Same as what Dave said. Liquid petrolium gas. Same stuff you can get in bottles to run a BBQ or patio heater. Not to be mistaken with butane! lol!! as it's a bit heavier..... that's about the extent of my knowledge
Currently in Mikey's garage: 1996 Mazda MX-3 V6, a 2002 1.2 8v Fiat Punto active (the Wife's!) and a 1998 Suzuki GN 125. Used to own a 2002 Laguna II Dynamique 1.8 16V (Now Sold yey!)
Yes it is called Liquified Petroleum Gas. I think it is a residue/leftover's from refining petrol or something like that. You have to have the filters changed about once a year, or more depending on the mileage, the garage told me that you will know as it will struggle going to and from gas and running, then bring it in, lol.
The filters clog up with a residue that solidifies or something. Thinnk he explained it like a dried fatty deposit.
Anyway, as promised, here are the pictures. I have also included a video of it changing from startup into LPG.
This is where I fill up, some cars have a round socket with the following adapter fixed into place.
This is the control in the car. A simple pressure pad button. The Video below show's it from starting the engine to going into gas. My engine was already warm so it didn't take too long to change. At the end of the video it is in LPG.
These are all the components under the bonnet within the engine bay. I hope the diagram isn't too confusing. If it is let me know and I will explain each pic for you.
Like I said, any more questions please ask and I will try my best to answer them.
Currently in Mike11's garage: An LPG powered Ford Focus ST170.