How do cars measure fuel level accurately when the fluid is constantly sloshing around?
In: 2855
Most sensors that display end user data are some form of average or composite. The accuracy is only enough for generalization, not sicence. This is for user ease of use and isn’t a problem in most consumer products.
Cars from the 90s were instant read. My race truck is infuriating bc it’ll get to a point in a stint where it’ll read out of gas while floored or in a tight turn, but read 3/4 tank under braking. Had a daily driver camaro from the 80s that was same way.
Low pass filter on the signal. ELY5: average the measurements over time — assuming the sloshing varies randomly it will ‘balance out’ and you’ll get a more accurate number. Unless you’re driving a rocket ship this averaging is still responsive enough, you won’t mistake how much fuel you’re using.
The most common type of fuel level sensor is the Resistive/Float Sensor. This is basically a float on a long lever. The float always sits at the fuel level, which moves a contact on the other end that’s touching a resistor. This is much like how dimmer switches work, where you change the part on a resistor that the switch contact rests. Depending on the spot the contact is touching the resistor, the resistance of the circuit changes, and the vehicle’s onboard computer can thus derrive how high or low the float sits, and thus the fuel level.
But you’re right, the fuel dows slosh around, so how does it accurately measure the fuel level? Short answer, it doesn’t. If you park on an incline, it’s possible to get a wildly different reading than what your true fuel level is. In general while some measures are taken to ensure accurate readings, ultimately the reading is never 100% accurate, and it becomes especially innacurate the lower the fuel level gets.
“Accurately” lol
In the old days they steadied the gauge by using a very slow moving needle. It would move but not quickly, it’d take half a minute for the needle to get where it was going. So as the fuel sloshed around, the needle wasn’t quick enough to catch the peaks and valleys, and would effectively just report an average.
Today’s digital gauges just replicate this behavior. Take an average over time.
Fuel tanks often have baffles to limit the sloshing, but yes they do intentionally slow down the reading so it doesn’t fluctuate on you.
A lot of people are saying baffles, and they are quite wrong, the vast majority of road cars do not have any fuel tank baffles of any kind. The signal input from the level sensor is just heavily dampened, giving the dash enough time to display a reasonably accurate average.
In over 20 years of being a mechanic I think I have only seen one fuel tank with baffles, and that was mini club racer with an aftermarket FIA tank, and it’s not baffles like you would think, more foam blocks.
The answers on here seem accurate for most fluids, but I’m assuming the engine oil works in a different way?
I’ve had the oil run super low in my car on a few occasions, and the dashboard light seems to be super sensitive. Like the first time it comes on will only be during sharp turns or hard brakes where the fluid seems to be shifted. This has happened to me in two different cars where I didn’t realize I had a slight oil leak until I started catching that light for a fraction of a second during turns and stuff. Why wouldn’t they have that light kick in before it’s reading completely empty?
I know this one!
It’s called a “low pass filter”.
There is an electronic filter that filters out high frequency signals. The more the fuel sloshes, the higher the frequency of high and low.
The goal of the low pass filter is to not allow the gauge to see the higher frequency high and low but only allow the low frequency high and low.
There is a more complex explanation however, that was the easiest that I could make it.
Fun fact, DSL filters on your home phone line work the same way.
Baffles help otherwise electronic filtering of the signal to average out the sloshing. Pretty simple
Imagine a can of soda and a hollow tube attached to it, welded together from top down. The welded area are holes even spaced apart and when you fill the can, the tube is also filled through these holes. In the tube is where the measurement is gauged and sloshing isn’t very noticeable in that tube as much as it is in the can when it’s moving around.
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