>I was wondering if someone out there could tell me the mechanism behind
>that fuel additive that you buy to remove water from your fuel system. I'm
>guessing there's some sort of hydrolysis (maybe make a fatty acid or
>something) or maybe addition to an alkyne or something. (I _am_ just a cell
>biologist though.)

No. it's purely to do with improving the miscibility of the normally-immiscible fluids. Let' start with the Gasoline FAQ, ( available from the FAQ archives at rtfm.mit.edu in
pub/usenet-by-hierarchy/rec/autos/tech.

[ begin extract ]

8.7 How can I remove water in the fuel tank?

If you only have a small quantity of water, then the addition of 500mls of dry isopropanol (IPA) to a near-full 30-40 litre tank will absorb the water, and will not significantly affect combustion. Once you have mopped up the water with IPA, small, regular doses of any anhydrous alcohol will help keep the tank dry. This technique will not work if you have very large amounts of water, and the addition of greater amounts of IPA may result in poor drivability.

Water in fuel tanks can be minimized by keeping the fuel tank near full, and filling in the morning from a service station that allows storage tanks to stand for several hours after refilling before using the fuel. Note that oxygenated gasolines have greater water solubility, and should cope with small quantities of water.

[ end extract ]

So your question may be " why IPA, why not methanol or ethanol, which are usually cheaper ?". The answer is dependent on two aspects, the actual ability to act as a co-solvent, and the effect of temperature on the phase separation point of gasoline/co-solvent mixtures. The aromatics content of the fuel also has a profound effect on the amount of water that can be tolerated, with 25% aromatics tolerating about 0.1% water and 40% aromatics tolerating about 0.2% water at 15C.

Adding 15% by volume of anhydrous methanol to a typical non-oxygenated gasoline will provide a water tolerance (the amount of water that can be added before phase separation occurs) to around 0.16% at 15C, but at 0C the tolerance would be close to 0%, i.e. the water/methanol would be a separate phase without the addition of water. Adding more that 15% of methanol can adversely affect drivability on engines without management systems designed for oxygenated fuels.

Adding about 3% by volume of anhydrous isopropyl alcohol will provide a water tolerance of about 0.35% at 15C and about 0.2% at 0C, thus its ability as a co-solvent is superior on a volume basis, and far less temperature sensitive than methanol.

Ethanol is between the two, with a 15% blend tolerating about 1% water at 15C. The best additives are the isomeric-propyl and butyl alcohols, and above them, the tolerance decreases as the alcohols become less water miscible.

The above numbers are from memory, and are approximate, but you get the general idea. The experiment is easy to do if you want real numbers for your local gasoline. Take 100 MLs of gasoline (remembering that it's highly flammable) in a stoppered glass ensuring cylinder and measure temperature.

Add one drop (ideally a 1ml syringe, but any narrow tubing or dropper can be used, and use 20 drops of water = 1 ml ), stopper, and shake. If the solution goes hazy, warm slightly in a bucket of warm water and measure the temperature that it clears.
If it stays clear, reseal and put it in a fridge or freezer (in a container that will contain the fuel should the glass break - and remember that all vapour will be flammable and that the controller and light use non-flameproof switches).

When hazy, remove and record temperature that the haze clears, and if it's still clear, add another drop and repeat.

Now repeat adding 5, 10, 15% anhydrous MeOH, EtOH, or IPA (remembering that many IPA products (e.g. as an antistatic) actually are not anhydrous, and can contain up to 40% water - which somewhat defeats the purpose :-) ). You can draw a graph of water tolerance versus temperature that shows the different positions and gradients of the alcohols.

Bruce Hamilton