2008 Gas Mileage: MFD vs. Purchased

Thanks for posting. For comparison and validation, I have attached my spreadsheet where I have documented the same thing since 10/07. Mine has a little less detail and a slightly different approach to the cumulative results: It adjusts each fillup based on miles driven for that tank and then averages the adjusted MPG figures.

Our numbers are quite close: I show a 2.32% difference since 10/07. For a calendar year comparison I temporarily deleted all the '07 fillups, which gave me 2.71%.

To help validate my formulas, I plugged your MPG and tank numbers into my spreadsheet, with a result of 2.596% -- close enough for government work.

Maybe others who are as anal about tracking this as we are () might consider posting their own results here. This would be a good thread to which to refer newbies to reassure them the MFD is quite close with its calculation and not to panic over substantial single-tank differences.

 

Interesting, I had a play with the chart maker, that was fun. You can see how winter affects mileage

 

My spreadsheet is not as detailed as yours, but here it is. I keep mine in "car years" instead of calendar years. The last column is for how many miles are left in a given year before the insurance goes up.

 

I just did some back of the envelope calculations about the difference between the actual lifetime mpg and the values shown on the mfd. Much to my amazement, despite the wild differences shown at each individual fill-up, the variance for the almost two years that I have owned the car is less than one percent. Actually .9780251%. I would have never thunk it.

 

i use Trip A for my mileage calculations so i calculate in 10ths and use the Trip A number for both calculations. my variance

2006: 1.92%
2007: 2.27%
2008: 2.48%

overall 2.31%

i am still using OEM tires... i wonder if that makes a difference? im sure it does since with less tread, i am going thru slightly more revolutions per mile. its been a while since i calibrated my odometer mostly because the first time i did it, i could find no real variation using a combination of data from two GPS systems and google maps (which measures distance in feet)

the only thing is that since i am using the same figures for mileage (trip A) the change in tire size should not make a difference right?

to be honest with ya, i attribute all the variance to pump inaccuracy

 

And it's all measured by duration of the injectors which have a known flow rate and fixed pressure across them. There is no flow measurement.

 

Pat, I don't understand what you're saying above. Can you expand your
comment?

FWIW, ScanGauge users can get access to injector timing data. Codes are:

TXD 07E021F3
RXF 0461 85F3 0000
RXD 3808
MTH 000A 0008 0000
NAM inj

Info on how to use the outputs in lieu of say RPMs are in the second half of this:

CAN splitter

 

Fuel use is calculated by the injector pulse duration. Fuel injectors, by their very purpose, need to precisely dispense fuel into the engine. This is done by using a known flow rate. Each injector is opened for a precisely timed pulse. The MFD keeps track of fuel consumption by totaling up these injector pulses and multiplying them against the know flow rate of the injectors. It's like using an eye dropper to measure the volume of a swimming pool. You suck the water out, count how many drops it takes, and multiply it times the volume of each drop.

Tom

 

qbee42,

Thank you, sir.

And I take it that a component of a fixed "flow rate" is a constant or fixed
pressure during the oh-so brief interval -- ~ 2-7 what? micro seconds --
that the injectors are open.

 

Correct. As far as I know, injectors are always driven from a fixed rail pressure. I suppose there may be an exception to this, but I haven't seen it.

Our marine diesel engine uses a single injector driven from a jerk pump. In this case the pump develops a variable amount of pressure, so the injector system uses a relief valve to spill off the extra pressure back to the fuel system. As long as the pump develops more pressure than the relief setpoint, the injector sees the same pressure through the duration of the pump stroke. The jerk pump mechanically varies the length of the stroke to very the length of the injector pulse. On the Prius it is a bit more sophisticated, but the principle is the same.

Tom

 

aforkosh,

I believe you are introducing a systematic calculation error/bias by adjusting the MFD with your 0.5 adder. The MFD isn't truncating its internal calcs as best I can tell. (The value will move up and down without the mileage changing with the ICE off when parking...had it move from 44.9 to 45.0 the other night when I lifted off the brake and put it in park.) The MFD value is the average since whenever you reset it.

At any rate, my MFD mileage average with stock tires at 42/40 is reading 0.17% low on average compared to calculation. I can more than account for this by variations in tank fill since this is only 4,000 miles--my last fill was clearly "fuller" than previous fills, moving the calculated average error considerably and switching the sign of the error.

 

Thanks Tom, saved me some typing.
The fuel injection pump in your diesel uses spill port timing. The actual stroke is always the same but the spill port rotates to align with a helix cut in the pump piston element to vary the effective pump stroke. As soon as the spill port closes the fuel is injected through the injector, opening it against a strong spring when the spill port is again open fuel circulates back around to the inlet side and the pressure to open the injector is lost. This is what I am familliar with but there may be otehr systems.

I'm not sure how Prius does it but in older systems a pressure relief valve would maintain a constant pressure across the injector by applying manifold vacuum on a diaphragm to assist the fuel pressure on the other side of the diaphragm overcome a spring opening a a return valve which would be set to about 30PSI from memory. The fuel pump would move fuel through the injector rail at about 2 litres per minute but the engine would never use this much fuel and most of the fuel would be returned to the tank to go around again. Unfortunately this resulted in the fuel heating up increasing evaporation. Prius doesn't circulate the fuel through a fuel rail to reduce HC emissions from the fuel system. I'm not sure how pressure is controlled or how manifold absolute pressure is sensed.

 

While I don't doubt that it is truncating reported miles (I see that on the 2008 as well) it is almost certainly not calculating instantaneous/MFD mileage the same way...unless it reads 0.0 mpg until you reach 1 mile driven.

Since it automatically resets for you, the interval is the same as what you recorded from the odometer. (So you should calculate the MFD's gallons used from the odometer and the MFD indicated.) There is no need to use the MFD's trip value to adjust the MFD reported mileage. Doing so is producing a bias nearly as large as the overall error I have observed so far.

 

Thanks for posting the spreadsheet.

Most interesting to me was the maximum MFD to Calculated MPG difference (9.8%). While your average difference is a bit over 2%, that a single tank calculation can be so far off is comforting.

I only have one tankful to compare and it came out 9% different, so I was upset that my car's MFD was so far off, but seeing that you also experienced one fill with a major difference, I can still have hope that my Prius will normalize with subsequent fills.

Question about the difference formula: you use (2*diff)/sum when neither measurement is known to be correct? This makes the difference related to the mean rather than either number. Seems like a weird value. (I got an A in statistics 40 years ago in college, but I admit I understood very little.)

Several folks have made claims that the MFD fuel measurement is accurate so I was using "(CalculatedMPG-MFDMPG)/MFDMPG" which is "calculated value related to 'measured' value".

Tovli
(WB0ZJJ)

 

If you look at mine you'll see a tank off by as much as 15%.

 

Your MFD average is only 1.5mpg more than your total miles over total gas 67.6 vs 66.1 or 2.3% high which is what all the "across a year of fills" are coming out to.

It seems like it is useless to calculate the per tank mpg. Seems like it is better to de-rate the MFD reading by 2% for a guess on any one tank full, and only calculate the actual once a year.

 

I too have been charting my MPG against the MFD. But I have this very basic question: what measure of gas used vs. miles driven is in your calculations? That is to say, the gas amount at the pump for the miles yet to be driven? How about the shrinking blatter problem?