I've had several experiences with propane and gas engines. When I want to start a lawn mower or snow blower, propane directly into the carb will allow the engine to start and run with no need for gasoline at all.
Down in Tennessee I saw the most remarkable natural gas practical application. A V-8 car engine was mounted on a skid with the fan belt connected to a high volume compressor (rather than the power steering). There was no gas tank at all. The air cleaner was removed and right in the four barrel carb was placed an outdoor spigot held in place with soft putty. Natural gas coming right out of the well at ~80 psi was directed into the carb. The volume of gas was controlled by the spigot handle. The engine ran smoothly at about 1000 rpm powering the compressor. It ran for weeks on end.
Down in Tennessee I saw the most remarkable natural gas practical application. A V-8 car engine was mounted on a skid with the fan belt connected to a high volume compressor (rather than the power steering). There was no gas tank at all. The air cleaner was removed and right in the four barrel carb was placed an outdoor spigot held in place with soft putty. Natural gas coming right out of the well at ~80 psi was directed into the carb. The volume of gas was controlled by the spigot handle. The engine ran smoothly at about 1000 rpm powering the compressor. It ran for weeks on end.