Fuel Vaporisation Question

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Kevin
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Post by Kevin »

If you increase the pressure then the boiling point will rise and so be less prone to boiling at engine bay temperatures.
That where I am getting lost as the as the summer fuel quotes a lower pressure :o
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ColinP
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Post by ColinP »

Oh dear,

Back to my long forgotten O level Chemistry......

Inside every liquid, the molecules are moving about - this is caused by the temperature - the hotter the liquid, the faster they move. If they're near the surface, they can fly off and escape. They become a vapour.

Now, most molecules also attract each other; inside the liquid one molecule is attracted to all the others, so there's no overall effect, but at the surface, the pulls are mostly down and sideways (which is the meniscus) and this causes the surface of the liquid.

The vapour pressure is a measure of how "keen" the liquid is to become a vapour.
as an example, acetone has a high vapour pressure (30 Kpa at 20C) and evaporates quickly;
water has a vapour pressure of 2.4 Kpa at 20C and evaporates quite slowly, at 100C it's 101Kpa (equal to the surrounding atmosphere = 14.5 psi) and it boils. If you can change the surrounding pressure e.g. put it in the radiator and have a 4.5 psi pressure cap, the water won't boil until it reaches a higher temperature. Take off the radiator cap & it will boil out over your hand! 'cos you've lowered the pressure

Any help?

Colin

If
SAE 30 oil has a vapour pressure of 0Kpa at 20C, and doesn't evaporate (just leaks out of the engine!).
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Post by doobry »

OK Colin if I have understood correctly (probably not) that when the external pressure rises to match the fuel pressure this is where vaporisation begins to start
not really!

The pressures quoted are in absolute, whereas most peple think of pressure in 'guage' (ie relative to atmospheric pressure of approx 1 atmosphere) and this causes a fair bit of confusion.
Atmospheric pressure is roughly 14.5 psi absolute, so to make the petrol boil at 38.5°C you need to lower the pressure - ie place it under a mild vaccum (suction from the fuel pump)

as the BS EN standard gives a wide range of approx 4psi then in the worst case you hardly need any vaccuum at all to make the petrol vapourise at 38.5 degrees.
at >40 degrees it will be worse, so based on that standard, modern fuel can be pretty useless in a hot moggy (if it is at the worst case end of the specification)
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Post by polo2k »

few ideas here. would it be feasable to re route the fuel line under the chassis leg and under the steering rack then through the opening below the damper
alternativly could a secondary pump be fitted closer to the tank with a bypass valve

Code: Select all

TANK--------//--------------PRIMARY PUMP (original) --------- CARB
       ¦                 ¦
        \SECONDARY PUMP/
the // is a bypass valve only allowing fuel to move foreward.
the secondary pump could be connected to a hidden switch under the dash and could be used to prime the fuel line thus flushing out the vapor lock.

picasso eh? i tell you i am artisté
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Kevin
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Post by Kevin »

But the main problem is caused in the pipe where it feeds the pump not from the pump to the carb, so I dont think the secondary pump would be much use, and when in some cases a mechanical fuel pump is mounted on the engine Marina style providing the opening is fitted the problem is eliminated.
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doobry
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Post by doobry »

Kevin - form your comments I expect that his diagram has 'word wrapped' on your screen. The 'CARB' should be on the far right!

I've wondered about that configuration also, and the // valve you mention is called a non return valve. The problem with NRV's is that they are either unreliable at low flow/low pressure or they are spring loaded against the seat. If it's a spring loaded version then it will require a fair old pressure differential to pull fluid past. If both pumps are working, you will solve the fuel vapourisation problem. However because of the NRV problem you then rely on BOTH fuel pumps to be working and if one pump fails (twice as likely as you now have 2 of them) then you will conk out.

To solve the vapourisation problem you would really need to abandon the suction pump and just use a rear mounted 'pusher' pump rather than try and use 2 pumps and a NRV.
Last edited by doobry on Thu Oct 28, 2004 10:05 am, edited 1 time in total.
polo2k
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Post by polo2k »

i ment the secoandary pump near to the tank so in effect it will push the air lock through the pupm to vent to the carb. the by pass valce is required otherwise the secondary pump will just circulate the fuel round its "take off" pipes and the intermedary piece
- Ash
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    [IGOR] - 1970 trav (In Surgery)
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Kevin
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Post by Kevin »

Kevin - His diagram has 'word wrapped' so the 'CARB' should be on the far right!
Yes but it still shows the 2nd pump feeding the carb, and the non return valve wont help matters as its not so much a flow problem as an evaporation one.
And the main problem is not the pump but the pipework feeding it, and if you use a pusher pump as Cam has done you reposition the pipework anyway, which if you can use a mechanical one fitted to the block does the same thing in a simpler way as the pipework is much lower in the engine bay.
Just seen your update Ash, it would be easier just to fit a pushed pump (facet type) as Cam has done and do away with the original however this does alter the engine bay looks if you want to keep it original.
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Kevin
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Post by doobry »

And the main problem is not the pump but the pipework feeding it
I think you're missing something there Kevin?
The problem is fuel vapouristation, which occurs because of the mild vaccum and raised temperature.
What the diagram shows is a way to keep the fuel under raised pressure instead of a mild vaccum so that it should NOT vapourise, and then original pump will not flounder when it tries to pump fumes.

Anyway - as poer my previous post I wouldn't recomend it.
Cam
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Post by Cam »

Well.......... I have a pusher pump located next to the tank which pressurises the fuel through a braided hose to about 5.5 PSI. This hose follows the original lines (secured in the original locating tabs) up to a fuel pressure regulator which is fitted in place of the original pump. This then drops the pressure to below 2 PSI so that the needle valve in the float bowl can cope with the pressure, as it 5.5 PSI it would not be able to close.

As Ray said, the additional pressure prevents (or helps prevent) the fuel from vapourising. If you wanted to retain the original 'look' in the engine bay, then you could use the original copper fuel line with a section cut out next to the tank and a pusher pump in place (rated 2 PSI or slightly lower), and just have a 'dummy' pump under the bonnet with inlet connected to outlet internally (should not be difficult to modify).
Kevin
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Post by Kevin »

As we agree it a air temperature problem caused around the pipe feeding the pump, what is needed is an effective way of keeping it cooler in the area of the pipe as even the Grumpy`s kit which is really a deflector does not completly solve the problem, there should be an insulating type of product that should be able to solve the problem but as to what it should be.......
And I dont fancy lagging the exhaust manifold.
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Post by doobry »

As we agree it a air temperature problem caused around the pipe feeding the pump
it's a combination of air temperature and pressure (vaccum) so yes, if you want to keep the suction pump then the temperature is the bit that needs sorting.
Would it be possibe to run the pipe up the inside of the bulkhead, or would that just be a bit too unsafe?
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Post by Cam »

What about the fuel additives?? do they make the fuel any less prone to vapourising?
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Post by Stig »

Kevin wrote:As we agree it a air temperature problem caused around the pipe feeding the pump, what is needed is an effective way of keeping it cooler in the area of the pipe as even the Grumpy`s kit which is really a deflector does not completly solve the problem, there should be an insulating type of product that should be able to solve the problem but as to what it should be.......
And I dont fancy lagging the exhaust manifold.
I've got a length of split heater hose round the pipe, not perfect but I don't get any major problems. i.e the engine still runs well enough after sitting in traffic, but I do hear the pump going a bit happy once I get out of the jam.
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Post by Kevin »

I reduced the problem by using seperate inlet and exhaust manifolds but in traffic jams on hot days the pump still clicks away far more than usual.
Cheers

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