Q. Why is water in the gas tank a problem particularly in winter, and how
does adding moisture (dry gas) to two liquids (gas and water) remove moisture
from the tank?
M.P.
Brookline
A. David Williams, a chemist with the Coastal Unilube Company, says you've
usually got water in your gas tank and your gas lines. Small amounts aren't a
problem most of the time. But in the winter, water can condense and freeze up
in your gas line. Then you've got a problem.
The water comes from humidity condensing into liquid, either in your car's
fuel system or in the underground tanks at the gas station.
Water is heavier than gasoline and normally settles to the bottom of the
filling station's underground tanks. But when the tanks are refilled, the
water is stirred up. The more recently your filling station has been
resupplied, the more water you'll be getting in the gas you buy.
Once it's in your car, water settles to the bottom of your gas tank, too,
but driving around mixes everything up. Small amounts vaporize out into the
fuel lines, and in the winter that condensate can freeze.
Now, how does a liquid added to two liquids remove a liquid? It doesn't.
Williams says so-called dry gas-type products are alcohols, usually isopropyl
alcohol or methanol. (The molecular composition of isopropynol, made up of
carbon, oxygen and hydrogen, is set up in such a way that one end bonds with
the carbon and hydrogen in gasoline, and the other end bonds with the hydrogen
and oxygen in water. You end up with one long chain. . .the gasoline on one
end, the water on the other, and the alcohol in the middle holding them
together. The methanol doesn't bond with the water, but it reduces its surface
tension so it stays suspended in the gasoline, kind of like little globs of
oil in vinegar after you shake up salad dressing.
Then, when the fuel pump draws gasoline to the engine, it draws the water
along with it, where everything gets burned up.
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