The Big Hisssssss

[attym3]

When I fill my Eunos as I remove the cap I get a big hiss. I usually fill it to the top and fill when just under a quarter.
Anyone else get this or do I have a breather blocked/inop/faulty?

Mark

[Munter]

Standard feature I believe. Not sure what the cause is (except obviously that a pressure difference must exist), but it's not a problem.

[youngsyr]

It's just a pressure difference caused by the petrol in the tank being heated/cooled to ambient, isn't it?

I've noticed it in practically every car I've driven for the past few years.

[Mr MXT]

youngsyr said:

It's just a pressure difference caused by the petrol in the tank being heated/cooled to ambient, isn't it?

I've noticed it in practically every car I've driven for the past few years.

I assumed it was the pressure difference changing as the tank empties i.e. negative pressure inside the tank, so its air rushing into the tank as you open the filler.

[youngsyr]

Mr MXT said:

youngsyr said:

It's just a pressure difference caused by the petrol in the tank being heated/cooled to ambient, isn't it?

I've noticed it in practically every car I've driven for the past few years.

I assumed it was the pressure difference changing as the tank empties i.e. negative pressure inside the tank, so its air rushing into the tank as you open the filler.

That makes much more sense as logically filling an empty tank wouldn't have much petrol in it to cause pressure changes!

[tuttle]

Pretty sure it's the other way round, petrol being a high expansivity (is that a real word?)liquid. When you fill up with cold liquid petrol its volume is minimum. Tank empties, air in tank warms,Petrol vapour expands.

Edited by tuttle on Friday 12th August 09:30

[youngsyr]

tuttle said:

Pretty sure it's the other way round, petrol being a high expansivity (is that a real word?)liquid. When you fill up with cold liquid petrol its volume is minimum. Tank empties, air in tank warms,Petrol vapour expands.

Edited by tuttle on Friday 12th August 09:30

Does the petrol vapour expand enough to more than replace the 50 litres or so of liquid petrol that has been removed from the tank though? I wouldn't have thought so.

Re "expansivity" I think the technical term you're looking for is "high coefficient of thermal expansion", but I could well be wrong as I've barely scratched the surface of understanding thermodynamics.

[denniswise9]

Used to get an even bigger hiss on my Focus when I had that, nothing to worry about.

[tuttle]

youngsyr said:

Does the petrol vapour expand enough to more than replace the 50 litres or so of liquid petrol that has been removed from the tank though? I wouldn't have thought so.

Re "expansivity" I think the technical term you're looking for is "high coefficient of thermal expansion", but I could well be wrong as I've barely scratched the surface of understanding thermodynamics.

You could well be right, & I am almost certainly musing out of my arse.
My thinking is going on no more expertise than schoolboy physics. Petrol=combustible liquid, therefore very high thermal coefficient thingy.
Rate of expansion for 1cm cubed of water is 1700:1 .Petrol must have a higher rate than that & becomes a vapour at much lower temperatures, so can easily effect pressure changes in an enclosed space i.e. 45 ltr petrol tank. IMHO

FWIW when I take my fuel cap off, air always seems to 'blow out'. Nothing to worry about.

[MX-5 Lazza]

The hiss is air/fumes blowing out, not sucking in.
I think it's something to do with emission control i.e. petrol tanks aren't allowed to be vented to air. There is a vent but that goes through a charcoal filter and into the intake manifold, I guess to burn up excess tank pressure. If the tank wasn't pressurised somehow then without being vented the pump would quickly fail to be able to pump fuel and the car would stall. I'm not sure where the pressure comes from though. Maybe one of our friendly mechanics could enlighten us

[youngsyr]

Right, I need to know the answer to this, so I'm putting it out to the wider PH massive:

http://www.pistonheads.com/gassing/topic.asp?h=0&a...

[tuttle]

Again, I'm just inexpertly musing here.

If fuel draining from a tank caused significant vacuum, would it cause the engine to stall as the pump would struggle to apply pressure?


http://www.consumerwatchdog.org/resources/DPReport...

[attym3]

Mr MXT said:

I assumed it was the pressure difference changing as the tank empties i.e. negative pressure inside the tank, so its air rushing into the tank as you open the filler.

This what I always thought, but some of the answers on here have got me thinking maybe that's wrong.

Mark

[Mr MXT]

We all have too much time on our hands.