Ask HN: Vague fuel gauge in cars

#1

Post by rayhano » Thu, Jan 05, 2012, 12:03 AM UTC

Why is it that we have this instrument cluster with so much detail, yet the key ingredient that makes everything work (and for which we pay by the litre/gallon) is measured with a vague needle or set of bars?

We fill a specific volume of litres/gallons, as represented by the fuel pump, but have nothing in the car to verify how much fuel actually went in. Is there a difficulty with including a sensor in the fuel tank to accurately display this information? Or do people not care out of habit?

Re: Ask HN: Vague fuel gauge in cars

#2

Post by jgeorge » Thu, Jan 05, 2012, 12:24 AM UTC

The specific amount of fuel in the tank is a number that I would think most people wouldn't really care about.

Some cars will give you calculated "miles/km till empty" based on average mpg calculations, which is arguably at least a semi-useful metric. My car gives me a vague gauge until I get close to zero, and then it gives me a marginally more accurate countdown from 1.0 gal to zero.

I think the reason that most fuel gauges are so vague is that fuel levels rise and fall pretty erratically, which would make a very specific gauge give you bad information at least as often as it would give you good. Fuel levels are usually measured with some type of float, and depending on if you were travelling or parked on an incline the float could register a fairly large swing.

In an old Honda I drive once in a while I could see the fuel gauge swing more than 1/8 of a tank depending on what direction I parked it on my sloping driveway (nose down or nose up).

Re: Ask HN: Vague fuel gauge in cars

#3

Post by pwg » Thu, Jan 05, 2012, 12:57 AM UTC

I suspect the true reason is expense based, with a bit of old habit thrown in for good measure. Most tanks are oddly shaped (to fit the confines of the space into which they fit) which means that they empty non-linearly. Most fuel sensors are a float on a pivot arm, which means the sensor also operates non-linearly. While it is quite technically feasible to compensate for both non-linear functions and produce a gauge that reads to the exact tenth, or hundredth, of a gallon/liter remaining in the tank, doing so would add extra expense. And while the expense per car might be small (a few dollars more per car), over the course of a years production run, a few dollars per car times several hundred thousand cars, makes the sum total a very large amount.

Plus, the vast majority of drivers very likely only really care about "when do I need to add gas" and we have a simple gauge marked in quarter increments, or a set of 6 or 8 bars that slowly disappear.

Re: Ask HN: Vague fuel gauge in cars

#4

Post by shadowzero313 » Thu, Jan 05, 2012, 3:02 AM UTC

Floats are cheap, simple, reliable and reasonably accurate. It's also adaptable to different models of gas tanks and cars with out a lot of re-engineering effort. Using flow meters would take at least 3 sensors (new gas, out to engine, unused in from engine), using a weight measurement system would require re-engineering the entire fuel tank mounting system, as well as dealing with the drift over decades of bouncing along the road.

Re: Ask HN: Vague fuel gauge in cars

#5

Post by rayhano » Thu, Jan 05, 2012, 10:43 PM UTC

shadowzero313 wrote: post #4

Floats are cheap, simple, reliable and reasonably accurate. It's also adaptable to different models of gas tanks and cars with out a lot of re-engineering effort. Using flow meters would take at least 3 sensors (new gas, out to engine, unused in from engine), using a weight measurement system would require re-engineering the entire fuel tank mounting system, as well as dealing with the drift over decades of bouncing…

Thanks, that is really interesting. Plus I heard that fuel volume or density is affected by temperature changes between night and day.