There is another area of the car that does periodic, not continuous measurements? Do you have OEM navigation?

Most people are surprised to learn that the OEM GPS Navigation does NOT make continuous GPS measurements. It reads GPS signals quite infrequently.
Every few minutes, not seconds in fact. Only on an "as needed" basis. Once the car knows where it is in LAT/LON, it uses speed, odometry, and gyroscopic motion to move your position on the map. This way, you get your position in tunnels, and turn-by-turn directions in "urban canyons" between tall buildings, and under forests where there is no view of the sky to get GPS signals, which are quite faint... Orders of magnitude weaker than cell phone signals. The Ford Navi only uses GPS sparingly to verify its position. It's also much better position wise than any other unit I've seen, and never "drops out"... even though the graphics aren't particularly good.

I've used hand-held GPS for hiking for example, and an unobstructed view of the sky is paramount. I've never used "portable" in-car GPS units that stick to your dash with suction cups and use AA batteries ( IE no interface to the car ) and I have to wonder how well they work under forests and in downtown Chicago.

I get the distict feeling many methods go into the MTE.

It may use ave. MPG to estimate, instant fuel consumption to deduct MTE, and tank level every 10%? to verify.

 

We should consider Occam's Razor: the simplest answer is likely the correct one.

The answers proposed that contemplate some esoteric correction algorithm, for example, are most likely the musings of a person that puts ultimate faith in the computer readouts on his scangauge (& squared if there are two scangauges on the dash).

Ford would likely keep such things simple. My observations over the summer towing I recently did, confirm a rather simple algorithm: a running average (over a long interval) of fuel mileage combined with a measure of fuel remaining in the tank to yield miles to empty.

I find the gas tank gauge to be repeatable and therefore likely accurate. Its output is converted to a digital number (0-255) and used by the software. There is no fuel flow transducer, if fuel flow is used it is inferred from fuel rail pressure & temperature combined with the flow characteristic of the injectors and time open.

My position is simple: anything complex, such as a "correction" method, is bogus.

This is based on my 06 FEH and my relatively close observation of what the readout provided across a couple thousand miles of towing where the answer that I chose to rely on made a difference in how my day turned out.

GPSMAN1:
- If you're going to respond to try and hammer me save it, I won't read it.
- If you're going to try to wow others with your digital acumen, have a ball.

 

So based on the pressure/temperature and injector characteristics the computer can make an accurate flow calculation? Seems reasonable to me. Is this documented anywhere? I have always had doubts about accuracy of "flow meters" anyway, though the technology has improved greatly in the past few years. Flow, either measured or inferred, is used for instantaneous FE calculation in some fashion, so they must use some method to determine it.

Based on my observations, though, I still think the rather simple algorithm I wrote down is at least close to correct.

RayP

 

RayP- The accuracy of the flow characteristics of the fuel injectors depends on how much Ford chose to spend on them. I agree with you that they may well be not so accurate. I searched for some kind of fuel flow transducer in the 06 FEH wiring diagrams, there is none.

So the remaining method of doing a flow determination is pressure across a known orifice combined with the temperature (density) of the fluid passing thru it. Then open/closed time can yield a flow number. That's the only way Ford could determine fuel flow in the 06 FEH.

In my experience the fuel tank level can be made more accurate, given the anti-bounce algorithm, by simply making the wire windings on the rheostat inside the tank very fine. This can be done inexpensively as compared to a calibrated flow fuel injector.

So I don't know how Ford implemented their software side of this. I can only observe how the instrumentation presents the info and then infer something about how Ford did this.

All of my postings on this topic were based on what I observed last summer when I towed a trailer. This gave me a significant drop in fuel mileage and, for the first tank, I had hopes of better fuel mileage than actually transpired.
This was because the display showed an average that only slowly dropped to the real mileage I was experiencing.

The MTE appeared to mirror this. Thus the info I have posted.

 

The anti-bounce algorithm has a long filter time constant. If fuel level was used to calculate instantaneous FE it would have to be based on differences in sequential realtime measurements. These two algorithms are incompatible, so I have to conclude there is no way to accurately use fuel level for instantaneous FE. So I must agree with you, if there is no fuel flow meter in the system, it must be calculated based on other quantities such as injector flow.

RayP

 

Well, I didn't mean to say that there was a flow meter, per se, in the system, just that they measure fuel flow to determine MPG, both instant and average.

Does anyone know how many miles are averaged in determining the MTE?

 

The system has some set number of "data points" that are used for average fuel economy and mte. I have seen the number posted here, but don't remember what it is and can't seem to find it.

MM

 

I've searched the Powertrain/Emission Control book for the 06 FEH and there is just no mention of mileage or MTE calculation. So I can find no credible documentation for the methodology for the calculations so far discussed.

There is another way to calculate fuel consumption than using a fuel flow transducer or inferring fuel flow from the Pressure/Temp of the fuel rail versus the fuel injector open time. It is to use the mass airflow reading.

Since mass airflow is controlled directly by the engine control system, it is proportional to fuel flow. This system measures oxygen in the exhaust stream to keep it around zero. Then it adjusts long & short term fuel trim to control the injectors and the throttle setting.

The air flow mass would then be a direct reading of fuel flow in most cases. Since this transducer is a fine wire (heated), it will respond quickly to changes in flow. Thus, I believe, it could well be the way fuel flow is determined.

Speculation on my part, but it is another method to get to the same place without relying on the fuel injector flow characteristic to measure something accurately.

Note that in terms of controlling the engine power and emissions output the control hardware does not need a calibrated fuel injector. So long as the fuel injectors are close to each other in response the software will properly control fuel flow and emissions without needing to know the precise flow thru the FI.

But to measure fuel flow Ford would need to have used a fuel injector with a good and measured flow. These can be expensive.

 

The MPG meter has a memory of 2000 data points.
This comes directly from Ford.
This is not in any maintenance manual, since there is nothing to maintain, or troubleshoot. How often data points are recorded is a bit of a mystery.
One per minute is an estimate, but has never been proven.
Thus, at highway speeds, the memory could last 2000 miles.
In the city, with slower conditions, this memory could be filled in less than 1 tank. The older data is dropped as new data is recorded.

We all know that MPG can change dramatically within the space of 1 minute.
I suspect, but have never proven ( it's just not that important ) that the computer takes the ave. MPG over 1 minute, and records that into one register of the 2000.

Could be one reason why a ScanGauge and the FEH never exactly agree with each other. They have different methods, and "resolution" for computing MPG.

 

One thing I know for sure is, the Ford MPG / MTE adjusts based on fuel trim... % fuel injector on time based on elevation and fuel and driving conditions.

The Ford on-board system adjusts for ethanol use, for example.
A ScanGauge does NOT adjust for ethanol, thus gives falsely high MPG numbers as the injector on time increases. A ScanGauge probably infers MPG from mass air flow. One thing a SG needs to know is cylinder displacement. If you adjust this on your SG, you can adjust your MPG in either direction.

The mass of air drawn into the engine does not change much when burning ethanol. But the fuel use ( injector on time ) increases. The SG is unaware of ethanol use. The FEH takes this into account.

HTH,
-John