LPG to NG

If a person had a source for free natural gas and could get the equipment to deal with compressing the natural gas, storing it and using it cheaply enough, it might make sense to try it in a tractor or other movable engines.

But natural gas: methane has a lot less energy potential than an equal volume of propane. I would think that the existing mechanism for using propane would have to be modified to add a lot higher volume of natural gas over what it now handles with propane. And of course you would need the proper pressure regulators to get the highly compressed natural gas pressure down to where the tractor could handle it efficiently and safely. All this sounds expensive and complex to me.

Propane or natural gas would have some advantages--they do have higher octane ratings than gasoline and would allow the use of higher compression ratios. And using a gaseous fuel cuts down on wear to the engine and much reduced contamination of the lubricating oil so in theory an engine should last longer. But at today's high propane prices, I doubt that I would ever convert any of my engines to run on propane.

One other thing about using compressed natural gas: it requires special equipment to compress it as you describe. I'm sure this would be several thousand dollars, unless you happen to have a CNG filling station next door. Propane, of course, can be filled directly from a bulk tank.