Propane can be carried in a bucket at about -40F. Above that temperature, it starts to boil (vaporize) at atmospheric pressure, but on engine applications, a vaporizer, sometimes heated by the coolant system is typically used to get enough vapor to run the engine. Liquified fuel is usually delivered to the vaporizer on the engine. We've worked with LP fueled generators for remote communications sites and in order to get enough LP from the tanks to the engines, had to go with liquid draw from the tanks and vaporizers on the engines. Temperature dictated that approach even to get enough fuel thru the pipeline Even making the pipe 6 inches diameter wouldn't have delivered enough gas, because there wasn't enough heat available to vaporize it at the tanks. These were only 12kw gensets also.
Natural gas (mostly methane) has slightly more energy density (power) than gasoline and propane has just a touch more, but the problem is getting that energy to the engine. Gas (vapor) weighs a lot less than liquid fuel.
