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Thread: Instantaneous economy

  1. #11
    Senior Member Maricopaman's Avatar
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    Hah, hah, well, I don't know where I got the idea you had an "L". The highest MPG I've seen was 40.3 downhill to Casa Grande (not a hill, just slope) with the A/C off in the morning. I do know that I would get better mileage if I didn't almost always press the peddle all the way to the floor on acceleration and hold the shifts to redline (I only use auto on road trips). The road to Casa Grande (18 miles) is 55MPH, the 4 lane to Maricopa (15 miles) is 65MPH. I plead the 5th as to whether I drive the speed limit or not!
    2015 Bianco Sport

  2. #12
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    its more likely to mean infinity or more literally an undefined number as it is a number divided by zero which isn't explicitly infinity but in this case it is sorta the same. the car will go into fuel cutoff mode in decel conditions at anything over 1500 rpm. i dont have a 500 so i dont know how it behaves but if you hit the clutch preventing fuel cutoff mode at a speed high enough for idle to still work out to over 99 mpg and it swaps out the 2 lines for the number 99 then it indicates fuel cutoff mode and an undefined value. if it still reads the 2 lines then it just means it exceed 99. my dart just tops out at 99 and doesn't indicate whether it is higher nor does it indicate fuel shutoff it handles the display differently than the fiat. how it play into the average i don't know. the fiat and the dart might not even be the same. i notice a bit of smoothing in my instantaneous gauge. maybe over 3 seconds or so. but how it plays into the average gauge is not clear. after about 500 miles the rate that the current state of driving alters the average seems to level off. there must be a limit to the data points it can record and calculate with. but there can be a few methods and whether it is recording those undefined data points or recording them as a manageable defined value may have a negative effect on the readout as anything compared to infinity is less but they might use values high enough for this to matter to a negligible amount or build in some optimism to the measurements to counter the effects or have multiple layers of averaging or some clever way to do it i haven't thought of.

    either way the gauge can only be so accurate and it seems to self calibrate over time. it may be more accurate with different driving styles. another thing is the instant gauge isn't always the best way to watch your mileage. understanding the physics and the engine are also important. for one thing fuel mass burned has more to do with speed than distance or g force of acceleration. more speed is more friction to the air and more speed means more energy in the car the amount of energy it took to get that energy in the car varies very little and is based on efficiency. the goal is not to keep the engine sipping gas all the time, it's to get the energy into the car with as little fuel as possible and to also coast as much as possible and use as little speed as possible. some of these things are contradictory. i mean coasting with the engine idling give more mileage while coasting faster than slower for instance unless you want to shut the engine off in which case speed doesn't matter, and engines make more power per fuel used when they are at torque peak. but being at torque peak rpm and not using all the torque doesn't work either. you want to use all the engines torque but only the power you need which means lower rpms and higher gears, the cylinder pressure and engine torque will come naturally with that condition. but that's not always possible. that all said the fiat's engine is pretty good at keeping things efficient no matter how you drive it. the body is not that areo. my point is really that you shouldn't worry so much about avoiding a dip in the instant gauge at all because using marginal power might keep you from coasting longer. you should instead worry about speed and coast down distance. if you are going stop sign to stop sign you might only want to get up to 10mph and put the car in neutral. anticipate your stops and coast down. if you can anticipate your stops to let the car coast to 10-15 mph before you hit the brakes a short distance away you will do good with mileage, if you need to stop in a shorter distance than that leave it in a low gear and decel to a stop to activate the fuel cutoff mode.

    because the engine is more efficient under high torque conditions aggressive starts aren't all that bad especially if you short shift it, however that's efficiency but mpg is not efficiency because distance is not a factor of energy like speed and height are. this is how things get weird, slower acceleration takes more distance to get to speed. the energy is the same, the efficiency is lower, but so perhaps a little more fuel per mph is used but the distance is greater before that amount of energy enters the car and the distance is greater before you pile up all the wind resistance subtracting from your energy so the mileage may be somewhat higher depending on the car despite lower efficiency in the powertrain... as you can see its complicated on the acceleration end of things but speed and anticipation will help you manage fuel consumption in any situation. coasting is better than decel despite fuel shutoff because turning the crank slower is better than turning it faster and it will prolong the distance the car travels for it's speed. but if brakes and decel are needed then use fuel cutoff mode all you can.
    Last edited by dart1.4t; 10-18-2015 at 01:53 PM.

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  4. #13
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    My impression is that while you are coasting in neutral, you're consuming as much fuel as you would at idle. While you're coming to a stop in any gear (engine breaking) the injectors do not fire, therefore the car uses zero fuel. It is much easier to hear the injectors on my TDi than the Abarth, but I think the same applies to both diesel and gas cars. Correct me if I'm wrong.

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    If your fuel injectors didn't operate ("fire") while coming to a stop, it would be like shutting off the fuel supply and the engine wouldn't be running at all. Taking your foot off the accelerator just means the fuel delivery goes to just above idle, depending on RPM and the the air moving through the MAF sensor.

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    I'm pretty sure with in an electronic flue injected car, that is exactly what happens; fuel is cut off from the car. The engine makes no power in that circumstance and therefore uses no fuel. The only thing keeping it "running" is the inertia of the car moving forward while in gear.

  7. #16
    Senior Member Doohickie's Avatar
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    ...and I'm pretty sure you're wrong. Next time you're coasting downhill on a straightaway, shut your engine off. You'll be able to hear the difference.

  8. #17
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    Quote Originally Posted by Maricopaman View Post
    I'm not sure how their system works, but I've noticed some strange readings on my automatic as well. Many times when I come to a complete stop the instant mileage will actually go up for half a minute and no throttle down hills the mileage will drop. After refueling and resetting the average will do the same kind of weirdness but levels off as the miles increase. I know some people get crazy about this kind of thing, to me, it's just a distraction and I concentrate more on the joy of driving the car.
    well, if you focus on NOT enjoying the drive, you'll get better mileage

  9. #18
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    Combustion is still occurring so it's still eating fuel - however, it's below the threshold for the computer's ability to meter it, or the computer's ability to display it. It's just "out of range". I had a VW that didn't have a metered cutoff, before an ECU reflash it'd display "430mpg" on a long downhill. After a reflash it would display 0 any time the number was over 99. It's normal.

  10. #19
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    Quote Originally Posted by fierostetz View Post
    Combustion is still occurring so it's still eating fuel - however, it's below the threshold for the computer's ability to meter it, or the computer's ability to display it. It's just "out of range".
    Combustion requires fuel, air and spark. Take any of the three away, no combustion. As Doohickie says, it would be the same and turning off the key going downhill, which in normal operation isn't what happens when you go downhill with your foot off the accelerator. The fuel injection, based on the throttle position encoder, goes to idle fuel delivery.

    One of the other replies was correct when they said the estimated instant MPG exceeds the amount of range the display can indicate with the engine at idle, yet the wheel speed sensors indicating forward motion. Has nothing to do with the fuel injectors not-firing, because they are as long as the car is running.

  11. #20
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    Quote Originally Posted by KellyfromVA View Post
    Combustion requires fuel, air and spark. Take any of the three away, no combustion. As Doohickie says, it would be the same and turning off the key going downhill, which in normal operation isn't what happens when you go downhill with your foot off the accelerator. The fuel injection, based on the throttle position encoder, goes to idle fuel delivery.

    One of the other replies was correct when they said the estimated instant MPG exceeds the amount of range the display can indicate with the engine at idle, yet the wheel speed sensors indicating forward motion. Has nothing to do with the fuel injectors not-firing, because they are as long as the car is running.
    Must be a diesel thing for me then; when I'm engine braking in the TDI, I can actually hear the injectors kick in as the rpm needle approaches idle. Thanks for clarifying!

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