Fuel tank size

KevR · Post by **KevR** » Sat Oct 23, 2010 1:25 pm

It had never occurred to me that the reason the fuel gauge sits on full for so long was that the main tank was full and it was running on the supplementary one - makes sense now!

kiwi · Post by **kiwi** » Sat Oct 23, 2010 10:53 pm

Keeping to the techinical details your main tain is 52 litres the supplementry tank and fuel line between them holds the remaining 14 litres. Giving the capacity of 66 litres.

The BX14 as already eluded to has only a 44 litre tank, I would think the original thinking behind that was a 14 is more efficiant than the BX16s when they were first produced, so gave them the same kind of range. The additional tank capacity would have been the result of the GTi and MKII upgrade.

One of the reasons for ther Fuel gauge accuracy as you pointed out is because of that expansion or reserve tank.

jacksun1987 · Post by **jacksun1987** » Sun Oct 24, 2010 9:44 pm

Ohh ya just wondering. When my light comes on the needle as still got a bit to go down . You know the thick red reserve line? Ive never seen the needle below that line. Like all the way down. The light comes on but the needle has bit togo. Wondering how much fuel is there left. Is the needle accurate or the light.

On other cars ive seen with the light on the needle is right at the bottom.

jacksun1987 · Post by **jacksun1987** » Sun Oct 24, 2010 9:57 pm

Just though up of a game. We could all meet up early in the summer. Full our cars right to the top. Pick a good bit of road, Or evern put some money in book a track for a day. Like on of them banked oneslol. Keep driving untill the last one standing. Then we could tell whos got the best bx in fuellol.

Philip Chidlow · Post by **Philip Chidlow** » Sun Oct 24, 2010 10:53 pm

Trouble is you might have someone in a diesel managing over 820 miles on a tank... a bit much for a track day.

Post by **mat_fenwick** » Sun Oct 24, 2010 11:03 pm

Or how about a contest to see who could finish their tank first, otherwise it would be a very dull track day!

jacksun1987 · Post by **jacksun1987** » Sun Oct 24, 2010 11:18 pm

Whats better on fuel the 1.9d or the 1.7 td

jacksun1987 · Post by **jacksun1987** » Sun Oct 24, 2010 11:48 pm

be a 24 hour race. That would be well good. Hey remenber that race top gear had the 24 houre one we should enter that with a bx lol.

electrokid · Post by **electrokid** » Mon Oct 25, 2010 12:01 am

When my light comes on the needle as still got a bit to go down

On the BX the light and the gauge are separate sensors in the tank - when the light comes on you have - if memory serves - about a gallon left.

A few months ago I had the instrument pod to pieces and didn't get the fuel gauge needle back in the right place - filling up makes it read quite a bit over full - but the red light still comes on at the right time and tells me it needs feeding.

kiwi · Post by **kiwi** » Mon Oct 25, 2010 2:20 am

electrokid wrote:On the BX the light and the gauge are separate sensors in the tank - when the light comes on you have - if memory serves - about a gallon left.

As long as your not talking short changed US Gallons

Handbook says 5 litres (1 gallon) approximately, that tends to place the permanant light on. On normal roads probably around the 10 litre margin the light might first flicker on
Although I got a rather step bit of hillside near me. Managed to get that light on with quarter tank showing, that light can be a bit unerving when you pretty much know how many ks it has been since the last fill up and now its making you think you got between 50-100kms left.

Best bit russian roulette been able to do without running out was fill up with 64.94 litres

Yea I play the roulette wheel on the fuel gauge quite a bit. Not the one that says fill it with a couple of quid but the one that goes as far as you can on a tank! Given the nearest sevo is 20kms away it is often unavoidable to play that game. The light is my best mate

kiwi · Post by **kiwi** » Mon Oct 25, 2010 3:03 am

Here is what is inside the fuel tank sensor

What I noted were three distinct marks on the way down the sensor which I marked two on the photo indicating 1/2 and 1/4 fuel points.

The trigger for the light is in the area circled. Which you can make out half the circuit is missing. On the float is a metal clip that floats up and down and from what I can see that clip when it reaches near empty no longer has a connection hence activates the light.

All boring technical stuff really.

JayW · Post by **JayW** » Mon Oct 25, 2010 7:58 pm

Potentiometer + 12v + Petrol = no bang!

The wonders of electrickery...

kiwi · Post by **kiwi** » Mon Oct 25, 2010 8:50 pm

JayW wrote:Potentiometer + 12v + Petrol = no bang!

The wonders of electrickery...

Petrol in liquid form is not flamable the vapour it gives off is

Electrical current does not cause the bang the spark does

Did make me wonder though when I saw those electrods submersed in fuel to wonder how often a fault has occured to cause a fuel tank fire.

electrokid · Post by **electrokid** » Mon Oct 25, 2010 10:19 pm

Potentiometer + 12v + Petrol = no bang!

Petrol needs air as well to go bang - it's a chemical reaction - Petrol + the oxygen in air = smoke and flames.

And it will only explode between certain proportional limits - the Lower Explosive Limit or LEL and the Upper Explosive Limit UEL expressed as a proportion of the fuel.

For petrol in air the LEL is around 10% and the UEL is around 20% so anything outside those proportions aint gonna pop - which is why it is easy to 'flood' an engine - over 20% fuel vapour and it won't ignite - and with the choke fully 'out' it's getting close to that. (The stoiciometric ratio - the proportions where theoretically all the petrol and all the oxygen react with each other and get completely used up is around 14%. Unfortunately chemistry doesn't work like that - reactions only 'proceed to their limit' which, as far as I'm aware, is never 100% - which is why it is almost impossible to get rid of 'unburnt fuel' in the exhaust system.

The concentration of fuel in the fuel tank is either 100% liquid or 100% saturated air above - again no pop.

Another thing you can do with petrol is to pump it with an electric motor as in injection fuel pumps and some rotary pumps for carbs - no oxygen to react with the fuel.

The wonders of electrickery...

If there's still a risk then the voltage and current can be limited to a level where it won't cause a spark - from memory (it's some time since I worked on oil rig electronics) it's <3 volts and <20 milliamps - a line limited to these values is called 'intrinsically safe'. This voltage and current is so low it's only practical use is for sensors.

Hydrogen is not only the most useful fuel - it is also the most dangerous - the LEL is just 1% and the UEL is around 76%.

RichardW · Post by **RichardW** » Wed Oct 27, 2010 8:10 am

The UEL for Acetylene is 100%....

Hence the long and involved procedure for cooling an acetylene cylinder engulfed in fire before you can deem it 'safe' again!

Vapour above the fuel in the tank will be approx 70% petrol, so it's well above the UEL - I watched cringing as my Dad drilled a hole in the top of a petrol tank on a Visa to fit a new earth wire for the gauge sender, but he was confident it was too rich and no problem

BTW I though the stoichiometric ratio for petrol was about 14:1 ie 7%...but I could be wrong!