RE: Overheating '67

[James MacNaughton <JMac@uglyduckling.com>]

Okay, I understand Chem 101, no problem; you speak truly to my understanding
(which is getting further away).  I will now have to see if I can't witness
this phenomena.  (do-do-dee-doo-do)

	-----Original Message-----
	From:	Jim Adney [SMTP:jadney@vwtype3.org]
	Sent:	Sunday, April 18, 1999 6:11 PM
	To:	type3@vwtype3.org
	Subject:	RE: Overheating '67

	On 18 Apr 99, at 11:34, James MacNaughton wrÝöK— 	<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*?  I really 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 beұIame (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 just below the 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
	-
	*******************************
	Jim Adney, jadney@vwtype3.org
	Madison, Wisconsin, USA
	*******************************

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