RE: Overheating '67

["Jim Adney" <jadney@vwtype3.org>]

On 18 Apr 99, at 11:34, James MacNaughton wrote:

> 	<My comments below...>
> 
> 	The theory is that when you are relying on the pump to SUCK the gas 
> 	from the tank that you can only such so hard before the gas 
> 	vaporizeds and you are sucking vapor. Now suddenly the only thing 
> 	coming out of the pump is vapor and this doesn't sit nicely in the 
> 	carb float bowl anymore. 
> 
> 	>>>>Okay, I definitely understand the principle (and big kudos to
> you, as you're the first in my relatively meager 16 years of working on cars
> that made an intelligent response to my dad's prejudices); but, how would it
> vapourise the fuel by sucking on it?  Wouldn't that require *some* air leak
> *somewhere*? \vÒy do want to know, because a *lot* of people believe in
> vapour lock, and while I've never (with the help of my dad) 'had' vapour
> lock, my dad *is* human.

Imagine a closed container partly full of gas. Suppose you removed 
all the air from above the container. When you do this, some of the 
gas would evaporate and the gas vapor would fill that space. The 
pressure that the vapor would make would be less than the 1 
atmosphere of pressure that the air had before. The pressure you get 
is called the "vapor pressure" of the gas, and that pressure is 
highly dependent on temperature.

If you can imagine trying to pump out the gas vapor, then you will 
see that more gas will just vaproize to replace that which you 
removed, so it will never be possible to pull the pressure lower than 
the vapor pressure of the gas.

Now lets say that the vapor pressure of gasoline at 0 C is 1/2 
atmosphere. That means that if my fuel pump has to pull 1/3 atm. of 
suction to get the fuel from the tank then the pressure at the pump 
inlet is 1atm - 1/3 atm = 2/3 atm. So the (absolute) pressue at the 
pump inlet is higher than the vapor pressure. In this case the gas 
stays a liquid and everything works fine.

Now let us drive on a hot day and the vapor pressure of the gas is 
3/4 atm. Now the pump inlet (absolute) pressure still needs to be the 
same (2/3 atm.), but the vapor pressure of the gas is now higher than 
that. In this case the gas will boil and only vapor will come into 
the pump.

As I write this, I realize that this is not going to be clear to most 
of you. So just let it be said that it is possible, and it does 
happen. It is worse on warm days or in warm engine compartments, but 
the only temp that matters is the temp of the gas in the input line 
to the pump.

Carbed engines almost universally use a mechanical fuel pump bolted 
to the engine, so these have to suck fuel from the tank. Once FI was 
introduced with the necessity of an electric pump, the pump could be 
mounted anywhere. The obvious choice is justÇÁ®¶he gas tank where 
the gas will just fall into the pump.

Modern FI cars almost all put the pump inside the tank on the bottom.

Jim
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Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA
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