::.. How is the avg MPG L/100K calculated in the OBC?

[Qube]

Thanks to ArtemLepilov's post, I remember what I wanted to ask for some time...

How is the fuel economy calculated in the OBC? How many data points does it store, or perhaps what type of formula is used to calculate a continually running average?

[Qsilver7]

if you have the link to the instructions on how to "unlock" the OBC...the formula for gas consumption is given in the TEST for gas consumption. You can search the archives here for the instructions...or go to Bruno's web site.

[Qube]

if you have the link to the instructions on how to "unlock" the OBC...the formula for gas consumption is given in the TEST for gas consumption. You can search the archives here for the instructions...or go to Bruno's web site.

On demand consumption is test 2 I believe... or test 8. One's speed, one is economy... so are you saying the formula is in the OBC display or on a site on how to access the 'unlocked' OBC?

[Qsilver7]

here it is...if you go down to the bottom of the page...(#2) there is a footnote on how to calibrate the avg fuel consumption...from the instructions given on how to recalibrate, you can extrapolate how the OBC formulates the average: http://home.iae.nl/users/bts/obc.htm

[Qube]

here it is...if you go down to the bottom of the page...(#2) there is a footnote on how to calibrate the avg fuel consumption...from the instructions given on how to recalibrate, you can extrapolate how the OBC formulates the average: http://home.iae.nl/users/bts/obc.htm

Okay I'll bite. Huh? The wheels go round, the horn goes honk. ;)

[Qsilver7]

In basic terms...it's calculating the mileage and gallons of gas being consumed.

From the web site:
*2
The average fuel consumption value can be calibrated from the OBC. Function 20 is used for this.

* Fill the fuel tank to the maximum.
* Reset the day trip counter.
* Run the fuel tank nearly empty and fill the fuel tank again to the maximum.

This will give you the fuel consumption.

New Correction Value = Old Correction Value * OBC Fuel consumption (in MPG or l/100Km) / Actual Fuel Consumption (in MPG or l/100Km)

[Qube]

Right right... so my question, updated is...

There are two OBC counters. I reset ONE of the two at every fill. How is the 'older' one being contantly calculated (and different of course) from the newer reset value?

Does this question make sense to anyone other than me, or am I delusional? Sorry for heckling if it seems so.

[Bellicose Right Winger]

Miles per gallon is calculated by summing the total miles and dividing by the total gallons consumed since the last reset. I believe that in order to keep Consum1 separate from Consum2, the values are simply stored in separate registers.

There are probably some constraints to prevent displaying a constantly changing average. My OBC won't change the reading until the change is greater then .3 MPG. If it hasn't changed for some period of time, then it will display smaller changes.

Paul Shovestul


Right right... so my question, updated is...

There are two OBC counters. I reset ONE of the two at every fill. How is the 'older' one being contantly calculated (and different of course) from the newer reset value?

Does this question make sense to anyone other than me, or am I delusional? Sorry for heckling if it seems so.

[uscharalph]

Right right... so my question, updated is...

There are two OBC counters. I reset ONE of the two at every fill. How is the 'older' one being contantly calculated (and different of course) from the newer reset value?

Does this question make sense to anyone other than me, or am I delusional? Sorry for heckling if it seems so.
The older one is being calculated by the fuel consumed to get the miles you've travelled since you last reset it.

[uscharalph]

Miles per gallon is calculated by summing the total miles and dividing by the total gallons consumed since the last reset. I believe that in order to keep Consum1 separate from Consum2, the values are simply stored in separate registers.

There are probably some constraints to prevent displaying a constantly changing average. My OBC won't change the reading until the change is greater then .3 MPG. If it hasn't changed for some period of time, then it will display smaller changes.

Paul Shovestul
Mine jumps or declines in values of .1 or .2

[infinity5]

Back to his main question, based on how fast the display changes right after you reset the MPG, it looks like it plots a new data point every second, maybe every half second. Each time it gets a new data point, it probably adds that to the previous data points, then divides by the number of data points it's take thus far. Or it might limit it to the last 500 data points, for instance.

Is that what you were getting at?

Since it changes slower the longer you drive after it's been reset, it's obviously doing a simple mean calculation. So after 50 miles of 22mpg, suddenly getting 4 mpg wouldn't change the display unless you consistantly did that for an extended period of time, becuase it has little affect on a mean composed of thousands of similar data points.

[Qube]

Back to his main question, based on how fast the display changes right after you reset the MPG, it looks like it plots a new data point every second, maybe every half second. Each time it gets a new data point, it probably adds that to the previous data points, then divides by the number of data points it's take thus far. Or it might limit it to the last 500 data points, for instance.

Is that what you were getting at?

Since it changes slower the longer you drive after it's been reset, it's obviously doing a simple mean calculation. So after 50 miles of 22mpg, suddenly getting 4 mpg wouldn't change the display unless you consistantly did that for an extended period of time, becuase it has little affect on a mean composed of thousands of similar data points.

Now this sounds like a viable strategy it's doing. I believe the simple mean calc holds more water in this case. There is just no way it could store an infinite number of running data points. Memory is expensive... and in 1989, more so!

Does anyone have any in depth idea of how it's really done? It's one of those things that have intrigued me about this car :)

[Kalevera]

Guys, the OBC AVG MPG isn't calculated from the gas tank levels -- it measures injector on time and then considers that against the amount of time that the engine has been running (since either or both of the two counters were reset).

This is one of the reasons why adding a 3.5 bar FPR gives the illusion of having better fuel economy on the 535: more fuel is injected over less time (EDIT: rather, the same amount of time), and time is the only consideration for the OBCs calculation.

best, whit

[Reuben Lim]

Is that also inaccurate? Thought there was some sort of sensor in the gas-tank that feeds the actual amount of fuel left to the OBC. Must admit there are times I'm on the red and the 20 or so miles left to travel is a comfort.

[uscharalph]

Now this sounds like a viable strategy it's doing. I believe the simple mean calc holds more water in this case. There is just no way it could store an infinite number of running data points. Memory is expensive... and in 1989, more so!

Does anyone have any in depth idea of how it's really done? It's one of those things that have intrigued me about this car :)
Why would it have to plot data points at all? It could just keep a running total of miles traveled since it was last reset and divide it by the running total of fuel consumed.

[Kalevera]

The gauge reads from the float in the sender -- go by the gauge if the inst cluster is in good working condition (ie - capacitors aren't messed up). I'm relatively certain that the OBC interpolates the info from the dash module, but I'll have to read up on it to confirm.

I usually think about fuel consumption based on how much I know is in the tank (from the last fill-up), how many miles I've driven since then, and what I think the average MPG is based on what the economy gauge is doing and how I've been driving the car. To help "calibrate" oneself, it might be a good idea to remove the sender from the tank when the gauge is on the red.

These days, I'm not keeping less than 1/2 tank in the car, anyway -- coldness + less than half filled tank = condensation.

best, whit

[Reuben Lim]

Yup, I get antsy myself when the needles gets to 1/2, but do get these really busy spells when I can't fill up as soon as I should. Thanks for the info, cheers!

[Paul in NZ]

the data input is the average of the guzzle o meter isnt it???When you next reset your consumption on the obc the guzzle o meter and obc dispaly are the same at first,if you are cruising on the highway they will sit on the same value.......

[Bellicose Right Winger]

Recognize that no two programmers think alike and that only the OBC programmer know exactly how the MPG calc is made, but I'm certain the OBC doesn't have to store an infinite number of data points. It needs at least two registers, one to hold the sum of total miles since last reset, one to hold the sum of total gallons consumed since last reset. The MPG figure is simply created by dividing total miles by total gallons.

This is of course duplicated for the MPG2 calc.

Think of the registers as a mail box where values are stored. The registers are continuously increased as you accumulate more miles and consume more fuel.

The sample rate that's visible after a reset is just the frequency that the MPG figure is calculated.

Paul Shovestul




Now this sounds like a viable strategy it's doing. I believe the simple mean calc holds more water in this case. There is just no way it could store an infinite number of running data points. Memory is expensive... and in 1989, more so!

Does anyone have any in depth idea of how it's really done? It's one of those things that have intrigued me about this car :)

[Qube]

Very good discussion! For some reason I just can't think this one through logically... and I'm not sure why :)

[realness]

Recognize that no two programmers think alike and that only the OBC programmer know exactly how the MPG calc is made, but I'm certain the OBC doesn't have to store an infinite number of data points. It needs at least two registers, one to hold the sum of total miles since last reset, one to hold the sum of total gallons consumed since last reset. The MPG figure is simply created by dividing total miles by total gallons.

This is of course duplicated for the MPG2 calc.

Think of the registers as a mail box where values are stored. The registers are continuously increased as you accumulate more miles and consume more fuel.

The sample rate that's visible after a reset is just the frequency that the MPG figure is calculated.

Paul Shovestul



How do you reset or What do you reset to calc the AVG MPG?