This engine looks as if it was built with a CO2 cylinder, but actually has a steel cylinder which was filled with gasoline, which was placed over a heat source in the steel tray below. The heat source caused the gasoline to vaporize and spray back into the tube at the rear, where combustion took place. Although the device worked, it didn't produce a lot of power and also doesn't really seem to be the something that kids should have played with, since it was not only a potential fire hazard, but possibly could explode.