i would say that an air fuel bomb is NOT hard to set off
getting the right mixture of air and fuel is the key
a grain elevator explosion is an example
sometimes a spark does nothing ,but if things are just right you get an explosion
put 25 lbs of calf milk replacer in a plastic bucket with a small charge and an electric squib
makes a great 4th of july bang
use a long wire tho
setting one up in a van with a gas would be a lot harder i would guess
but i do know a man who knocked his house off its foundation trying to light a propane water heater with so little gas in the basement that he could not smell it
he was lucky he lived