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They're inaccurate for a few reasons. Firstly, calculating
fuel flow is quite tricky, you could do it at the injectors
by measuring duty cycle. This leaves you dependent upon
accurate sensors for temperature & pressure in the fuel
rail. Then you need a calibration table for flow Vs duty
cycle. I wouldn't be surprised if the accuracy and
precision of each injector was out by >5%. Then you
multiply that error by any problems in temp/pressure
measurement. Then you multiply that by the number of
measurements made, which is 0.5 x number of injectors x
total number of revolutions in the journey. So for a
standard econobox car 0.5*6*50000 = 150,000 for a 20
minute drive. |
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Other options would need something like a specific fuel
flow meter in the main fuel line. These aren't simple, you
get a stated 3% error, which will degrade with time, in a
~$200 part. For a car it would need to be cheaper, and
will still be subject to temperature induced density
variations. |
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//they typically read 10% high// |
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The main reason for that, and one you can do something
about if the motivation is there, is because they usually
don't include idling. I can set my car computer to
measure the fuel economy of a 10min drive, then I idle
for 5 mins at the end and the trip fuel economy stays the
same. Sneaky. |
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A simple spreadsheet on your phone will do this much more efficiently, shirley ? |
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//calculating fuel flow is quite tricky//
The fuel has to come from the fuel tank; therefore measure
that. Measuring at the cylinders/injectors is like measuring
the position of each toe to get the distance you walk.
I would suspect, given the inaccuracy of car speedometers,
that distance travelled is the more difficult; although these
days GPS could do better than measuring wheel revs (apart
from that pesky built-in error...). |
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//fuel has to come from the fuel tank; therefore measure
that.// |
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Now you're in a real mess, because most fuel systems
pump fuel to the engine where a fuel pressure regulator
will portion the incoming fuel to either the fuel rail, to
replace that the engine has burned, or return it to the
tank via the return line. They run as a sort of endless loop
with a variable fraction diverted off to the fuel rail for
injection to the cylinders. You could measure flow after
the regulator, but then you have to compensate for
pressure/density/temp issues. Those vary pretty wildly
given cold start-up/heat soak etc. |
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There are return-less systems, but they rely on computer
control*. That computer is largely concerned with
maintaining pressure and if it cares about flow, it derives
it from injector duty cycle. |
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*And are therefore not to be trusted. |
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When measuring fuel economy you don't need a moment-to-moment analysis. Start at empty. (it doesn't have to be absolutely empty) Drive the car 400 miles in city traffic. Drive until empty again. Now calculate the total miles and add up all the fuel that was put into the car. That's the mileage, to within a very small percentage. If you want to adjust for different driving styles change drivers every 100 miles. |
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Or just get the actual numbers from 100 people driving that model over a thousand miles each, drop off the 50 least likely to be reporting actual mileage, drop off 10 outliers, and perform a regression analysis on the remaining 40. |
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Start at full - brim the tank. That is an easy, highly repeatable datum. |
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Drive the car, using whatever distance-measuring system you favour (but using multiple sources, i.e. GPS, GLONASS and the odometer is best) for an arbitrary distance. |
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Brim the tank again. Calculate ratio of distance traveled to fuel consumed. Weep bitterly into an alcoholic beverage and curse the repressive regime that taxes fuel unreasonably. Plot the overthrow of the regime by violent means. Rebel. Kill them all. Leave the corpses dangling from lamp-posts as an example. Assume power and decree free fuel for everyone. |
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//curse the repressive regime that taxes fuel
unreasonably.// |
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Tell me about it. It's only just dipped below £1.60 a gallon.
They should be paying me to dispose of that ethanol-tainted
crap. |
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No integrated vehicle display seems to factor in changes in altitude, and they can have a significant effect on consumption. |
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Driving in mountainous terrain is likely to use more fuel than travelling a similar distance on level ground, so a metric which includes both dimensions would be more enlightening for the sophisticated user. |
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For substantial changes in elevation, such as from the California coast up to the Sierra Nevada, engine performance degrades noticeably, although turbocharged powerplants suffer rather less. |
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Basically if the driver's math consistently shows 10% lower
economy than the vehicle's math then the driver could
access the appropriate vehicle menu and adjust the
displayed value to reflect the actual economy, i.e. 400
miles driven / 12 gallons to refill the tank = 33.3 mpg, not
the 37 indicated. |
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The system would continue to measure fuel consumed per
duty cycle, or whatever it does, but for the average
economy readout figure it would apply the appropriate
error correction based upon the value the driver entered
at the most recent fill-up, thus the 37 in the vehicle's
mind is adjusted (down, in this example) to 33 to reflect
my math. |
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Most people simply drive until they need to refuel,
whatever their economy, and I'm no different but a
consistently true indicator helps as a vehicle health
monitor: when the economy dips for no apparent reason
there might be something that requires attention. |
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True, a consistently inaccurate monitor would serve the
same purpose regarding vehicle health but now we're
using it solely for that alternate purpose because it
doesn't work for the factory intended one. |
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If the head keeps sliding off the hammer I suppose you
could use it solely as a projectile weapon but it was really
meant for driving nails. |
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// A consistently false indicator can serve the same purpose, as long as it really is “consistent” // |
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That's partly correct. Consider two analog clocks, both set correctly at midnight; one is stopped, the other loses 10 minutes per day. |
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The stopped clock will indicate the correct time twce per day; the "losing" clock will be right only once in 144 days ... |
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[+] If the car had an accurate fuel gauge, it could
calculate the long term average consumption and
compensate automatically. With current technology, it
seems like an accurate capacitive fuel gauge shouldn't
cost more than a few dollars per unit in mass production. |
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I like the idea of using GPS to compensate for inaccuracy
in the odometer (and speedometer) caused by tire
pressure and wear. // (apart from that pesky built-in
error... ) // What built in error? If you're talking about
selective availability, that got turned off in 2000 and is
supposed to stay off. GPS accuracy is claims to be better
than 16 feet. That's plenty good enough for this purpose,
and most of the error will average out anyway. |
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//recalibrating the dashboard indicator after every fill up would be a waste of time// Could be totally automated. Fuel pump wirelessly transmit volume of fuel delivered, to car system. Car system calculates difference between dead-reckoning and measured values and adjusts conversion scale appropriately, Display then shows calibrated value. |
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Also if all displays would just show error estimates as well e.g. "46 +/- 3 mpg" we could all relax a lot more. |
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Don't forget relativistic effects - as you go faster,
your car, and your car's fuel, will get heavier,
which may affect your fuel consumption calculations. |
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The manufacturer's calculation assumes consistent fuel quality, which is an unrealistic assumption at the stations* near Camp Teacup. |
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Knocking, pinging, and 'bucking stallion syndrome' tell us which stations have water in the underground fuel tanks (by accident or design is unclear; research is ongoing), which have added corn (ethanol, whether advertised or not), and which have inadvertently filled the storage tanks with diesel (not really, but attendants lurking around just out of visual range make us wonder). |
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*We have accounted for condensation in the car's line or tank due to seasonal temperature changes, since the fuel never sits long enough between the 40C of summer and the -40C of winter. |
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[kdf], I see this monitor thingy as something designed to
assist me but which fails consistently and in the same
direction and should be adjusted to serve me better.
Idling, route, terrain, fuel quality, use of a/c, driving
style, tire size, all the various things that define my per-
tank driving situation could be summed up in one
correction such that the displayed value is closer to the
truth. Your driving situation in your vehicle might be
different and so your user-inputed correction would bring
your readout closer to your math. |
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I have a .22 rifle that used to print high and left. Perhaps
another user would have found it to be dead nuts on but
for me it was high and left so I did a sight adjustment and
now I'm in the ten-ring. |
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//Fuel pump wirelessly transmit volume of fuel delivered,
to car system. Car system calculates difference between
dead-reckoning and measured values and adjusts
conversion scale appropriately, Display then shows
calibrated value.// |
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That's reasonable. The fuel pumps are supposed to be
calibrated and temperature compensated. |
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//I have a .22 rifle that used to print high and left.
Perhaps another user would have found it to be dead nuts
on but for me it was high and left so I did a sight
adjustment and now I'm in the ten-ring. |
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That's fair enough. They're clearly overrating the figures
to make people feel better about their car/driving rather
than giving the best possible information. They'd have to
be forced to change that. |
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A solution maybe to drive at a consistent speed on a
totally level road in Nevada or somewhere. Film several
hours of it as data collection. Then, brim the tank and
state calmly and confidently "At the indicated fuel
economy, I should easily arrive at the next fuel stop with
a 7% reserve" Then drive off at the same consistent speed
and indicated economy filming all the way. Then, run out
of fuel 3% short of your target and over the next day or
two, die of dehydration. That should leave you with a
pretty winnable argument in court. |
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Yes, your legatees will undoubtedly welcome your selfless act when the class action pays out. |
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