[Zappo]

Originally Posted by PRADEEP KUMAR

It is a myth that shaking a vehicle will enable to top up on the fuel.Liquid by default will fill up and no amount os shaking will enable you to add on more.

Pradeep you are not entirely correct. Shaking a vehicle actually lets add some more fuel to the tank. The reason is that there are lot of turns and bends in the hoses that carry the fuel from the tank. The shaking thing lets the fuel to flow through these and fill up the hoses, thus letting your say 35 lts tank to hold 38 lts. To make an analogy have you seen the western style comodes? Why do they always hold water to certain height? Liquid should have flown away down the sewage pipes, right? Its because the bend at the joint does not let it flow. And yet when the force of running water is made to go through the water does starts to flow through the bend.

Originally Posted by suman

Zappo, I don't know about you but I've always asked the attendant to fill to the brim

Maybe you do suman, but most people even on this forum have always used the auto-cut to auto-cut mechanism to find out the "exact" FE. Search through the threads and you will know what I mean. But yes... now I am very skeptical about it all. However I am not convinced that the hilao-dulao way of filling till it starts flowing out is the best way to measure FE either.

[ksethuram]

I never believe in auto cut, Once it gets cut i tell the guy to wait for 30 secs and then fill in slowly. Nearly 4-5 litres will be filled in extra each time when the fuel comes to visible level.

[suman]

Originally Posted by Zappo

However I am not convinced that the hilao-dulao way of filling till it starts flowing out is the best way to measure FE either.

No need to hilao-dulao buddy; once the auto-cut happens, give it 5 seconds and ask him to keep filling slowly till it comes up to the outer rim and doesn't go down, thats all. No need to shake, rock or roll.......!

At least, this way, I know I'm filing to the same level each time with no "hidden" cut-offs.

Its exactly like Sethuram says...

[Zappo]

Originally Posted by suman

No need to hilao-dulao buddy; once the auto-cut happens, give it 5 seconds and ask him to keep filling slowly till it comes up to the outer rim and doesn't go down, thats all. No need to shake, rock or roll.......!

At least, this way, I know I'm filing to the same level each time with no "hidden" cut-offs.

Its exactly like Sethuram says...

Hmm... gotta do something of the sort the next time. By the way, what is the assurance that this way also every time we fill the same amount of fuel? I mean, since they do that rocNroll stuff to push in some more even when it is filled up to the brim it means there is always the chance to pump in some more. So probably even without that rockNroll stuff the ltrs of fuel that goes in everytime is different. Is not that a possibility? I wonder...

Sometimes I wonder why can not these car manufacturers give a simple reserve type mechanism in the cars as well! It is soooo very simple to implement... only thing that one will need to do is that once the fuel level falls below a level in the tank a light will come on (instead of the car stalling like a bike ).

Edit: I just realized after posting it that there actually exists that mechanism in the cars!!! Wonder how we never think about it. In fact therein lies the answer... That light comes on so very late in the day. Probably there should be another light to mark half tank consumption.

[SLK]

Originally Posted by Zappo

but most people even on this forum have always used the auto-cut to auto-cut mechanism to find out the "exact" FE.

Well then... that just says... the so called 'exact' FE is not exact!

Also people do a very obvious mistake.... SEE THE EXAMPLE BELOW

Source Data
Tank size-----------------35
Fuel in tank on start------ 33 (due to previous auto cut off)
Car's actual fuel economy--15

Case 1
Kms run-------------------60
Actual Fuel Consumed-------4
Fuel filled (auto cut off)-----3 (due to auto cut error)
Total Fuel in tank----------32

Calculated FE--------------20


Case 2
Kms run-------------------105
Actual Fuel Consumed--------7
Fuel filled (auto cut off)------6 (due to auto cut error)
Total Fuel in tank-----------32

Calculated FE--------------17.5


Case 3
Kms run-------------------300
Actual Fuel Consumed-------20
Fuel filled (auto cut off)-----19 (due to auto cut error)
Total Fuel in tank-----------32

Calculated FE-------------15.79


Case 4
Kms run-------------------465
Actual Fuel Consumed-------31
Fuel filled (auto cut off)-----30 (due to auto cut error)
Total Fuel in tank-----------32

Calculated FE-------------15.50


Points to note
1) Auto Cut error is SAME in all 4 cases, BUT you get different fuel economy.
2) More kms you run the error in topping up gets distributed and the per km error is substantially less.
3) No matter how careful you are while topping there is an error for sure. The error is more when relying on autocut and less when on fill to the brim.

Moral of the story: You run as much as possible on a tank then fill up.

[turbo_c]

A petrol tank (expect deisels are the same) is designed to allow some space for the expansion of the fuel. The way this is done is quite simple: The fuel inlet pipe (the metal part on the tank itself) extends a little below (about 1/2" in my Esteem) the top of the tank body. This allows a 1/2" thick air pocket to form above the fuel since once the liquid level covers the bottom part of the fuel inlet, the fuel / foam will tend to overflow, trigering the auto cut-off.

I don't think it is a good idea to "force feed" the tank after the auto cutoff by shaking the air out of the air pocket, because you would be defeating the tank's design and risking an overflow.

Try this: Fill a 1L mineral water bottle with 1/2L of petrol. Cap it tightly and then shake it vigourously for just 5 seconds. Hold the bottle away from yourself and carefully open the cap... hear the vapours hissing on the way out? Now imagine what happens to 40 odd liters of the same stuff in the tank of a car going over a potholed road hour after hour!

Thankfully, petrol tanks are allowed to breathe and are not really air tight, so they "should not" get pressurized, but the air pocket is still important to allow the expansion of the fuel.

- T u r b o C -

[jat]

Originally Posted by turbo_c

.....................
I don't think it is a good idea to "force feed" the tank after the auto cutoff by shaking the air out of the air pocket, because you would be defeating the tank's design and risking an overflow.................

- T u r b o C -

You are absolutely right. People should avoid filling to brim as a rule.

But if somebody wants to measure Fuel Economy and compare his driving style, then he has to fill up with all the methods described above.
But
1. He should leave a small amount of space for initial expansion and avoid potholed roads (tough job)
2. He drives for some distance to consume around 1~2 litres of fuel to allow for expansion.
3. Does not park his car in sun for long time.
4. Calculates FE when he has got around 1/4 tank reading on fuel gauge.

This way he would be able to get FE with good accuracy without causing spill.
IMO, people trying to get a lot of accuracy should add fuel with measuring cans, fill up the tank by putting the wheel closest to the filling cap on a ramp and fill slowly, {take into consideration expansion of fuel due to temperature changes and loss of fuel to evaporation when the vehicle is parked for long time ;-)}, otherwise for general comparison the average method with auto-cutoff with number of fill should suffice.

[SLK]

I wonder if someone has tried.... do anything you can.. and fill it to the brim!
drive just 1-2kms... (you will have to if you don't live next to the petrol pump). AND I bet you can't find the fuel anywhere near the brim!

No amount of shaking can substitute for car's vibrations while being driven. So the moment you start to drive.. the fuel goes in.

Please try it yourself... before putting up that it would spill off on things like potholes!! ... and expansion due to heat! (unless the pothole is right next to the petrol pump)

I always fill it to the brim (no shaking but also not relying on auto cut)... and never has my fuel spilled. (Delhi being as hot as it gets).

Every time.... I fill, my car takes 27.5-28.5ltrs.... with no exceptions!!
but the autocut off.... sometimes stops.. at 24ltrs !!!.. or sometimes at 27ltrs.

Autocut off is a safety mechanism!.... not a way to fill up a tank!

If you may note!... with autocut.. the fuel gauge may not even show the 'F' mark.

[normally_crazy]

I think these days you have a button on the side when you open your fuel cap, thats supposed to be pressed to remove air from the tank (The Octavia has it). That way you can get in more fuel without actually physically shaking the car. The Octavia takes in always >55 litres , I fill it up as soon as the low fuel indicator light gets activated.

Also I have heard there is a chance of vaccum being created if the fuel is filled up to the brim , and fuel not flowing to the engine resulting in stalling. True ?

[santosh.s]

Originally Posted by jat

d
4. Calculates FE when he has got around 1/4 tank reading on fuel gauge.

This way he would be able to get FE with good accuracy without causing spill.

In general fuel gauges are very inaccurate, they are meant to give just a rough idea of available fuel. Also since the resolution of these gauges is quite coarse(not more than 4-5 lines in the entires range from empty ot full!, slight diff in needle reading may translate to a few litres of error) and its indication is non-linear , measurements based on the gauge could be far from accurate.

However, measurements made after several top-ups should be quite accurate since it will filter out all mistakes in idividual calculations made for each filling. But that is purely an average FE over long period, not exactly what we are looking for

[santosh.s]

Originally Posted by n_aditya

sometimes the auto cut off does not work....

I had this horrible experience yesterday. Since I always keep a watch while filling, the guy quickly stopped it and there wasn't much of wastage. I asked him to wipe the spilt petrol immediately as I knew it can spoil the paint (so expensive ), worried whether the stain will show up little later?

[suman]

Originally Posted by Zappo

... That light comes on so very late in the day. Probably there should be another light to mark half tank consumption.

In the Fiesta, the warning light comes on when you have 80kms driving fuel left in the car (the DTE meter shows 80 kms); that should be adequate warning though not half tank

[DRC]

Yesterday night, I had one more refuel.

Same scenario.. Had almost run out and barely reached the station.
Same bunk, same pump, the gun cut off at 40.20Litres.. and asked to top up!!

All he could manage to force in is another 2 l..So 40+2... Last time he m anaged 40+5,, Now is topping up reliable??

[swathyd]

It hasnt worked in any petrol pumps i have filled except from one pump. It is a feature which seldom works , some times overflows occured after false assurance from pump guys about auto-cut of.

[mail4loys]

I normally fill petrol to the brim and had noticed that after cut-off my
Car can take another 9-11 Lts when filled slowly.