Use Lysol spray. I used to make road rockets by punching a hole in the cap of a plastic 2 litre pop bottle with a nail and then spray 3 half second shots of lysol spray into the bottle. Put the cap back on, lay it on a smooth surface (on its side) and light it with a match. It makes a very loud swoosh with lots of blue flame as it rockets away. If you get the mixture of air and spray right the bottle will actually collapse after the fuel/air mixture is burnt due to the vacuum created inside the bottle.