fuel sensor 6

[atlaskar1]

Hi all I am new in this forum.
I have a question about how it works when one is fuelling a car at the gas station.

How does the pump know that the tank is filled ?

Could somebody help me with this ?

Kindest regards,
atlaskar1

 

[TheBlacksmith]

There's an elf that live in the nozzle and when his feet get wet, he kicks a lever that trips the nozzle closed. Or maybe its backpressure as the tank reaches full and all the compressible air is forced out causing a shock wave.

Blacksmith

 

[mixiesam1]

I like the elf idea (do you have to go to an Elf petrol station for it to work?), but there are enough gremlins in cars these days without adding elfs.
The fuel tank has a dip tube which keeps a void at the top of the tank (~10% vapour space). This is important since the fuel is pumped in cold from an underground tank, and can then sit in the vehicle tank and get hot in the sun. Without the compressible expansion volume the tank could split with hydraulic pressure.
When the tank is full the fuel starts to come up the filler neck and the pump nozzle detects the back pressure and clicks off (the sensor in the nozzle is down the small pipe that you can see below the main nozzle).

 

[patprimmer]

Thanks Blacksmith

I needed a good laugh to start the day

Regards

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[atlaskar1]

Hi, again and thanks for the answer mixiesam1.
Could you perhaps point me to a picture of this ?

i am a little bit confused about how to detect backpressure...
Allow me to think loud:
The fuel comes into the tank with a certain pressure which could be detected by a simple manometer.

Are you saying that one needs to detect a decrease in pressure from the manometer ?

And if this is what you mean, do you know any "hard and fast method" to estimate this backpressure ?

Regards,
atlaskar

 

[mixiesam1]

Ok I give up, I don't have any more info on this, but somehow it detects pressure (if you pull the nozzle out the filler neck you can put more fuel in). You'll need a reply from someone who works on the pump design, I work on the car side. Sorry.

 

[patprimmer]

I don't know how they work, but it wont be pressure in an open neck. It will be some sort of chemicals sensor that measures impedance, induction or something like that.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Warpspeed]

It must work on the aura of the rising bubbles that return back up the fuel filler neck.

These travel back up that small tube creating a psychic resonance. That is the sudden shock you feel when the fuel shuts off.

 

[jons999]

The gas flowing through the nozzle passes by some venturi holes, the flow of gas creates venturi vacuum in a diaphram. About a 1/2 inch up from the end of the nozzle there is a small hole called a sensing port which is also connected by a tube to the diaphram. The sensing port bleeds off the vacuum to atmosphere while you are filling. When your tank is full fuel starts to rush up the filler tube which closes off the sensing port which allows the vacuum in build up and move the diaphram to shut off fuel flow.

Sometimes you here people complain that the pump keeps shutting off, that can be caused by two things. If the end of the pump nozzle is damaged the fuel won't flow out smoothly and may splash around in the filler neck and cover the sensing port long enough to move the diaphram. Another common cause is that the cars fuel vent tube, rollover valve, or evap cansiter is restricted which allows pressure to build up in the tank and causes fuel to rush back up the filler neck and cover the sensor port shutting off the pump.

-Jon

 

[rmw]

jons999

I am giving you a star. While I, too, as a typically curious engineer as to how things work, had always wondered what was the mechanism that shut off (or not) the pump nozzle, I found it curious that among such a group of such brains, it took this long to get to the bottom of this mystery.

Thanks for enhancing the knowledge base of this group.

rmw

 

[patprimmer]

Me to. I now no longer believe in elves nor Santa Clause.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Warpspeed]

Definitely worth another star.