He carries the balloon to his girlfriends place of work, and puts the balloon into the backseat of his vehicle. Both he and his girlfriend then proceed to drive to the site of their party. While making a turn, the balloon slides across the backseat and pops. The male reaches to open the electric window when the vapor mixture explodes and bulges the doors and roof, as well as blowing out all the windows. Glass was thrown about 200′. Fortunately no one but the occupants were injured. Slight burns and the Manufacturer of said device has a ruptured eardrum.
Gas/vapor explosions usually occur when you have the proper gas-air ratio, as well as an ignition source. The concentration of gas in the mixture must be within a certain range. If there is too much or too little gas a flame will not propagate through the mixture.
EXAMPLE: Natural Gas (Methane) has a lower Flammable limit of 5% and an upper of 15%, mixtures between 5% and 15% will propagate a fire or explosion depending on the mix. Methane Gas at ideal burning velocity is 10%.
Keeping the above explanation in mind we look at Acetylene: Lower limit of 2.5%, upper limit of 80%. Gas at ideal burning velocity is 9.3%
As you can see the mixture range for Acetylene is a better ratio for a fire or explosion. Anyone who welds knows the potential for problems with acetylene. In this case the mixture was powerful enough to blow all the windows out of the vehicle, and cause a reshaping of the roof and sides.
The information on flammable limits was obtained from:
The Investigation and Control of Gas Explosions in Buildings and Heating Plant
by R. J. Harris
This is a book published by “British Gas.”