How do cars know how much fuel is in the tank?

[Clarkey]

Yes I have, both scabby old jap scrappers with steel tanks. Both times, the bottom of the tank was seen to be absolutely gleaming clean.

 

[Mickie]

some tanks can rust internally, then you have carbon build up and maybe bits of dusy in the air or bits of cap seal.. over a long time little bits of anything can get in..

though when i stripped down my tank on the 205gti their was hardly anything there and their is also a filter around the fuel pump to stop things getting dragged through..

im sure all modern cars do the same thing..

an inline flow meter wouldnt cost that much to make but its a lot easier to make a float which does a good job..

plus a flow meter on the filler neck would probably slow filling times down unless they were sealed against the pump..

 

[DampDog]

Yes I have, both scabby old jap scrappers with steel tanks. Both times, the bottom of the tank was seen to be absolutely gleaming clean.

Cool, glad you've had no problems. I'm not trying to win you over, I've seen clean and dirty.

 

[opstrat]

Is that not why fuel filters should be on your yearly service list?

 

[Jokester]

Do petrol tanks have sumps?

 

[[TW]Fox]

Is that not why fuel filters should be on your yearly service list?

A filter is at its most efficient when dirty isnt it?

 

[malfunkshun]

Yeah, but the pump flow meters are quite expensive to get the required accuracy, but yeah there's nothing stopping the pump communicating that info the car afterwards though.

hmm A spot of NFC when you first put the nozzle in and then a final amount when you put it back.

 

[bullit in head]

Because dirt and grime tend to be solids so collect at the bottom of the tank. When you're running on the dregs, the chances of sucking them up increases. The fuel pick up is normally at the bottom of the tank.

so ergo any dirt will get sucked up as soon as it hits the bottom, so just cos it is empty it isnt going to suck up any more dirt than it ushually would.

Do petrol tanks have sumps?

generally yes. and the fuel pump sits in it( lowest point in the tank)


bullit

 

[Clarkey]

By no means have I seen them all, but the ones I have, the pickup is at least 10mm from the bottom of the tank, and has it's own sock filter on. I honestly don't buy the whole fuel tank dirt thing and consider it a myth. Until I see otherwise with my own eyes I'll stick to that.

I will open the filter when I get around to changing it though, be interesting to see.
That might be sooner rather than later, the location of it is very silly if you ask me, under the floor near the lowest part of the chassis. Alarmingly it actually has a bit of road rash on the bottom

 

[DaveyD]

Not sure how? The pump dispensing it manages fine. Heck, if measuring it as it enters the car is that hard, pumps and cars could be fitted with NFC

The fuel flow from the pump will likely be nowhere near accurate to meatie the flow of the fuel through each injector, the ecu will keep an average fuel flow from your injectors over a set period of time. Will then take the average speed and work out your range.

A flow meter would never work in a car alone, you need a level reading.

As an extreme example or two, you fill your tank with petrol then leave the cap off accidentally for a week. Loads of fuel evaporates and your car still thinks its full.

 

[PistolPete]

As an extreme example or two, you fill your tank with petrol then leave the cap off accidentally for a week. Loads of fuel evaporates and your car still thinks its full.

Or less extreme....a fuel leak.

 

[Bear]

Because dirt and grime tend to be solids so collect at the bottom of the tank. When you're running on the dregs, the chances of sucking them up increases. The fuel pick up is normally at the bottom of the tank.

That doesnt make any sense. If dirt and grime are solids and sit at the bottom of the tank, then the pick up from the fuel pump would pick that stuff up regularly as the pick up has to be near the bottom of the tank else you wouldnt be able to suck it up in the first place.

Grot at the bottom of the tank -> fuel pick up at the bottom of the tank = grot being able to be sucked up at any point in time and fuel level is irrelevant.

The only time fuel level would be relevant would be the grot that floated at the top of the tank surely as that is the stuff the pick up wouldnt reach normally.

 

[DaveyD]

Or less extreme....a fuel leak.

Yeah, I did forget to finish that point! Fun of typing on a phone.

But either way, fuel flow is not the answer!

 

[Jonnycoupe]

Not sure how? The pump dispensing it manages fine. Heck, if measuring it as it enters the car is that hard, pumps and cars could be fitted with NFC

How big and heavy is the unit in a fuel delivery pump though?

Theres no point it existing really. For MPG etc to be accurate they should first sort the speedo accuracy out, although that would be easier on cars with sat nav/GPS.

 

[Jonnycoupe]

You're kidding me. Have you ever emptied an old patrol tank?? Do you think the fuel doesn't contain dirt grime and even water? You should see the sludge at the bottom of the average patrol station storage tank.

I can't imagine the orders of magnetude between how much a personal car uses a year compared to how much fuel goes in and out of that in a week...

The fact the crap is at the bottom of the petrol storage tank suggests its quite happy staying there anyway.

 

[Jez]

LOL at all of the suggestions of flow rate meters and GPS speedo gizmos.

How many of you actually struggle to work with the standard float fuel readings? Its a total non issue.

 

[DampDog]

That doesnt make any sense. If dirt and grime are solids and sit at the bottom of the tank, then the pick up from the fuel pump would pick that stuff up regularly as the pick up has to be near the bottom of the tank else you wouldnt be able to suck it up in the first place.

Grot at the bottom of the tank -> fuel pick up at the bottom of the tank = grot being able to be sucked up at any point in time and fuel level is irrelevant.

The only time fuel level would be relevant would be the grot that floated at the top of the tank surely as that is the stuff the pick up wouldnt reach normally.

It works like this. Fill a bucket full or water and throw in a bit of fine sand/grit whatever and allow it to settle to bottom. Then agitate the water in the bucket a bit. The stuff at bottom barely moves if at all. Then empty off 95% of the water and agitate it again and you'll see it stirs up nigh on all the crud up into solution. Obviously you're not sticking you hand in the tank, but driving around sloshes the fuel around in the tank in the same way.

It goes without saying the older the car the more chance it will have had to build up sediments and grime at the bottom of the tank. Plus old steel tanks can rust on the inside, though most modern car have plastic tanks. That's why cars have fuel filter/filters to catch all the particulates before they reach the expensive bits like fuel injectors and pumps.

 

[Dogbreath]

That's why cars have fuel filter/filters to catch all the particulates before they reach the expensive bits like fuel injectors and pumps.

There isn't much of a filter before the pump, usually just a plastic mesh though it is obviously good enough.

 

[Dr Who]

But there is usually a filter between pump and fuel rail.

 

[DampDog]

There isn't much of a filter before the pump, usually just a plastic mesh though it is obviously good enough.

That's true but on all fuel injection systems the high pressure precision sensitive/expensive stuff is on the other side of the filter/filters

I have also read that because most fuel pumps now run submerged in the tank they rely of the fuel to cool them, and running the tank down to empty can cause them to overheat and fail. No sure about that one though, sounds more like urban myth. I don't like sound of an overhearting fuel pump in a petrol tank, sounds a bit iffy to me.