Spray and pray
Spray and pray (or spray & pray or spray 'n pray) cannons are the most basic combustion cannons. It is the simplest way of fueling a cannon, and can be very inconsistent. You first spray in fuel at the back of the cannon. This basic fuel can be anything that is combustible and in a spray can, aerosols like hairspray or deodorant are mostly used. The cannon has a threaded end cap which is removed and is put back right after the fuel was sprayed in. Because getting the right amount of fuel (stoichiometry) in is very hard with this method you often have got a poor mix. It doesn't always have the same power, and sometimes the amount of fuel is so much off that the cannon wont even fire.
Its called spray and pray because you first spray in your fuel, and then pray because you don't know if its going to work and just hope for the best.
An upgrade from this type of fueling is building a fuel meter on your cannon. With that you will always inject the right amount of fuel.