Fuel Vaporisation Question

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Pyoor_Kate
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Fuel Vaporisation Question

Postby Pyoor_Kate » Thu Sep 30, 2004 5:11 pm

So,

Is the fuel vaporising in the pipe or in the pump, or in the carb? I've always kind of presumed it was in the carb and have been pondering rather radical solutions based on this, but wanted to check before I attacked the problem from this angle.

[This is because, if I'm suffering, as I appear to be, from quite noticable fuel vaporisation problems when it's just *warm* I fear next summer....]
Pyoor Kate
The Electric Minor Project
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1974 & 1975 Daf 44s, 1975 Enfield 8000 EV, 1989 Yugo 45, 1981 Golf Mk1, 1971 Vauxhall Viva, 1989 MZ ETZ 125, 1989 Volvo Vario 340, 1990, 1996 & 1997 MZ/Kanuni ETZ 251s
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Cam
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Postby Cam » Thu Sep 30, 2004 5:25 pm

It's in the pipe going to the pump and so the pump is trying to suck on fumes as opposed to fuel which it is not very good at. The copper pipe is situated quite close to the exhaust manifold so is prone to getting hotter than it should!

rayofleamington
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Postby rayofleamington » Thu Sep 30, 2004 11:08 pm

You will probably also get some in the carb, however if it was just int he carb, then the steady flow of cooler petrol would sort it out.
I can't see how it can be just the carb because if so the pump could push fuel through flat out and you would get steady flow of fresh fuel straight from the tank. If the carb was hot enough to vapourise all the the fresh fuel from the pump (over half a litre a minute) then your car would be a very different shape. BOOM! :o (and even without the explosion the whole street would smell of petrol)

Anyway - that's my theory on why it can't be just the carb.

If it's the fuel in the pipe (as per Cam's suggestion which I agree with) this accounts for the pump not being able to supply enough fuel, without a strong smell of petrol.

Pyoor_Kate
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Postby Pyoor_Kate » Thu Sep 30, 2004 11:28 pm

Yes, yes, that makes more sense.

Just seems so far away from the exhaust.....still, should be easier to cure :-)
Pyoor Kate
The Electric Minor Project
The Current Fleet:
1969 Morris 'thou, 4 Door. 2010 Mitsubishi iMiEV. 1920s BSA Pushbike. 1930s Raleigh pushbike.
The Ex-Fleet:
1974 & 1975 Daf 44s, 1975 Enfield 8000 EV, 1989 Yugo 45, 1981 Golf Mk1, 1971 Vauxhall Viva, 1989 MZ ETZ 125, 1989 Volvo Vario 340, 1990, 1996 & 1997 MZ/Kanuni ETZ 251s
Desires:
Trabant 601, Tatra T603, Series II Landy, Moskvitch-401, Vincent HRD Black Shadow, Huge garage, Job in Washington State.

Kevin
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Postby Kevin » Fri Oct 01, 2004 10:23 am

Its the sheer heat from the manifolds and it does radiate a fair bit and because the grill is open with air coming through the heat goes backwards in the direction of the pump and pipework, see how hot :oops: it gets in that area after the car has been run.
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MilitantGraham
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Postby MilitantGraham » Sun Oct 10, 2004 5:06 pm

I have heard that modern unleaded petrol is more prone to vapourising than good old fashioned 2 star.
I think most modern cars have an electric pump mounted near the tank to eliminate the sucking vapour problem.
One exception was the early Range Rover which had an electric pump under the bonnet and a return pipe with a restricter in it. This meant the fuel was continually circulating, the carbs would take what they needed and the warm petrol would get returned to the tank to cool.
Even when towing or offroading when the engine is working hard with no significant airflow from the movement of the vehicle, vapourisation is not a problem.

It would be possible to fit a return pipe to a Minor, but I don't know if the SU pump would like to work full time like that rather than the normal "as required" method.
Graham [img]http://www.aecmilitant.co.uk/pictures/animated.gif[/img]

Kevin
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Postby Kevin » Mon Oct 11, 2004 11:55 am

It would be possible to fit a return pipe to a Minor,
As the Moggie vapourisation occurs before the pump this wont help as there is nothing to return, the main problem is the location of the fuel pipe to the nearness of the manifold, on some engines its possible to fit a mechanical pump (marina type) if there is a blanking plate (not all engines have one) and then the fuel pipe is at a much lower position and the problem is removed.
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rayofleamington
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Postby rayofleamington » Mon Oct 11, 2004 12:06 pm

As the Moggie vapourisation occurs before the pump this wont help as there is nothing to return
not strictly true.
The heat in the pipe is part of the problem - the other part is that the fuel is not moving as you are at idle and not using much fuel. This gives the fuel a long time to heat up so recirculating would be an improvement.
I doubt the moggy pump would cope with it for 2 reasons. Firstly it does not create very highh pressure so the return pipe would create pressure loss and maybe you would end up with fuel starvation at motorway speeds. Secondly the extra duty to run the pump continuosly at speed would reduce the life of the SU quite dramatically.

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Postby MikeNash » Mon Oct 11, 2004 10:23 pm

I think this problem of vaporisation is worse than we know. This summer I've been trying to measure fuel consumptions at various speeds by measuring the fuelpump stroke rate. Great fun but bedevilled by the vapours even though the car (1963 1098 Traveller) has never, ever suffered a stoppage due to it. Fitting one Mr Grumpy's kits reduced it and wrapped both exhaust pipe and fuel pipe with bacofoil (to act as reflectors) where they come close. But I find that whenever the air temp is over about 20C I get still multiple pump strokes and even below it, it still occurs every time I pull away after waiting even for short spells at lights, roundabouts, etc. Now its cooler its generally better but today with the air temp at about 14C I got it again when bowling down the M3 at 60+ 'cos I've got my rad blank in to get the water temp up to 85C! The real complete cure I suspect means routing the fuel and placing the pump on the off side of the engine. The real cause I suspect is modern unleaded fuels. Is there a fuel chemist out there to advise?
You're not alone, Kate!

rayofleamington
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Postby rayofleamington » Mon Oct 11, 2004 10:36 pm

1963 1098 Traveller) has never, ever suffered a stoppage due to it
well it's not been such a big problem then. Any liquid will have a higher tendency to vapourise when it's under vacuum, and this tendency rapidly increases with heat. It's only really a problem if/when you get fuel starvation.
Unfortunately for those folks who commute in bad traffic it can be a pain, and reminds me of when I was commuting at 18 (15 years ago) and I would get fuel starvation at the end of a 25 minute queue to a busy roundabout (the queue was there every day, but fuel starvation only came on the hot days). I could always feel the harsh glares as my moggy spluttered around this busy roundabout at barely walking pace. The spluttering continued until 100 metres later when some fresh fuel got through.

If it's worse or not with modern fuels would be an easy question for the fuel companies to answer, however the traffic is steadily worse so it makes little odds what the fuel company says.
For anyone with a heavy commute in the uk I guess the heat shield will be enough - for the folks in Japan (very hot and much worse traffic) something much more drastic would be needed.
Last edited by rayofleamington on Tue Oct 12, 2004 9:00 am, edited 1 time in total.

MikeNash
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Postby MikeNash » Mon Oct 11, 2004 10:56 pm

Ray.
Yeah, perhaps I've been lucky. I'm not in big towns. But my point is that even with the kit fitted and the pipes wrapped it still goes on, and on days in the summers of air temps in the upper twenties any journey could easily double the number of pump strokes. So for parties who can't keep moving most times the problem is probably acute. I've only done crude fuel temp measurements (which is the critical thing) and suspect that when the fuel gets above 24C we're in trouble. But more measuring's needed to be sure.
Regards, Mike N.

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Postby MikeNash » Tue Oct 12, 2004 8:08 pm

Thinx. If we speed up the flow up the pipe to the pump then it'll have less time to evaporate. How about a smaller bore pipe like brake pipe? A quick look suggests its about 2/3 the external diameter, so say 1/2 the internal cross section, so the petrol goes up at twice the speed, i.e. in half the time! Gotta be good! Regards, Mike N. (And insulate and shield as well of course.)
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Postby ColinP » Tue Oct 12, 2004 8:38 pm

Mike,

If you have 1/2 the internal cross section, how much extra "suck" do you need to pull the fuel through the pipe? Can the old SU cope?

Perhaps we should all check our fuel lines to see if they are slightly kinked or flattened, then blow through them to make sure there's no grit/rust flakes clogging them up?

Colin

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Postby Chris Morley » Wed Oct 13, 2004 1:25 am

I wonder why some Minors have this problem? It clearly isn't a basic design problem because my Minor (without any shielding) doesn't have this problem in normal summer conditions. The only time I ever had it was a day in July 2003 in the early afternoon when the ambient temperature was between 36 and 37C (the record heatwave).

Perhaps it's down to poor efficiency of the engine cooling system, allowing under-bonnet temperatures to rise beyond normal? Or perhaps the use of 95 octane fuel which also causes the engine to run hot?
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Postby Pyoor_Kate » Wed Oct 13, 2004 11:10 am

Chris, certainly this problem coincided with my change to Unleaded+additive (instead of LRP). I'd not encountered even the slightest problem with LRP fuel.

But it also coincided with filling the car up at [a cheaper supermarket] instead of my normal [other supermarket]; so I'm suspecting it might also be related to that. We'll see, I've run the tank right down (I usually do anyway); so this next lot should give me a bit information. On the other hand it's definately colder than it was. So, it might tell me nothing at all.
Pyoor Kate
The Electric Minor Project
The Current Fleet:
1969 Morris 'thou, 4 Door. 2010 Mitsubishi iMiEV. 1920s BSA Pushbike. 1930s Raleigh pushbike.
The Ex-Fleet:
1974 & 1975 Daf 44s, 1975 Enfield 8000 EV, 1989 Yugo 45, 1981 Golf Mk1, 1971 Vauxhall Viva, 1989 MZ ETZ 125, 1989 Volvo Vario 340, 1990, 1996 & 1997 MZ/Kanuni ETZ 251s
Desires:
Trabant 601, Tatra T603, Series II Landy, Moskvitch-401, Vincent HRD Black Shadow, Huge garage, Job in Washington State.

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Postby ColinP » Thu Oct 14, 2004 10:08 pm

Well,

here's a couple of external links:

http://www.chevrolet.com.au/articles/petrol.html

http://www.british-cars.org.uk/kimber/m ... 990710.htm

It looks as if the fuels may have changed from the 50's

Colin

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Postby ColinP » Tue Oct 26, 2004 8:50 am

Ok,
with thanks to Shell technical helpline:

1) Modern petrol meets the standard of BS EN 228.
This has requirements for the vapour pressure (i.e. the pressure of the vapour of the liquid. If the vapour pressure = the external pressure the liquid will boil).

As far as I can make out, there are three standards for vapour pressure:
a) summer 45 - 70 (kPa) - I convert to 6.5 psi - 10 psi
b) Winter 70 - 100 (KPa) - 10 psi - 14.5 psi
c) intermediate 45 - 100 (kPa) - 6.5 psi - 14.5 psi
The temperature of this measurement is 38.8C (100F). So, both the winter grade and the intermediate will boil at 38.8C (14.5 psi = "normal" atmospheric pressure.

That explains the comment earlier about "40C" temperatures, and the vapour locking.

If you can find a garage that stocks "summer fuel", it will be much more resistant to vapour lock than the winter. Unfortunately, the BN is set as a standard, so all the petrol producers produce petrol to this standard....

COlin

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Postby Cam » Tue Oct 26, 2004 9:54 am

Blimey, 100F sounds quite low. Any idea what the boiling point of the old style 4 star was? (or the earlier 50s petrol if it was any different) as a comparison?

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Postby Kevin » Tue Oct 26, 2004 9:56 am

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 ie: 14.5 psi = 14.5 psi, so the summer fuel (if it available in reality) would be at a lower pressure so the problems will not occur in spite of the external pressure being higher ie: 14.5 psi = 10 psi.
So does this mean we need to keep the external pressure higher to avoid vapourisation and if so what is the best way to achieve it as the heat shield kits dont seem to be effective.
As you can tell atmospheric pressure is an area I am not at all familiar with as I would have thought that the higher external pressure would have caused more problems than lower ones.
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Cam
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Postby Cam » Tue Oct 26, 2004 10:01 am

Well, I understood it as 100F is the temperature at which it starts to boil IF the external pressure (atmospheric) is 14.5 PSI.

If you increase the pressure then the boiling point will rise and so be less prone to boiling at engine bay temperatures.


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