If you read the fuels pages it is pretty well explained why it is the volume of the air (hence the amount of oxygen) in the chamber that is by far the most important factor affecting the amount of energy you can get in the chamber. Different fuels typically have relatively little affect on the potential energy. The only real outlier in the spectrum of fuels is Hydrogen. Hydrogen is unique in two ways; (1) how fast it burns (very fast) and (2) how forgiving it is in the amount of fuel vs. the amount of oxygen in the chamber for a combustible mix. (But a stoichiometric mix will give the most energy in the chamber.)
http://www.spudfiles.com/spud_wiki/inde ... n_Spudguns
http://www.inpharmix.com/jps/Combustion_fuels.html
http://www.inpharmix.com/jps/Liquid_Com ... fuels.html
There is nothing intrinsically wrong with a liquid fuel but in practice there are at least two challenges;
1) The amount of fuel needed for a stoichiometric mix is typically very small. For a 100in^3 chamber the amount of liquid fuel is in the range of two or three drops or so.
2) It is difficult to accurately measure volatile liquids (like gasoline, alcohol etc.). If you try to use something like an eye dropper the fuel will partially vaporize in the dropper and be constantly piddling out even without squeezing the bulb (chemists have tricks for minimizing this problem). A syringe often has the same problem with a volatile liquid, you can't keep the liquid in the syringe because of its high vapor pressure.
The energy content of liquid fuels is no greater (and no less) than gaseous fuels like propane so there is no real reason to hassle with liquid fuels.
A reasonably accurate liquid fuel injector would be a pretty slick setup but with that setup you run into the third major challenge with liquid fuels -- making sure the fuel as completely vaporized before firing the gun. In an internal combustion engine the temperature is so high that vaporization of the fuel is not a problem. In a spud gun at typical temperatures complete evaporation is more of a problem. For a typical spud gun, especially with a chamber fan, there is probably enough time between fueling and firing for complete vaporization. Without the fan I would expect consistency problems caused by incomplete evaporation of the fuel.
