# The Naked Scientists Forum

### Author Topic: How does a car measure how much petrol it has left in the tank?  (Read 9370 times)

#### Chemistry4me

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##### How does a car measure how much petrol it has left in the tank?
« on: 27/01/2012 12:05:00 »
When I am nearly out of petrol (gas) I often see the yellow petrol tank symbol telling me to fill up the car, but I have noticed that when I park on a gradient for a while, the little symbol is gone but after a bit of driving it comes up again. This made me curious as to how the car detects the amount of petrol, and why this is affected by parking on a slope.

#### Don_1

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##### Re: How does a car measure how much petrol it has left in the tank?
« Reply #1 on: 27/01/2012 14:28:44 »
I think it works on a float system summat to this effect:

A slope can effect the float depending on its position and how much fuel is in the tank.
« Last Edit: 27/01/2012 14:31:23 by Don_1 »

#### Geezer

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##### Re: How does a car measure how much petrol it has left in the tank?
« Reply #2 on: 27/01/2012 18:27:05 »
My first car didn't have a fuel gage. The filler cap was big enough to let you put your hand in and "dip" it with your finger.

#### SeanB

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##### Re: How does a car measure how much petrol it has left in the tank?
« Reply #3 on: 27/01/2012 18:52:30 »
Low level is detected nowadays by measuring the level via a float. Older vehicles had a physical switch that was operated when the float dropped below a level, and when driving near the switching point you would see the light flash as you turned corners or braked. Newer vehicles use the dashboard computer to determine the low level point, and are programmed to operate the light only after the float has been below the level for around 5 minutes or so continuously, or if the vehicle is turned on with the level below the mark. If you park not level, then the float can be either higher or lower, depending on the float positioning in the tank, and the angle of the vehicle. Thus you can have  the light off when you start and come on 5 minutes later when you are driving level, or the opposite happen if parked the other way.

#### neilep

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##### Re: How does a car measure how much petrol it has left in the tank?
« Reply #4 on: 27/01/2012 19:09:49 »
My first car didn't have a fuel gage. The filler cap was big enough to let you put your hand in and "dip" it with your finger.

Surely you just opened the grate and viewed how much coal was left !!

#### RD

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##### Re: How does a car measure how much petrol it has left in the tank?
« Reply #5 on: 27/01/2012 20:25:32 »
I think it works on a float system summat to this effect:

A series of (reed) switches are more reliable than a potentiometer (variable resistor)

http://www.thenakedscientists.com/forum/index.php?topic=40019.msg360683#msg360683
« Last Edit: 27/01/2012 20:27:06 by RD »

#### Geezer

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##### Re: How does a car measure how much petrol it has left in the tank?
« Reply #6 on: 27/01/2012 21:51:22 »

#### syhprum

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##### Re: How does a car measure how much petrol it has left in the tank?
« Reply #7 on: 27/01/2012 22:38:21 »
I had an early Volkswagen with no fuel gauge only a reserve tap, that worked fine until you got in the car and found it was already switched to reserve then you had no idea how much you had in reserve and had to carry some fuel in a seperate can unless you did not make it to the "gas station".

#### Chemistry4me

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##### Re: How does a car measure how much petrol it has left in the tank?
« Reply #8 on: 27/01/2012 23:24:44 »
Awesome, thanks for the replies.

#### Geezer

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##### Re: How does a car measure how much petrol it has left in the tank?
« Reply #9 on: 28/01/2012 04:22:23 »
I had an early Volkswagen with no fuel gauge only a reserve tap, that worked fine until you got in the car and found it was already switched to reserve then you had no idea how much you had in reserve and had to carry some fuel in a seperate can unless you did not make it to the "gas station".

Yup! Mine was a '55 VW. If you recall, the reserve switch had three positions. Main, reserve, and off!

The switch was conveniently positioned to allow you to operate it with your foot. Not that I ever did it myself you understand, but some rotters took advantage of this arrangement to "run out of petrol" at very opportune times.

#### The Naked Scientists Forum

##### Re: How does a car measure how much petrol it has left in the tank?
« Reply #9 on: 28/01/2012 04:22:23 »

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