measuring fuel efficiency

Nowadays even economy cars (e.g.: 2010 Sentra) measure gas flow and whatever may be derived from it (mpg, average mpg, "distance to empty" etc). How do they do it? My (uneducated)guess is either of the following :

  1. accurate level meter in the gas tank (accuracy may be affected by temperature and pressure - the tank is not stiff)
  2. flow meter (how accurate are those things??)
  3. count number of fuel injectors squirts (how accurate/repeatable is the squirt size?) Any comments/ideas/suggestions/referrals to articles will be appreciated P.S.: I understand that this is not automotive group, but it's the one I regularly visit. Engineers are engineers after all.
Reply to
Michael
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I would guess 3, because for the EFI to work correctly the squirt size _must_ be highly repeatable (and easily controllable), and is probably at least fairly accurate. So the only thing that stands between you and a fuel consumption estimate is software.

"Distance to empty" would require an accurate fuel sensor, but the other two could just come from the odometer and the fuel flow.

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Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

I'd be surprised if the accuracy between "height of a float gauge" and "gallons of fuel remaining" were worse than, e.g., 10% accuracy over temperature and pressure. ...although since the software already corrects for the arbitrary shape of the tank, correcting for temperature as well would be straightforwrad anyway.

I also figure that their idea of "empty" is just like the idiot light's: You still have perhaps a gallon or so left, it isn't the very last drop of gasoline going into the engine right as the "distance 'til empty" gauge hits

  1. :-)
Reply to
Joel Koltner

Distance to empty could be a sensor fusion problem-- use the level sensor to detect the fuel remaining (it only needs to be accurate, and a bit conservative, near the bottom) and use the fuel injectors and speed reasdings to estimate average consumption.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

Well, the "distance to empty" gauge on a Nissan I'm familiar with cuts out when the distance drops below 60km = 38 miles. Too close to call, or whatever.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

I doubt (3), probably (1) because...

Take as rough guesstimates: 40MPG, 40MPH, 2000RPM... 4000 "squirts" per minute, each only 0.016mL in volume. Injector isn't that precise, but is tweaked via PWM using O2 sensor to maintain stoichiometric mix.

The first "miles-to-go" stuff I worked on (probably ~40 years ago) used a calibrated tank float.

My 2¢ worth :-) ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

It uses #3 by using the injector pulse width and the rate (from the RPM). I have one of the Scan Gauge units that connects to the ODB port and that is how it works. You get instantaneous and average MPG readings.

tm

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ODB reading is probably coming from car computer. There is no accuracy to talk about unless "squirt" volume is known. The car has mpg gauge that is very (overly) sensitive to gas pedal, road incline, etc. It makes me think that they measure flow, not gas level in the tank (it is needed as "reference point"). I just learned that there are so many ways to measure the flow:

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... The turbine flow meter seems to be the most accurate. I would think that this is the way to go. BUT... How does gasoline viscosity depend on temperature? Diesel (and vodka) get really viscous when cold. Is it another engineering challenge?

Reply to
Michael

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From floscan.com, at least that many are in use:

International leaders in fuel flow technology We've been connected to some well-known engines. FloScan is the world's #1 producer of fuel flow sensor systems - over 500,000 used on cars, trucks, boats, and airplanes. (As a matter of fact, we invented them for each of these industries). Mercury Marine uses our fuel flow instruments to test engines. So does Power and Motoryacht Magazine for their boat test reports.

Over the road, in the air There are more than 100,000 FloScan sensors aboard such aircraft as Cessna, Piper, and Beechcraft, and such trucks as Kenworth, Peterbilt, Freightliner, and International. Remember in 1986, the record- breaking, non-stop, around-the-world flight of the "Voyager'" in which error-free fuel monitoring was absolutely paramount to the mission? Pilots Yeager and Rutan were guided by FloScan fuel flow meters. We'll give you the same security at sea. Whether you're fishing the Canyons, the Keys, Cabo, Catalina, or the Kenai, FloScan can extend your range and give you peace of mind - with an instrument that pays for itself.

Reply to
linnix

'High end' and safety critical vehicles probably use flow meters, but normal consumer cars mostly use injector on times.

Nial.

Reply to
Nial Stewart

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From experience from both my Q45's, there seems to be a learning/self-calibration. If I reset the fuel display it seems to take maybe 20-30 minutes for it to stabilize. Which makes me suspect that raw volume and odometer readings, are stored and calculated against PWM to give you that hokey/for-show instantaneous MPG display. ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

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it takes time to stabilize because its needs a resonable amount of data to average, the instantaneous mpg is all over the place depending on wheter you accelerate hard, cruise at constant speed, go downhill in cut off etc. you wouldn't want the average to show 10mpg just because you floored it going on to the freeway, same for the miles-to-go

injector pwm should be quite accurate, the fuel pressure is regulated to manifold pressure the orifice is know and the opening/closing can/is calibrated out.

if mapped correctly the car would run in open loop just fine if it had to

-Lasse

Reply to
langwadt

  1. Fill up the gas tank at a certain pump at a certain station, just until the nozzle clicks off - don't top it up. Record the mileage from the odometer.
  2. Drive. Note driving conditions, if desired for more granularity in your mileage assessment. ;-)
  3. Fill up again at the same station and pump, again just until the nozzle clicks off. Note gallons.
  4. Subtract previous mileage (see step one) from current mileage.

  1. Do the math. ;-)

Cheers! Rich

Reply to
Rich Grise

The more interesting question is how accurate the results are with a random select of stations and pumps, since this is what the vast majority of people use...

Reply to
Joel Koltner

I think station pumps are quite accurate, at least in those states that penalize heavily for inaccurate delivery... like AZ :-) ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

Oh yeah, certainly the "gallons delievered" read-out is accurate... but if you're doing as Rich says (and this is what my parents taught me to do decades back), it relies on your filling your tank up to the same level as it previously was, and hence the question is how consistent are the auto-shut-off detection devices from pump to pump? (...this is why Rich says to use the same pump at the same station, to avoid this variation...)

Reply to
Joel Koltner

I can get average or instand MPG (Volvo V70) , the instant does go all over the place when you floor it.

Nial.

Reply to
Nial Stewart

you're doing as Rich

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This doesn't really matter as long as you finish at the same pump you started at!

Nial.

Reply to
Nial Stewart

It's not just the number of squirts but the squirt duration which is how the air/fuel ratio is controlled depending on engine speed, throttle position etc.

It should be simple enough for the ecu to maintain a running total of injector on duration -> fuel used.

Nial.

Reply to
Nial Stewart

"Nial Stewart" wrote in message news: snipped-for-privacy@mid.individual.net...

But if you finish where you started the mpg = 0/gallons :-)

Michael Kellett

Reply to
Michael Kellett

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