How do cars measure fuel level accurately when the fluid is constantly sloshing around?
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Even if there is sloshing as long as the average is correct you can filter the output. You can add analog or digital filters that limit the rate the level can change so what is shown is an average of recent measurements. It would be possible with a mechanical system too.
I would disagree taht the measurement is accurate, if you introduce a constant error from going up or down a hill, part at an incline or even accelerating or breaking there is some change. It is most of the time hard to spot because the gauge in the car does not have a fine graduation, the time you notice the problem is if just reach the low fuel light level then it will turn on and off when you drive. In a modern car where everything is computer controlled, you can hide that blinking by adding some hysteresis like turning on the light when the level reaches 10% and keeping it on until it reaches 15%.
I am also skeptical if the fault is linear or not. Tanks are often not completely symmetrical so before there was a computer in between I suspect there was not the same amount of fuel change for the same amount of gauge movement along the whole range. I suspect that is why gauges often just have quarter tank steps.
Cars do not use the fuel gauge to calculate fuel usage, they use the pumped volume which is a lot more accurate.
Modern fuel tanks have a partition that envelopes the float to minimize sloshing (think of a harbor where a jetty comes out to minimize ocean waves). Floats also have a delay so they are not constantly changing readings when moving back and forth
In my 1971 classic… by using a thermal gauge that has a slow response. Hell… you could run out of gas 7 hours before that sucker showed “Empty”.
The gauge in your dashboard responds to change very slowly. So it is too slow to show the sloshing. This is on purpose.
If you leave the power on while filling the tank, it will not show full for a few minutes or more after you are done filling it.
If you turn the ignition off for a few seconds(or more) the gauge will reset, and show the current level – full instantly.
It’s simple, they don’t accurately measure it, at least not on semi old cars. Notice how whenever you’re driving downhill/uphill the gas tank fullness changes.
Some of these posts are correct.
Older cars you *could* watch the fuel level change as you accelerated or braked. Newer cars would have a delayed reaction to the output from the float to minimize this, and baffled tanks to cut down on sloshing.
However – cars in the last few years have started to not actually measure the level in the tank constantly. Instead they monitor driving behavior and guesstimate the amount left based on MPG usage. So it’s not a direct tank-to-gauge display of fuel remaining, instead it’s the computer telling you how much it thinks is left.
Baffles. The fuel tanks is generally not just a big open void. It has interior walls and contours to subdivide the tank. That contains the sloshing.
If you have a large bucket and shake it, the water level will slosh quite significantly compared to if you had a 1cm test tube. For the same tipping movements the sloshing is much less.
Put that test tube inside your fuel tank along with your measuring device, when the bulk of the fuel is sloshing a lot, the tube where the measurement is taking is sloshing only a tiny amount
You think it’s tricky in cars? Motorbikes have fuel gauges now too.
Essentially [in cars and bikes] they make the gauges slow to react to change so the sloshing averages out to roughly the right amount. This can be seen rather well on a motorbike as the fuel level displayed when you first turn the bike on will be lower [or higher, but usually lower] as the bike has been on the side stand and as you ride off the gauge will catch up to the higher level.
My bike range increases by about 30 miles according to the computer when I get on it.
With the power of averaging.
There are ways to make any indicator to work at a slower speed. You can do it electronically or electrically or mechanically. But the logic is to have the indicator “sleeping” somewhere between the various different high and low peak readings.
An incredibly simple mechanical way is to surround a float sensor with baffle plates, so it seats in a dedicated undisturbed chamber barely connected with the rest of the tank.
For electric indicators, if you can get a dc voltage out of the sensor, you put a big capacitor on the line to the indicator, that will absorb all the ups and downs. But there are other methods.
Electronically, you just program the indicator.
This said, no car indicator is really accurate. If you park your car on a appreciable slope, you can see the indication being affected. The system is just calibrated for your normal horizontal position. And in average, your car is horizontal most of the time.
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