Gasoline liquid evaporates very easily to gasoline vapor. Gasoline vapor is highly flammable. So if you take gasoline vapor, mix it with air, and then cause a spark, it will burst into flames. If you put that explosion into a sealed container, that burst of flame becomes an explosion. If you only partially seal the container, plugging one side with a movable piston, the explosion will push the piston outwards. If you hook up that piston to a rotating crankshaft, the motion of the piston blowing outwards from the cylinder can be used to turn the crankshaft.

If you use some mechanical or electronic systems to regularly put gasoline vapor and air into the cylinder, spark it to ignite it, and then open a vent to get rid of the combustion gases, then you can turn that gasoline liquid/vapor into steady rotational energy in the crankshaft. You can hook the crankshaft up to whatever you want; a transmission to power a car, or an alternator to generate electrical power.