Fuel Tank Vent Valve

[Ferrino]

I have an injection tank and the Caterham aero filler neck assembly. My understanding is that (on the smallest of the ports on the neck assembly) you connect a small piece of hose, which has a one-way valve attached. And that this valve allows air to enter the tank (to prevent a vacuum forming as fuel heads to the engine), while not allowing fuel vapors to escape.

Is there not a requirement for another valve which does the opposite? i.e. allows air to escape when the tank gets hot (presumably while idling on a very hot day). Or is this positive pressure condition not really a problem (compared to the negative pressure condition)?

I guess I'm a little confused as I've seen some folks are using a 2-way valve, while Caterham uses a 1-way valve.

Thanks!

[Jonathan Kay]

Discussion following Workshop Notice 371. Please let me know if you'd like a copy.

Full search of the archives.

Jonathan

[Ferrino]

Jonathan, thanks for the link! I have read the "371" notice and it appears to describe a one-way valve that allows air in, but not vapors out. I was wondering if there is a separate valve that does the opposite, but I'm thinking that it doesn't exist, presumably because it's not an issue. 

[SM25T]

You don't need to let vapours out ... otherwise you risk dropping fuel too, which is what happens on left hand bends if the car just has an open hose on that spigot.

[revilla]

Since I fitted a one-way valve as per WN371 (because I was sloshing fuel out of the breather on left hand bends) I do indeed get quite a lot of positive pressure build up on a hot day but it doesn't seem to cause any problems. The filler cap positively pops out with a whoosh of escaping gas when refuelling. Worth noting that if you get the whoosh without the cap feeling like it is popping out, you've fitted the valve the wrong way around!

[ECR]

I suppose the pressure relief valve (engine end) deals with an increase in tank pressure on an injected engine. What about a carburettor engine?

[7 wonders of the world]

Fuel PRV merely bleeds excess line pressure back to the tank so you have a closed loop in effect, the only way would be to have a PRV venting to atmosphere  in the same manner as is adopted on your central heating system.

[ECR]

Yes, i see what you mean....

[Grant Dryburgh]

Have a look at the Newton TPV8 Vent Valve, I've bought one for my 7 but havent yet installed it.

Its a twin ball design which allows air in but also allows excess pressure out and as soon as fuel reaches the valve it shuts. 

https://www.newton-equipment.com/tank-vent-valves-inline-tank-valve-model-tpv8.html

[Ferrino]

Grant - thank you. This 2-way valve looks ideal. I live in a warmer climate, so there is definitely potential for a positive pressure condition when leaving the car in the sunlight on a hot day. If anyone else is looking, it seems that is available rebranded as a Mocal at DT for even less than then Caterham 1-way valve:

https://www.demon-tweeks.com/us/mocal-fuel-tank-pressure-relief-valve-moctpv8/