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Old 09-12-2004, 10:34 PM #1
Iowa4Runner Iowa4Runner is offline
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I finally got my MPG readout adjuster installed...

I filled the tank today and reset everything to see if this nutty idea of mine has even a chance of working. After a few miles of driving around, the MPG reading seemed to be in the neighborhood (~12% lower than normal) of what I was hoping to see. I'll keep close tabs on things for the next few tankfulls and see how it does with mixed highway/city driving and varying driving habits.

Here's a picture of the "adjuster" and a scope shot at idle that shows the change in pulse length (of fuel injector #1) that's sent to the MPG computer.
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I finally got my MPG readout adjuster installed...-mpg-readout-adjuster-jpg 

Last edited by Iowa4Runner; 09-12-2004 at 10:36 PM.
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Old 09-12-2004, 10:34 PM #2
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.

.
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I finally got my MPG readout adjuster installed...-scope-jpg 
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Old 09-12-2004, 11:20 PM #3
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Keep in mind... the MPG computer keeps a running average of the your MPG. Its calculation has no concept of "how many miles in this tank."

You (we) are comparing its readout to when we fill up the tank. When you divide your "Miles driven" by your "gallons filled", you're calculating a long term average.

Consider the following series.

1,2,3,3,3,4,3,2,1

The running average (n=1) is: 1.8

The overall average is: 2.44

It'll be interesting to see the results of what kind of adjustment you need. I wouldn't be surprised if a "constant" adjustment was still off...

Nice job, btw.
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Old 09-12-2004, 11:34 PM #4
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/me wishes he was an EE
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Old 09-13-2004, 01:33 AM #5
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Nice work. I was just wondering what was happening with this project.

Are you going to post the schematic and uCode? I'd be interested in checking it out.
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Old 09-13-2004, 01:52 AM #6
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Quote:
Originally posted by CMiYC
Keep in mind... the MPG computer keeps a running average of the your MPG. Its calculation has no concept of "how many miles in this tank."
Is this published somewhere?

Since the computer has access to the fuel pulses and speed sensors it would only need to keep a cumulative total of fuel consumption and distance traveled since it was last reset (until the counters overflow) to produce the long term average.

Can you explain how you came up with 1.8? Running averages of size (n) are usually computed by sliding a window of (n) points over your input sequence. For n=1, the running average is the same as the original sequence. Am I confused?
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Old 09-13-2004, 07:48 AM #7
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Quote:
Originally posted by CMiYC
Keep in mind... the MPG computer keeps a running average of the your MPG. Its calculation has no concept of "how many miles in this tank."

You (we) are comparing its readout to when we fill up the tank. When you divide your "Miles driven" by your "gallons filled", you're calculating a long term average.

Consider the following series.

1,2,3,3,3,4,3,2,1

The running average (n=1) is: 1.8

The overall average is: 2.44
I'd agree that the MPG algorithm has no concept of "what's in the tank", it doesn't need to know that. It's different than the range calculation.

If you reset the MPG computer each time you fill the tank, you can check it's accuracy against miles/gallons because both averages use the same miles traveled and gallons used.

By the way, how did you get 1.8 with n=1 in your example above?
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Old 09-13-2004, 07:54 AM #8
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Quote:
Originally posted by rando
Nice work. I was just wondering what was happening with this project.

Are you going to post the schematic and uCode? I'd be interested in checking it out.
Thanks, I'll post everything later today when I get home from work. It'll be a little embarrassing though, my "algorithm" is caveman simple.

Later....
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Old 09-13-2004, 10:41 AM #9
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Quote:
Originally posted by Iowa4Runner
Thanks, I'll post everything later today when I get home from work. It'll be a little embarrassing though, my "algorithm" is caveman simple.

Later....
Cool thanks! Don't worry about the algorithm. Based on the description of what you originally planned to do, I expected it to be simple. I presume you just take readings from a counter in your MCU to determine the pulse length. You then divide the count by 1.12 (or some integer math approximation) and output your fixed pulse while counting down.

Simple things are good. Simple things that work are even better!

Also, did you find a source of harness pins? Cut wires? Disassembled the trip computer and did some soldering?

Last edited by rando; 09-13-2004 at 10:50 AM.
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Old 09-13-2004, 01:15 PM #10
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Nice work. I'll be interested to see how this affects the readout over a full tank.
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Old 09-13-2004, 02:32 PM #11
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Quote:
By the way, how did you get 1.8 with n=1 in your example above? [/B]

Start with 1 and average that with the second number (2) for a running average of 1.5. Then average that (1.5) with the next number, and so on....
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Old 09-13-2004, 06:51 PM #12
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Quote:
Originally posted by YZThumper
Start with 1 and average that with the second number (2) for a running average of 1.5. Then average that (1.5) with the next number, and so on....
Ahh... I get it now.
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Old 09-13-2004, 07:20 PM #13
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Here's the schematic:

Because the country I live in is overrun with lawyers and adults who never grew up, I should at least attempt to provide a warning about implementing this device. I make no guarantees of any kind regarding this device. Installation of this device requires modification of the vehicle's wiring which may void all warranties and create hazardous situations. Such hazards include, but are not limited to, significant vehicle damage, personal injury, and death.
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I finally got my MPG readout adjuster installed...-schematic3-jpg 

Last edited by Iowa4Runner; 09-18-2004 at 08:30 AM.
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Old 09-13-2004, 07:21 PM #14
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Here's the microcontroller code:
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File Type: txt code.txt (1.3 KB, 450 views)
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Old 09-13-2004, 07:41 PM #15
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Quote:
Originally posted by rando
I presume you just take readings from a counter in your MCU to determine the pulse length. You then divide the count by 1.12 (or some integer math approximation) and output your fixed pulse while counting down.

Simple things are good. Simple things that work are even better!

Also, did you find a source of harness pins? Cut wires? Disassembled the trip computer and did some soldering?
The algorithm increments a 16 bit counter and turns on the output to the MPG computer when the injector is open. Once the injector closes, the counter decrements at an adjustable rate of 8 times the count up rate. Once the counter hits zero it turns off the pulse to the MPG computer. All it does is make the fuel injector pulse to appear ~12% longer than it really is.

As for the wiring... I'm ashamed to say I did cut one wire. The dashed line in the schematic shows the original path of the injector signal before I got my hands on it. I can restore the wiring to near mint condition with minimal effort if needed.
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