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Old 22nd March 2006, 19:20   #1
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Lightbulb How effective is the auto-cut mechanism of the fuel service stations?

For the 1st time since last three or four fills I have started noting the FE of my car. Till now I always use to walk in and ask for 15 liters of diesel. That used to fill up the tank as afterwards the car's fuel gauge needle used to show FULL.

Now since last 3 times I asked them to fill up till auto-cut. The first time I was mentally prepared to fill more than 15liters. However to my astonishment I have filled 10.098 lters, 11.038 and 13.29 liters respectively on these three occassions. On each of these 3 occassions the car had more or less run the same kms. Now I noticed two things,
  • When the auto-cut shuts off the filler the fuel-gauge does not show the tank to be full!!!
  • More interestingly, the auto cut appears to be a crude mechanism. If you have the fuel level a little lower the filling up creates a lot of foam in the tank (due to height from which the fuel is falling, a-la pouring of beer in a glass) and hence the foam level rises faster and the moment it touches the filler nozzle/pipe the auto-cut jumps in. On the other hand if the fuel level is higher in the tank lesser foam is created and hence more fuel is filled until the filler pipe touches the actual fuel and auto-cut switches of the pump.
In such a case I reckon that even though auto-cut shuts off the pump the tank may not have necessarily got filled to the capacity. The reason why some people feel that they manage to push in so much more fuel (Fuel tank capacity measurement, how?) even after the "tank full".

Also, a more thoughtful matter is that if my observation is correct then the way we measure FE is also not very perfect as we now have a coefficient of fuel-foam factor in the equation.

Think about it!
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Old 22nd March 2006, 19:33   #2
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Quote:
Originally Posted by Zappo
Also, a more thoughtful matter is that if my observation is correct then the way we measure FE is also not very perfect as we now have a coefficient of fuel-foam factor in the equation.
What you have said is correct BUT if I have been using the same petrol station and always use the auto cut-off, my FE measurement would be consistent if not perfect, wouldn't it?

In Madras, I almost always use only 1 petrol station (BP) and sometimes 2 others (both IOC).

First tankful at odo reading 46
Latest tankful at odo reading 56416
Distance between tankfuls: 56370 km
Fuel filled between these tankfuls: 5044.89 litres
Overall FE: 11.17 kpl (which is what I quote everywhere)

A/C is always on. My driving over this distance has been 90% city and 10% highway. I shift up at 15-30-45-60 for 2-3-4-5. I never switch off the engine or A/C at traffic lights or while waiting for someone in the car.

Perfect? No. Consistent? I think so. What do others think?

Last edited by hrag : 22nd March 2006 at 19:39.
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Old 22nd March 2006, 19:40   #3
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See posts.. 309 to 319 here (What is your Actual Fuel Efficiency?)

I still say.. Auto cuts.. are never consistent... whether same pump or not.
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Old 22nd March 2006, 19:46   #4
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@hrag:
Yes... you are right mostly. Except for one small fly in the ointment. That foam-factor I talked about. If the foam created is more, either because the height of the fuel pipe from the top level of the fuel in the car's tank, the speed at which the fuel is coming out from the piep, or for some other environmental factors the fuel filled may not exactly be what it should have been.

Take this example. After the last tankful suppose I consumed 14 liters of fuel. Now when the second time I fill the tank the foam created causes the auto-cut to jump in at 13 liters... I get a feeling that I consumed only 13 liters for the distance travelled whereas the actual should have been 14 liters. This will also make me feel that my car is more FE than it actually is. And the cycle would continue.
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Old 22nd March 2006, 21:13   #5
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Auto cut-off is only a safety device to avoid spillage. At times just the fumes of the fuel may activate the trigger to shut down the flow to avoid spillage. You can always shake the vehicle a little and continue from where the fuel flow stopped. I don't think it has anything to do with your FE calculations, unless I have misunderstood your problem.

For diesel vehicles no matter how much you shake the vehicle there is still a possibility of few air pockets. For perfect calculation, I suggest you fill your tank doing all these antics. Drive the car for atleast 2 Kms after your initial topup. You will be surprised to see it taking in another couple of litres. Now note down this reading. After having run the vehicle till your next fill, repeat the same process and note down the reading at your second topup. Its cumbersome alright but you can't get better perfection than this.

My vehicle has a capacity of 60 litres. I run the car for about 300 kms each time and topup with about 30-35 litres every time. This gives me a fairly accurate picture of my cars condition. I have afuel chart, which gives me the entire history of the vehicle since it was purchased ( 3 years back)
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Old 22nd March 2006, 22:57   #6
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Auto cut off is only a safety device.

I have this experience during one of my week end trips in a hired Tempo traveller..

Auto cut off kicked in at 5 liters on an empty tank, at a reliance station..
Reason, Filler in that van is located very far from the tank, teh driver told, it happens because some times the fuel comes up as the pipes can not swallow the fuel at the rate it is being pumped into by the nozzle.

This could well be the case in cars too
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Old 22nd March 2006, 23:04   #7
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here is the link how it works

http://auto.howstuffworks.com/question25.htm
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Old 23rd March 2006, 11:41   #8
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Whoa! So already we have a new approach to calculating FE (as SOCCER wrote) and a corroboration to the fact that the auto-cut does not necessarily helps us to calculate the correct FE.

@Soccer: The auto-cut is indeed used by most of us to calculate the FE of our cars. On a top-up to top-up basis. Basically after the top-up we drive some 200-300 kms and then again get a top-up. This way we assume that the liters that went in on the second attempt is the amount of fuel the car consumed for the kms travelled. Only that now I am finding out that when the auto-cut cuts in probably the tank could have easily taken in another couple of liters.

Let it come guys... keep the ideas flowing
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Old 23rd March 2006, 12:06   #9
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sometimes the auto cut off does not work....

on 2 separate occasions I had my tank overflow resulting in wastage of precious fuel.... the attendant sheepishly said sorry....

i made a scene and didnt pay for a litre.... i make it a point to always step outside my car and oversee the activity... if i was sitting comfortaly inside i would have been cheated...
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Old 23rd March 2006, 14:07   #10
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Another problem with the auto cut off is the fuel station guys using it to rig the meters. When the fuel pump cuts off automatically and when the guy starts the pump again you will notice the meter reading jump by about 3/4 of a ltr or sometimes even a ltr.
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Old 25th March 2006, 16:46   #11
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Quote:
Quote:
Originally Posted by n_aditya
i made a scene and didnt pay for a litre.... i make it a point to always step outside my car and oversee the activity... if i was sitting comfortaly inside i would have been cheated...
What is worse is that it leaves a mess on the paintwork. It eats up the wax polishing in those areas of spillage and the difference of colour show up the next day. Its even worse if it is diesel, because no matter how well you clean the area, it attracts dust and looks awful.
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Old 31st March 2006, 12:51   #12
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the auto cut off is not one bit effective. it cut off at 24lts on an empty tank and i mean completely empty that i had to push car to fuel station.
p.s my cars fuel tank capacity is 35lts.
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Old 31st March 2006, 14:09   #13
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Quote:
Originally Posted by cheap_deal
the auto cut off is not one bit effective. it cut off at 24lts on an empty tank and i mean completely empty that i had to push car to fuel station.
p.s my cars fuel tank capacity is 35lts.
Whoa... its getting more and more scary! All of a sudden I am really worried... guys... its worrisome, considering that this is how we have been measuring the FE till now. We always believed that the best way to measure FE is on an auto-cut to auto-cut basis. In light of these problems it is obvious that auto-cut may actually be a very optimistic way of measuring the car's FE. Whaddya guys say now?
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Old 31st March 2006, 14:24   #14
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Zappo, I don't know about you but I've always asked the attendant to fill to the brim; this is to compensate the effects of faulty rigs, air locks in the pipe etc. How much of petrol goes in after the auto cutoff also depends on the vehicle - I know that in my Ikon, 5 to 7 litres ALWAYS go in after the auto cutoff, in the Fiesta 3 to 4 is what I've seen.

In the light of the above (filling to the brim irrespective of auto cutoff), I have not faced any problem in accurately determining FE.....thats my experience
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Old 31st March 2006, 14:50   #15
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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.
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