Venting the fuel tank

[wai]

Not applicable for diesel.

I'll have to remember that for the next 100 underground bulk diesel fuel tank installations I design.

[kaanage]

Thanks for you "contribution" - we are talking about totally different applications
Can you answer my question one way or the other.

[wai]

I thought I did. All tanks have to be vented otherwise they will collapse when you pump the fuel out of it. If you fill the tank to try and get more fuel into it, then if it expands beyond the available vent space, the excess fuel will end up wherever the vent system goes to. This varies from manufacturer to manufacturer.

On page 215 of my Caddy manual, it says under Refuelling, "The fuel tank is full when the properly operated automatic filler nozzle clicks off for the first time." It goes on to say "Do not continue filling up after it switches off! The expansion space in the tank will otherwise fill up and the fuel could spill out. This could also happen when the fuel warms up and expands."

It cannot be any clearer than that.

[Idle]

I thought I did. All tanks have to be vented otherwise they will collapse when you pump the fuel out of it. If you fill the tank to try and get more fuel into it, then if it expands beyond the available vent space, the excess fuel will end up wherever the vent system goes to. This varies from manufacturer to manufacturer.

On page 215 of my Caddy manual, it says under Refuelling, "The fuel tank is full when the properly operated automatic filler nozzle clicks off for the first time." It goes on to say "Do not continue filling up after it switches off! The expansion space in the tank will otherwise fill up and the fuel could spill out. This could also happen when the fuel warms up and expands."

It cannot be any clearer than that.

In days gone by (before they started putting expansion space in tanks) I've often seen fuel running down the gutter from a just-filled tank.

All it would have taken was a cigarette butt...

Having said that, I've also been known to run my rear wheel up on a brick to cram a bit more fuel in before heading off on a long run — relying on consumption to outstrip expansion — which fortunately it always did.

When filling the TDI I'll usually trickle diesel in until the second click, which gets rid of most of the foam (or at least moves it into the expansion space, where of course it does no harm and eventually condenses) and hasn't caused any problems so far.

[kaanage]

All tanks have to be vented otherwise they will collapse when you pump the fuel out of it.

Derr.. Or the pump simply won't pump fuel against the lowered pressure.

I've found from another site that the MkVI Golf's fuel tank cannot be vented when filling. Thanks for everyone's "help"

[Idle]

Derr.. Or the pump simply won't pump fuel against the lowered pressure.

I've found from another site that the MkVI Golf's fuel tank cannot be vented when filling. Thanks for everyone's "help"

I don't quite get that — if it can't be vented whilst filling, where would you suppose the air goes?

It doesn't start to be compressed until the fluid level reaches the expansion (vent) space, so the displaced air simply comes back up the filler tube until such time as inflow ceases and the filler cap is replaced.

It's my understanding that this is how the auto shutoff nozzles work — when fluid reaches the vent hole in the end of the nozzle the back pressure triggers the release. That's also why many poorly designed filler tubes won't take the full flow and have to be filled slowly, and why you can usually get a few more litres in with a trickle flow.