No, air flow is measured by the MAF sensor - mass air flow. Typically this is done by measuring the replacement current required to keep a wire heated, while the air flowing past removes some of the heat. There is a real sensor involved here.


And I would say that it knows how much fuel was supposed to have gone through the injectors. A minor clog or a bit of erosion would change what really went through the injector.

As far as I'm concerned, I buy gas at the pump, and consume it as I drive. Since both the computer and I use the same basis for measuring distance traveled, I figure this is not a source of the difference in our numbers. I only record how much gas I buy to the nearest 1/100th of a gallon. But I calculate my average milage over multiple tankfulls, so I could probably get the same result if I only recorded it to the 1/10th of a gallon. Single tank milage is rather inaccurate - as many have noted.

 

I "trust" my lie-o-meter. It is what it is, and if I was worried about MPG's I would take the time to ook at it more closely. I'm not sure I'd trust a 2000 model year still.

I do note the differences between empty and towing my trailer to see those differences in MPG's. The smile I get driving comes at a cost that I'm happy to pay as long as I can.

 

The PID in most scan tools calls it "Mass Fuel Desired". It is not measured but the effects are observed and then corrected via fuel trim values. If the algorithms are so accurate, then why the need for fuel trim?

The fuel station dispensing pumps are pretty darn accurate. They are certified scales of trade and heavy fines are issued for non calibrated pumps.

 

you are kind of illustrating my point. it takes a very large sample to begin to eliminate the large degree of variability inherent to a fuel dispenser.

if you are recording your data to the nearest 1/100th of a gallon, then you are recording it to the nearest 27grams, at 60*, plus whatever calibration tolerance is allowed (1% maybe? not sure).

so did the pump dispense exactly the amount of fuel displayed on the readout and the PCM fouled it up, or is the PCM getting it right and the pump is introducing error? which is more likely?

this comes down to accuracy and precision. where accuracy is the ability to get the right answer, and precision is the ability to hit the same number, regardless of accuracy. a fuel nozzle has neither, which is why you cannot obtain accurate fuel mileage numbers from fuel station numbers, especially if you are using different nozzles and different stations. take a large enough sample and you can begin to see some precision, but accuracy is still in question.

like it or not, the number displayed on your dash is far more precise and accurate than what the fuel nozzle is telling you was added to your tank. To me, the fact that the two cannot correlate just points to the inherent error associated with selling a liquid by volumetrics estimated from flow rate, especially when the pumps are only calibrated every couple years.

 

We use a 5 gallon "prover." 5 gallons is 1155 cubic inches. Pumps are condemned when they are off by 25 cubic inches (about 2%), maintained when they are off by 6 cubic inches (0.5%) and accepted when they are within 3 cubic inches (0.3%).

When was the last time your injectors were calibrated? How accurate are those measurements?

 

I asked my LOM who was going to win the election and it said "MITT". Maybe because it was calibrated in a red state (Kentucky)?

So no, I don't trust mine.

 

i think these two questions are saying the nearly the same thing.

As stated previously, the mass of fuel delivered is measured indirectly via flow rate based on known variables. knowing that fuel injector flow rate can be somewhat of a moving target, realizing that injectors can wear, clog, or otherwise degrade in performance, the PCM also trims the pulse width to achieve a specified AFR.

someone can correct me if i am wrong, but as i understand it, the fuel trim is used to update the fuel injector flow chart. So, that being the case, the injectors are calibrated continously by the PCM by updating the look up tables for injector pusle width.

knowing that an acceptable pump is calibrate to 0.3%, we know the error associated with using the number on the pump. so if we added 25.000 gallons, as indicated by the pump readout, we know we added 25.00 +/-0.075gallons. if we drove 375miles we know our mileage is between 15.045 and 14.955 MPG. assuming the pump is acceptable.

this assumes, of course, that you filled the tank to *exactly* the same level as before. *This* is where i think the major source of error is, and where the discrepency lies between the readout and hand calculations.

in all honesty, you have no idea how much fuel was used, only how much was added to the tank.

the MPG readout on my work truck is usually within .3-.5 mpg of my hand calculations. which, for the most part, is within 2% difference. depending on which pump i use, it sometimes reads high, and sometimes reads low compared to hand calcs. The PCM is calculating MPG the same every time, only the fuel dispensor has changed, hence the reason i say that the pump is the source of the error, in particular the level of "full" acheived at each fill-up.

if you really think that a fuel nozzle is more accurate than a fuel injector, then take about 15 droplets of fuel directly from the nozzle and see if it registers on your display, or even your prover canister. now take those 15 droplets of fuel and pour them directly into the throttle body of the engine. which system do you suppose would react more noticably to the unaccounted for fuel?

taking it a step further, do you think that the PCM would allow the engine to operate at a 3% tolerance? it would never meet emissions if it did.

fuel injection is a far more accurate and precise way of measuring fuel than a fuel nozzle. given the choice between the two, i'm saying the PCM is more accurate.

i think it would be fun to run exactly 5 gallons of fuel through the tank and compare the numbers. but i can't see anyone actually going to the trouble.

 

I think I just stumbled onto the set during the filming of the Big Bang Theory.

Makes me miss Calculus262 NOT!

 

My last set was rebuilt on 8/18/12. Flow tested and balanced prior to re-installation. 140cc per thousand strokes. OEM spec is plus or minus 5%. My set was calibrated for plus or minus 3%

Prior to being rebuilt...4 of the 8 injectors were flowing less than 80% of spec. One was only flowing 62%. Strange thing was the PERDEL test didn't show any imbalance between cylinders. Nozzle flow must have been adequate at idle but not under load. The engine ran fine unloaded. Still towed a fair amount but consumed a ton of fuel in doing so...that was my clue that the injectors may not be very healthy. The injector control pressure system can easily compensate for worn out injectors and the operator may never know there is any issue while driving in light to medium capacity situations.

 

I agree...and there's entirely too much Sheldon and not nearly enough Penny!

 

Guess we know the answer to that question, now.

 

for a diesel, yep. the PCM knows how much fuel the engine used +/- 3%

the pump only knows how much fuel was added to the tank +/- .3%

gasoline, for which i was speaking, has yet to have numbers verified.

How are you measuring how much fuel the engine used?

i'm telling you, believe it or not, unless you are filling the tank to the exact same level every time you fill, your MPG number is being drastically influenced by the automatic shut off level. generally speaking, i can squeeze another gallon or more into the tank from the first time it clicks off, so it is not completely full. in my case that extra gallon, 5% of the tank volume, means about 1.5 mpg difference. which number is is right? they both are, but the accuracy is only with about 5%.

this difference is why my mileage on one tank is 18 or better on one tank and 14 or so on another. some pumps are better at getting the fuel tank full.

until you can somehow guarantee that the amount of fuel added to the tank represents exactly the amount of fuel consumed, then you cannot say that that number is more accurate than the PCM which is directly calculating the amount of fuel used. even though the precision of the pump may be better, the amount of fuel added to the tank has rather poor accuracy due to the differences in the automatic shut-off's

the only way you are going to be able to do it, is to feed the engine exactly 5 gallons at a time and compare that way. or, put a sight glass on your fuel tank to visually verify that the fuel level is exactlythe same at each fill. other than that .... sorry, the computer is going to be more accurate, regardless of the precision of the pump.

 

That is quite a leap of faith in the on board computer.

So far I have purchased 19,194.82 gallons of fuel for my truck. At the time of that purchase the odometer read 298,765 miles. I hand calculate the lifetime fuel economy to be in the 15.5 mpg range ( includes any towing ).

My wife has an identical truck except it is a Lariat version. The overhead computer says 24.5 mpg. Are you suggesting that the computer is more accurate than nearly 300,000 miles of documented fuel logs?

 

I agree that for short periods of time, the computer display will show the trend of milage increasing or decreasing depending on loading, towing, heavy foot of the throttle, etc. It has better knowledge of the current fuel consumption. Also, over a short period of time, one can assume that the injectors continue to deliver the same amount of fuel per millisecond, or whatever time unit is used.

Without knowledge of EXACTLY how much fuel the injector delivers (calibrating), the mpg displayed can deviate from the true mpg, but the offset should be consistent. This is because the computer believes the injector is delivering a particular amount of fuel, but has NO WAY of knowing how much fuel is ACTUALLY delivered. This is because neither the volumetic flow rate of the fuel, nor the mass flow rate of the fuel is ever measured. It has been established that air flow is measured, DESIRED fuel calculated, and injector opening time commanded.

When the desired amount of fuel delivered and the actual amount of fuel delivered differ, how does the computer know? It doesn't. Until the difference becomes so great it impacts drivability or triggers an emission control problem.

I expect most folks would agree that tank to tank milage estimates are inconsistent, but multi-tank milage estimates are much better. Over multiple fill-ups the difference in where the nozzle cuts off the fuel becomes less significant.

The example Shake-N-Bake cited for lifetime milage is kind of an extreme example - the difference is where the nozzle cuts off the fuel is nothing compared to over 19 thousand gallons!

I use the computer as a guide for milage getting better or milage getting worse. I use my hand calcs for what my actual average milage is.

 

Forget 5 gallons. Fill the tank to the same point by removing the flapper in the fill tube, scribing a mark you can see near the top of it, and fill it to that point. That gets the repeatable fill you are seeking.