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Posted by Curbie on August 22, 2009, 3:40 pm


I also may be way off, but to my way of thinking, that seems like
what's going on. I would think we would want to fix volatility
regardless of temperature, and adjust viscosity based on temperature?


Posted by Bruce Richmond on August 22, 2009, 5:37 pm

I think the two are tied together so the viscosity is being used as a
measure of volatility.  It may not be perfect but it gets you in the


Posted by Curbie on August 22, 2009, 11:57 pm


This is the notion that has me confused, maybe you can clear my
confusion up by explaining how fuel volatility and viscosity are "tied

http://en.wikipedia.org/wiki/Volatility_ (chemistry)

To my way of thinking, volatility needs to be fixed to the point of
clean fuel internal combustion, viscosity needs to be maintained at a
point for proper external distribution and internally for proper
injector fogging.

Hopefully, you can clear this up for me.



Posted by Bruce Richmond on August 23, 2009, 1:58 am

If we were talking about entirely different chemicals there would be
little connection, but here we are just talking about hydrocarbons.

Just to be sure we are talking about the same thing


1. evaporating rapidly; passing off readily in the form of vapor:
Acetone is a volatile solvent.


a. the property of a fluid that resists the force tending to cause the
fluid to flow.

Methane has one carbon atom with four hydrogen atoms.  Ethane has two
carbon atoms connected together and three hydrogen atoms attached to
each carbon.  Octane has eight carbons plus three hydrogens attached
to each end carbon, plus two hydrogens attached to each of the other
carbons.  There are also more complex hydrocarbons that form rings and
such.  The important thing here is that the more atoms you hook
together the more difficult it is for them to move around as a
molocule.  Methane can bounce around easy and is normally found as a
gas at room temp.  Octane C8H18 is a liquid at room temp.  C20H42 is a
solid wax at room temp, but can be suspended in a liquid of smaller

Gasoline is a mix of many different length molocules, as is diesel.
The molocules in gasoline tend to be shorter making it evaporate
easier.  This is particularly important for spark ignition engines.
If the fuel stays in liquid form and isn't near the sparkplug when it
fires there will be a misfire.  And it needs to be mixed fairly
uniformly at near the righ ratio for it to burn correctly.

For a diesel a large area of the combustion chamber is hot due to
compression.  Spray a little fuel in there and it will burst into
flame reguardless of the air/fuel ratio.  And spraying it into the hot
chamber under high pressure breaks it into small enough droplets to

The sunflower oil contains more of the longer chains than diesel and
fewer of the short chains.  Some of the heaviest molocules are more
complicated than just a simple string and when burned tend to burn
slower and cooler.  Those are the ones that leave heavy carbon
deposits in the combustion chamber.  Adding gasoline *could* break
some of the more complex molocules into smaller ones.  In refineries
they do what is called "cracking" to get more short chains out of long
ones.  The short molocules in the gasoline bounce around with a lot of
energy and could break up the longer chains.  At the very least they
burn quickly in the chamber and will promote more compleate burning of
the heavy molocules.

As far as volitility is concerned I think it is easy to see how the
small molocules can break free easier than a long chain.  The moving
air molocules can knock the small ones right off the surface of a
liquid while the longer ones stay tangled.  Think of pop corn kernels
vs string beans or spaghetti.  If you leave a container of gasoline
open the lighter parts of it evaporate leaving it thicker.  Because
the parts that evaporate easy have left it, it will be more difficult
to get an engine to start.  The long molocules are more likely to
break up under high compression (which is good for deisel but bad for
spark ignition) but at the same time burns more slowly increasing the
chance of detonation of the end gases in the chamber.

For viscosity, I think it is easy to see how the longer chains tangle
around each other and slow the flow.  Spaghetti just doesn't flow
through a funnel nearly as well as corn.

I know some of the comparisons are pretty lame but I think it gets the
idea across.


Posted by Curbie on August 23, 2009, 8:23 am


Thanks you for all your time in presenting such a detailed
explanation, I greatly appreciate it!

I still have one more piece of the puzzle that doesnt fit, its
probably the crux of my confusion issue, and could you please take a
whack at clearing this up for me also???

When we add heat to SVO, as in a duel tank environment, we do so to
increase the fuels viscosity, correct???.

Is the heat shortening the hydrocarbons chains that recombine when
cooled or is something else at play there???



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