it's pretty accurate or at least that it wants you to believe that it is!

If you reset the computer and the work out your own mpg between two fill ups, you can compare and get an idea of how good a job it's doing.

Lotus will know the technical answer to how it actually measures mpg
sfax,

Thanks for the vote of confidence. Just remember, anything you read on the Internet is worth exactly what you pay for it.

I haven't seen any data on precisely how the X3 specifically computes fuel useage, but I'd be willing to make a small bet it's the same as all other contemporary cars that I've had detailed info on. The typical method for port injection engines is for the trip computer to keep track of the fuel injector on-time or "duty cycle". On a port injected engine the fuel rail pressure is kept very constant, so the "on time" of the injectors is a very accurate measure of the total fuel used when integrated over time.

The N55 (35i) engine is a direct injection engine which uses a high pressure fuel pump (HPFP) running at something like 40X the pressure of a port injection system, and under some conditions, can generate multiple fuel injection "bursts". I believe it's also a constant pressure pump, but it may be modulated (am not sure, and BMW doesn't supply details of their direct injection system. If It is a constant pressure pump, then the "duty-cycle" measurement should work OK, if not, then a pressure measurement also is needed to plug into the fuel usage algorithm.

Either way, the computer knows what the injectors (magnetic for a port system and piezo for a direct injection system) are being "told" by the fuel management system, and as long as the injectors are not dirty, plugged or leaking, and the fuel pressure regulator is working correctly, that signal is proportional to the actual fuel used.

On a different thread, I once mentioned that I accurately measured the actual fuel used for a tankful and determined my 35i's mileage according to the odometer and the fuel consumed by measuring the refill amount required at the pump was 18.23 mpg.

For that same tankful, the vehicle trip computer indicator read 17.5 mpg avg. or 4.5% lower than real. However, the odometer was actually reading 1.5% high. So the bottom line was: For that particular tankful, my car's trip computer mpg measurement was reading about 3% lower than reality.

All things considered, I think that's pretty accurate.

Hope this helps answer Brutus1's original question.

Last edited by Lotus7; 03-06-2011 at 02:54 AM..
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      03-06-2011, 04:16 AM   #5
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They use a flow meter who measure the fuel flow and driven distance for average fuel consumption. In the previous x3 (diesel) the mass meter was mount in the end of the high pressure pump. The BMW efficient dynamics show the instant fuel consumption in the dash like in this thread http://x3.xbimmers.com/forums/showthread.php?t=497181

Last edited by perto; 03-06-2011 at 04:41 AM..
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      03-06-2011, 07:06 AM   #6
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Quote:
Originally Posted by Lotus7 View Post
sfax,

Thanks for the vote of confidence. Just remember, anything you read on the Internet is worth exactly what you pay for it.

I haven't seen any data on precisely how the X3 specifically computes fuel useage, but I'd be willing to make a small bet it's the same as all other contemporary cars that I've had detailed info on. The typical method for port injection engines is for the trip computer to keep track of the fuel injector on-time or "duty cycle". On a port injected engine the fuel rail pressure is kept very constant, so the "on time" of the injectors is a very accurate measure of the total fuel used when integrated over time.

The N55 (35i) engine is a direct injection engine which uses a high pressure fuel pump (HPFP) running at something like 40X the pressure of a port injection system, and under some conditions, can generate multiple fuel injection "bursts". I believe it's also a constant pressure pump, but it may be modulated (am not sure, and BMW doesn't supply details of their direct injection system. If It is a constant pressure pump, then the "duty-cycle" measurement should work OK, if not, then a pressure measurement also is needed to plug into the fuel usage algorithm.

Either way, the computer knows what the injectors (magnetic for a port system and piezo for a direct injection system) are being "told" by the fuel management system, and as long as the injectors are not dirty, plugged or leaking, and the fuel pressure regulator is working correctly, that signal is proportional to the actual fuel used.

On a different thread, I once mentioned that I accurately measured the actual fuel used for a tankful and determined my 35i's mileage according to the odometer and the fuel consumed by measuring the refill amount required at the pump was 18.23 mpg.

For that same tankful, the vehicle trip computer indicator read 17.5 mpg avg. or 4.5% lower than real. However, the odometer was actually reading 1.5% high. So the bottom line was: For that particular tankful, my car's trip computer mpg measurement was reading about 3% lower than reality.

All things considered, I think that's pretty accurate.

Hope this helps answer Brutus1's original question.
I knew you wouldn't disappoint Lotus!!
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      03-06-2011, 08:30 AM   #7
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Quote:
Originally Posted by sfax View Post
I knew you wouldn't disappoint Lotus!!
Should have noted that my comments apply to gasoline (Petrol) 28i & 35i cars only. Obviously, no duty cycle signal with Diesel engines using mechanical injectors and a mechanical injection pump. Since we don't have the "d" cars here, I don't automatically think about them. Apologies to all the high-efficiency oil burners out there.

As perto comments, on Diesels typically a "differential" mass flow meter is used that measures total flow and then subtracts the pressure bypass regulator flow to give the net amount sent to the mechanical injectors. No need for a flow meter when you have electronically driven injectors.
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      03-06-2011, 10:08 AM   #8
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you guys are great. I learn so much reading these posts - thanks
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      03-06-2011, 07:44 PM   #9
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Quote:
Originally Posted by sfax View Post
If you reset the computer and the work out your own mpg between two fill ups, you can compare and get an idea of how good a job it's doing.


Thanks all for the info. Since I have not reset it , my current average is for all miles driven. Where is the reset button for the computer and I assume you do not mean the trip odometer reset?
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      03-06-2011, 08:20 PM   #10
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Quote:
Originally Posted by Brutus1 View Post
Thanks all for the info. Since I have not reset it , my current average is for all miles driven. Where is the reset button for the computer and I assume you do not mean the trip odometer reset?
iDrive ---> Vehicle Info --->Reset
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      03-06-2011, 08:28 PM   #11
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Or, in the US, hold the button on the signal indicator stalk while mpg is displayed until it resets. That resets whatever's displayed only.
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      03-06-2011, 09:02 PM   #12
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Quote:
Originally Posted by Nahoa View Post
Or, in the US, hold the button on the signal indicator stalk while mpg is displayed until it resets. That resets whatever's displayed only.
Neat! You can learn something every day on this forum. How did you find that out?

The computer button info display section (X3 Manual pp. 70) does not mention that capability. It's like they designed hidden features in the system that they don't want anyone to know about.*

I'm sure there are more and hope someone discovers them.

*I'm still looking for the one that resets the MAIN odometer (very useful at resale time)
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      03-08-2011, 06:45 AM   #13
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In addition, that number is based on a rolling 30 mile period approximately. I have had BMWs for about 6 years, and on each I have kept a spreadsheet to manually track mileage the old fashioned way and also record the computer results. They are accurate to withing about 2% - which is extremely good.

Also, the "miles until empty" number uses this same method of calculation.
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