Lotus Elan

Petrol evaporation

PostPost by: mikealdren » Sat Dec 19, 2009 6:58 pm

Peter's comment on another thread and the perenial questions on tank venting have had me wondering about this one.

I can understand why the +2 needs the two large tubes linking the sides of the tank to avoid vapour locks while filling with the low entry to the tank. You can get round that by routing the tank side vents to the filler neck as per the Spyder mod. You then have the issue of expansion and contraction of the fuel and this can be vented either through the cap or by a small diameter pipe linked into the vent pipes and going out of the car through a standard sill outlet. This should only need a small diameter pipe as the change in tank pressure should be slow except when filling and the Spyder mod takes care of that.

Having said this, why is the +2 such a problem? Is it the high mounting of the tank in the cabin that allows the fuel to get hotter than in other cars causing more evaporation?

Mike.
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PostPost by: andyhodg » Sat Dec 19, 2009 7:12 pm

With the original system I wonder if the fuel vapour (heavier than air) is syphoning out of the tank? with the outlet well below the tank and potentially leaking fuel cap could one get a contious "flow" causing higher than normal evaporation?

Certainly I don't experience a lot of evaporative loss but I have modified the system much as you describe with a 4mm bore vent to the outside of the car and a 12mm bore vent to the filler neck. I have also sealed the fuel filler cap with a removeable O ring plug in the top of the filler neck.

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Andy
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PostPost by: kstrutt11 » Sat Dec 19, 2009 10:46 pm

The two large pipes are required so you can fill the tank up, this is because the filler neck comes into the tank part way down.

On my drophead I have routed these into the top of the filler neck just below the cap so it can vent when filling but is sealed when the cap is fitted. I have added a small bore (brake pipe) tube for venting which seems to work fine, the garage certainly no longer smells of petrol so it seems to reduce the vapourisation.
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PostPost by: bengalcharlie » Sun Dec 20, 2009 4:46 am

Andy,

would you have a pic of the o-ringed plug you made in the filler neck??
cheers
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PostPost by: andyhodg » Sun Dec 20, 2009 8:09 am

Robin

With a bit of luck I will have some time today and will post a picture or two.

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Andy
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PostPost by: stugilmour » Sun Dec 20, 2009 3:56 pm

For folks that have installed the small bore vent line, how did you route it to the outside of the car? Is it OK to just run across top of the tank and down through the boot floor, or does it have to be inserted in one of the stock larger diameter vents?
Stu
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PostPost by: andyhodg » Sun Dec 20, 2009 7:57 pm

Stu

I inserted mine through the existing large bore vent lines as I did not want to disturb my headling.

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PostPost by: andyhodg » Sun Dec 20, 2009 8:15 pm

Attached are some photos of the petrol filler plug I made. The O ring is a spare webber isolation plate O ring. Its been working well for more that 2 years. The only thing that is surprising is when using the car in heavy rain I actually collect water above the plug! Just think where that was ending up before fitting the plug.

Regards

Andy
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PostPost by: davidj » Tue Dec 22, 2009 1:02 pm

I think the tank "two bore vent design" was so you could park and fill and car on a slope. This would result in one of the vents being blocked if the tank was over half full, but the other one should always be clear.

I am guessing the reason the pipes were routed over the roof was so if the car turned over or was on it's side, petrol would never leak out of the pipes, as the exits would always be above the tank. This may be why the cap is also non venting.

I have also carried out the pipe mod but used plastic tee pieces.

I also have a spare tank, if anyone would like it for dimensions; otherwise it is going to the tip. Not sure if is would actually hold petrol!

David
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PostPost by: john.p.clegg » Tue Dec 22, 2009 4:56 pm

.....and I'm sure we all know of petrol filling stations built on a "slope".....?

John :wink:
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PostPost by: theelanman » Tue Dec 22, 2009 7:25 pm

andyhodg wrote:Attached are some photos of the petrol filler plug I made. The O ring is a spare webber isolation plate O ring. Its been working well for more that 2 years. The only thing that is surprising is when using the car in heavy rain I actually collect water above the plug! Just think where that was ending up before fitting the plug.

Regards

Andy


I made something very similar when I first got my +2 about 10 years ago....and its still in too.......
fits nice and snug under the locking filler cap...
I machined a nice bit of ali...n fitted an o ring....I then put 2 short allen screws just enough to get your fingers under and hook round it to give it a quick pull out..:)
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