MFD accuracy over long distances (many tanks)

<div class='quotetop'>QUOTE(ystasino @ Oct 16 2007, 04:13 PM) [snapback]526507[/snapback]</div>

Per Prius specialist, if MFD MPG is not manually reset at time of fillup, it will carry over the fuel milage from the previous fill(s).

In other words, reset on the MFD is required to obtain correct fuel mileage for any single fill.

 

<div class='quotetop'>QUOTE(ystasino @ Oct 16 2007, 04:13 PM) [snapback]526507[/snapback]</div>

Since April '07 22 fill-ups; nearly 10K miles driven my manually calculated MPG = 55.6 vs. the weighted average of MFD for individual tanks of 56.1 MPG. Yes the MFD is slightly higher but that is a 99.1% accuracy.

On an individual tank basis there maybe much larger differences due to when does the pump shut-off; temp; bladder etc. - that's why I don't necessarily "trust" manually calculated individual tank numbers. If I find that a manually calculated MPG is out of the norm I always check the tank before and after and take an average of 2 or 3 tanks for comparison with the MFD #.

Gabe

 

The MFD displayed mileage is calculated by totaling fuel going through the injectors, and as such, should show the same accuracy over one tank or a thousand. In other words, running the MFD totals over a large number of tanks will not increase accuracy as it does with manual calculations.

Tom

 

<div class='quotetop'>QUOTE(qbee42 @ Oct 18 2007, 02:57 PM) [snapback]527402[/snapback]</div>

I don't think this is what happens. There is no fuel flow meter in the Prius: what the MFD computer does is see how much of the tank has been consumed, and assumes this represents volume of gasoline used. It then calculates mpg according to the miles travelled (i.e. at 50% tank level according to the fuel level indicator in the tank, MFD assumes 11.9/2 = 5.95 has been used. It then looks at miles travelled and calculates mpg). This is why mpg varies with temperature (bladder volume), gas pump shutoff pressure, and so forth.

In fact, the only really accurate way of calculating mpg is through the use of a fuel flow meter as you alluded - unfortunately, these are pretty expensive (I know, I looked into it).

 

<div class='quotetop'>QUOTE(JimboJones @ Oct 18 2007, 08:33 PM) [snapback]527554[/snapback]</div>

Not sure where you got your info, but this can't possibly be true.

If I put 3 gallons in a just about empty tank and then drive 10 miles, if the system worked the way you suggest, the MFD would register 10 miles and 7 gallons. It would then indicate that I am averaging 1.4 MPG. Obviously this isn't what happens.

 

<div class='quotetop'>QUOTE(Danny Hamilton @ Oct 18 2007, 09:46 PM) [snapback]527558[/snapback]</div>

You've misread what I wrote. If you put 3 gallons in an almost empty tank, the tank level indicator will read roughly a quarter full. If you then drive 10 miles, the MFD will calculate how much fuel is left in the tank, subtract this from the amount BEFORE you started the 10 miles trip (i.e. 3 gallons), and determine mpg.

I don't know how you think the MFD will register 7 gallons used from 3 gallons fill up.

 

<div class='quotetop'>QUOTE(JimboJones @ Oct 18 2007, 09:33 PM) [snapback]527554[/snapback]</div>

The fuel flow meter, as it were, is part of the engine controller. All electronically controlled fuel injected engines have to know how much fuel to pump in based on the amount of air pulled through the mass flow meter. Basically it just counts how many times each injector squirts and how much fuel should be going through on each squirt. It's very accurate, but due to the vary large number of times the injectors fire, the small errors accumulate. Most owners find the MFD to be within 2% of mileage calculated over a large number of tanks.

Since the injectors fire at a very rapid pace, it takes a very short period of time to accumulate many samples. The percent error over a five minute period will be very close to the percent error over ten tanks.

Tom

 

<div class='quotetop'>QUOTE(qbee42 @ Oct 18 2007, 10:17 PM) [snapback]527574[/snapback]</div>

I think it's not a fuel flow meter as such, but rather a fuel metering device. The difference is that a fuel flow meter calculates the rate of flow of fuel going through the fuel line from the tank, whereas the fuel metering device injects fuel when it reaches a known volume/pressure combination. This is what electronic controllers use to apportion fuel into the cylinders.

A fuel flow meter is a passive device - it merely indicates fuel flow, and outputs this as an electronic signal. I don't think the Prius has this as an input to the MFD computer. A fuel metering device on the other hand is an active device which physically controls and diverts fuel into the injectors, and has no signal outputs (not usually anyway).

 

Fuel flow is measured by the ECU but not displayed on the MFD. If you have CAN-View or Scanguage, you can find out that info.

Here's CAN-View on the MFD:

 

<div class='quotetop'>QUOTE(ystasino @ Oct 16 2007, 04:13 PM) [snapback]526507[/snapback]</div>

I have tracked my mileage from the get go for over - recording the actual mpg and noting the calculated mpg and the number of miles driven ( to get the weighted average). It's accurate within 1 mpg over 40,000 miles.

 

<div class='quotetop'>QUOTE(pocono prius @ Oct 18 2007, 07:31 PM) [snapback]527611[/snapback]</div>

Other than losing data for the first 2-3 tanks, I've done the same for my 2005 Prius, and I'm also at just over 40k miles (42900 today, IIRC) in 2.5 years. The weighted average of MFD MPG is 47.7, while the computed average is 46.2 -- a 1.5mpg difference (so closer to 3%). More data at: http://scrye.com/~tkil/prius/gas.html

 

<div class='quotetop'>QUOTE(Presto @ Oct 18 2007, 11:27 PM) [snapback]527607[/snapback]</div>

That is interesting - looking at this http://www.autoshop101.com/forms/Hybrid12.pdf, there is no fuel flow meter as such in the engine. However, looking at the extended data list for the engine control system (p.2.17), the ECS uses the fuel pump speed as the flow indicator.

The question is whether the MFD uses the fuel flow to calculate mpg - it would only make sense, and if so, then the variances in mpg reported by people here must be due to using average mpgs rather than tank-by-tank mpgs (i.e. resetting the mpg in the MFD per tank), and the bladder volume effect shouldn't matter at all.

 

The PDF you posted looks to be just a training guide, which doesn't even look like it's made by Toyota. I'm sure the shop manual goes into far more detail than what's there.

 

OK, there's lots of conjecture in this thread. (isn't that what the interwebs is for?) but the typical car MPG display uses the duty cycle of the one of the fuel injectors to calculate fuel flow. It's a pretty accurate measurement assuming all the injectors are fairly well matched, clean and in spec.

I've owned 2 Prii (is that the plural?) The 2005 had a very optimistic MPG gauge, it usually read about 5 to 7 percent high. My 2007 is pretty accurate, reading about 1 percent high (averaging about .6 MPG high over the first 16000 miles.).

 

<div class='quotetop'>QUOTE(JimboJones @ Oct 19 2007, 09:01 AM) [snapback]527718[/snapback]</div>

I'm not sure what variances you're talking about.

I'm assuming you mean the variances people report between what the MFD shows for a tank and what they compute for the tank by dividing miles traveled on the tank by gallons pumped into the tank to refill.

If this is the variance you are talking about, then a significant part of the problem IS the bladder volume effect. However, the bladder volume isn't reducing the accuracy of the MFD, it is reducing the accuracy of the calculation made by the car owner.

Example senario:
1) Fill the tank and because of the bladder have 11.2 gallons in there
2) Drive 475 miles at 50 MPG using 9.5 gallons of gas, and leaving 1.2 gallons in the tank.
3) Refill the tank, and, because of the bladder, the pump shuts off after adding 10 gallons.
4) The tank now has 11.7 gallons and the fuel gauge registers full.
5) Car owner assumes they used 10 gallons of gas to travel 475 miles and computes 47.5 MPG
6) Car owner trusts their hand calculations more than the MFD and decides that the MFD is high by 2.5 / 47.5 = 5.26%

 

<div class='quotetop'>QUOTE(JimboJones @ Oct 18 2007, 11:06 PM) [snapback]527598[/snapback]</div>

Do you possibly think we don't know what a fuel meter is? Many of us are engineers that design this sort of equipment. I'll try restating this information one more time and see if it takes: The Prius doesn't need a fuel flow meter to calculate fuel usage because it knows how much fuel is used for each injection. As stated by an earlier poster, it's just a matter of looking at the duty cycle of each injection. It's a very accurate method of determining fuel usage, and happens instantly, which is how the MFD can display current MPG in addition to average MPG.

Tom

 

<div class='quotetop'>QUOTE(qbee42 @ Oct 19 2007, 12:17 PM) [snapback]527780[/snapback]</div>

Do you design fuel flow meters? If so, why did you state
"The fuel flow meter, as it were, is part of the engine controller. All electronically controlled fuel injected engines have to know how much fuel to pump in based on the amount of air pulled through the mass flow meter. Basically it just counts how many times each injector squirts and how much fuel should be going through on each squirt."

If you say you are an engineer who designs this sort of equipment, then you'd know that a fuel flow meter measures fuel flow RATE directly through a passage such as a pipe or tube. Counting fuel injector squirts and assuming how much fuel SHOULD be going through each squirt does not constitute a fuel flow meter. The amount of fuel in each squirt is assumed by the computer, not measured directly.

In fact, the Prius uses the fuel pump and its speed to calculate fuel flow and displays it in the extended data display.

<div class='quotetop'>QUOTE(qbee42 @ Oct 19 2007, 12:17 PM) [snapback]527780[/snapback]</div>

We just agreed then that the Prius does NOT have a fuel flow meter.

<div class='quotetop'>QUOTE(qbee42 @ Oct 19 2007, 12:17 PM) [snapback]527780[/snapback]</div>

No, the computer assumes the volume used for each squirt. The computer does not measure each squirt directly. The total fuel flow rate to all the injectors are calculated from fuel pump speed.

<div class='quotetop'>QUOTE(qbee42 @ Oct 19 2007, 12:17 PM) [snapback]527780[/snapback]</div>

That's fine, if you have verified the accuracy of that system. It's important to note the distinctions of how mpg is calculated, however.


<div class='quotetop'>QUOTE(Presto @ Oct 19 2007, 11:18 AM) [snapback]527742[/snapback]</div>

In fact, the PDF is what Toyota uses for its training manual. The guy who owns the website is a certified Toyota Master Training Instructor:

Professor Kevin Sullivan is a Toyota Certified Master Training Instructor, an A.S.E. Master Technician and A.S.E. Advanced Level Technician (L-1 Certification). He holds a Designated Subjects Credential in Automotive and Diesel Technologies and has over twelve years field experience as a Journeyman Automotive Technician.

One of the first graduates of the Automotive Technology Program at Skyline College, California, in the late 1970's. Kevin attended San Francisco State University to work on his Bachelor's Degree and Teaching Credential in Vocational Education. In 1989, he was hired to develop and coordinate the Toyota Technical Education Network (T-TEN) program at Skyline College.

From 1989 to 1999, he served as lead instructor for the Toyota T-TEN program. He also served on the Toyota Motor Sales National T-TEN Advisory Committee which represented T-TEN programs throughout the nation.

Currently, he is the Senior Automotive Electronics Instructor for the Automotive Department at Skyline College, California. He also serves as the Industry Program Coordinator, he directs the Toyota, Japanese, and Evening Automotive Programs. In addition, he is a member of the Joint Apprenticeships Committee which oversees Automotive Apprenticeships for the Counties of Alameda, Marin, San Mateo, San Francisco, and Northern Santa Clara.


<div class='quotetop'>QUOTE(narf @ Oct 19 2007, 11:26 AM) [snapback]527747[/snapback]</div>

Are you talking about the Prius specifically, or just a typical car? I'm happy to believe your statement, but note that there is still no measurement involved in it, rather an assumption of those things you mentioned (injector condition) plus assumption that the fuel is distributed evenly among injectors everytime.

 

<div class='quotetop'>QUOTE(JimboJones @ Oct 19 2007, 10:03 AM) [snapback]527801[/snapback]</div>

Jimbo, the assumptions are mainly coming from you, since an earlier post in this thread, you assumed that MPG calculation was based on how much fuel was left in the tank. You should have a look at some more threads and do a bit more research so you can understand the systems in the Prius instead of making assumptions based on what you know about other vehicles. I'm sure it's been stated before, that the fuel flow is measured at the injectors.