Fuel Used

My VMSpc displays the total miles and fuel used on my coach. I was wondering how the Cummins ECM monitors/logs how much fuel is used and if it's accurate. I calculated by average MPG for the life of the coach and if it's accurate, my Cummins ISC has gotten almost 10MPG over it's life of 6 yrs. and 27K miles.

From personal experience

From personal experience Cummins is overly optimistic on their fuel economy. I had to tweak my fuel rate and finally got a fairly accurate reading, not withstanding generator or hydro hot use. Use the parameter editor that comes with the VMSpc program. My ISC 350 was initially indicating I was getting over 9 MPG but actually it was closer to 7 MPG. The only problem with doing this is you may get a little depressed when you see your true mileage.

Don't Believe the ECM Can Do This

I am on the learning curve of VMSpc and have been waiting for this question to be answered. From all I have learned, the ECM does not compute either current or past MPG. VMSpc calculates Rolling, Instantaneous, and "since last fill-up" MPG. Now I will see if I am wrong.

Jim Walker
Northern Virginia

Re:Don't Believe the ECM Can Do This

Well, like most things, yes and no....

The ECM does keep Current MPG, it is called "Instantaneous MPG" on PID 184. This is Highly Variable, so we Average it, and give you a "Virtual PID" called "Rolling MPG" on PID 9. The "Rolling MPG" Averaging "window" is controllable on the "Engine Correction" Dialog Box on the "Advanced Menu".

The ECM does NOT keep Past MPG, but it keeps information which the Past MPG can be derived from..

Your Engine keeps track of Total Hours Run, Idle Hours Run, Total Fuel Gallons, Idle Fuel Gallons, and Total Miles. (Please note that on the Cummins Engines that ALL these values can be set back to 0 (AKA ZERO), on an Engine Software "flash")

We take the Total Fuel Gallons, Subtract the Idle Fuel Gallons (which give you the Fuel Gallons consumed while Driving), and Divide this into the Total Miles, giving the Net MPG (for the Life Time of the Engine, or since it was reset to Zero during a Engine Software "flash"). IF the mileage was set up correctly on the Engine (Tire Size, Differential Ratio, Transmission Gear Ratio, etc...) this will be a very accurate measure of your coach's MPG.
Please also note that the Net MPG, The Trip MPG, The Rolling MPG and The Instantaneous MPG, at a "given moment", Can All Be Different.

Mark Overholser

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Mark D. Overholser
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SilverLeaf Electronics Inc.
2472 Ferry Street SW
Albany, OR 97322

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Still a Question

I find Total Fuel and Total Miles, but don't find Net MPG on VMSpc. I can calculate average MPG from Total Miles and Total Fuel but I can't find how VMSpc does this or how it subtracts idle gallons.

Jim Walker
Northern Virginia

It isn't there.

We have never implemented a Net MPG on the VMSpc. Frankly, I don't think anyone ever asked. Instead, with VMSpc you can define an odometer that shows the MPG since "day one", but it doesn't subtract the idle fuel.

Personally, I've always felt that subtracting the idle fuel is cheating. Is the fuel you burn at the red light free? We put a Net MPG in the in-dash VMS models, but it's buried in a subsidiary screen, and I rarely mention it in our seminars.

"Interval" MPG

Now that I read how "rolling" MPG is derived (from the ECM's MPG's), it's a little different than what I expected.

I thought it would represent the actual MPG over the past "N" seconds (as represented by the rolling MPG buffer size). This can be misleading. To take a simple example: if you travel 50 miles and use 10 gallons (up a grade maybe) [5 mpg] then go back the 50 miles and use 5 gallons [10 mpg] your real fuel mileage for the 100 mile round trip is not the average of 10 and 5. (10+5/2) results in 7.5. Rather the average mpg is: (100miles/15gal) = 6.67 mpg. When grades, or other conditions, cause greater mpg variation this becomes even more misleading. e.g. If you use 25 gallons on the first 50 mile leg (a steep grade perhaps) [you get 2 mpg] and only 2 gal back [25 mpg], averaging the two {25+2/2) gives 13.5. In fact you used 27 gal for the 100 mile trip so the REAL mpg is 3.7!

I suggest another method of calculation, call it "interval mpg" for lack of a better term: Record miles traveled and gallons used in each second for a user-defined interval of "N" seconds. These would be stored in an array (or rotating buffer if you like). Then just calculate "Interval" MPG as (sum of all the miles in the array)/(sum of all the gallons in the array), giving the real MPG for that N second period. Each time you re-calculate, throw away the data from oldest one-second interval and substitute the data from newest second. If every second is too often you could use, say, five, ten or whatever seconds for each calculation.

"Interval" MPG

This question has been answered in the Rolling MPG Buffer Size topic recently. The buffer gives approximately 1/10 second for each buffer unit so you do have an interval MPG already.

Jim Walker
Northern Virginia

"Interval" MPG

Let me clarify. I had thought that "rolling" MPG was calculated as (sum of all the _Miles_ data in a buffer)/(sum of all the _Gallons_ in a buffer).

A posting earlier in this thread lead me to think that instead, "rolling" MPG is calculated as the average of all the MPG values in a buffer. If the calculation is, in fact, done by the latter method, the resulting number will be misleading. As I tried to illustrate with the example, such a calculation does not accurately represent true fuel economy and will yield a higher than actual MPG value.

.. rp

"Interval" MPG

I didn't read far enough to see your example. The kind of average you illustrate would give a false MPG for a time period. What must happen for Rolling MPG is the miles for the time interval are divided by gallons for the time interval. I can't testify that VMSpc does this but it would be the correct way.

Jim Walker
Northern Virginia

There is only one man who can settle this issue . . .

. . .me.

And I'm profoundly disappointed that anyone on this board would entertain for even a moment the thought that I, the author of VMSpc, would fail to properly average the fuel economy over an interval. Of course VMSpc sums the fuel and distance over the interval and reports the ratio. Thus it really is Miles/Gallon.

(It does filter out low-speed driving, which brings it "up to speed" quicker when you hop onto the freeway.)

But I'm amused and pleased by the fact that several people even understood the problem. We supply special test equipment to Kenworth and Peterbilt, and I have had to explain to more than a few engineers about how to make these calcuations. I would have thought that an Engineering Degree would require at least one class that mentioned Dimensional Analysis - I remember learning it in high school Chemistry. But apparently that is not the case. So the next time I get a call from those guys, I'll forward them all to you.

MPG Calculation

Hi Martin,

No offense intended. Like I said, I _expected_ that Rolling MPG would be calculated as (Sum of Miles)/(Sum of Gallons). Obviously this is the correct way and you, Jim and I agree. But somewhere earlier in this thread, someone said "so we Average it, and give you a "Virtual PID" called "Rolling MPG" on PID 9..."

I should have realized that you wouldn't think of doing the calculation (by averaging the mpg's) in the incorrect way.

.. rp

PS: When I taught Physics, I was surprised to find many of my students didn't understand the concept either.