I would agree with the previous two posters, both out of respect for their vast knowledge of tractors and related equipment and my own knowledge of the chemistry of gasoline, propane, and diesel. Of the three, diesel has the most energy per given amount of fuel and propane has the least.
Propane is stored under a lot of pressure as it is. I don't know how much success you would have with trying compression ignition with the stuff. I suppose it would work, but it seems kind of redundant to have a highly pressurized fuel, decompress it to feed it into a cylinder with some air, then recompress it to cause it to ignite. The decompressing process absorbs energy in the form of heat from somewhere, either the atmosphere or the engine. Then you would need to put mechanical energy into it to compress the fuel/air mix enough to cause it to ignite to get chemical energy out in the form heat that is converted by the movement of the engine parts into mechanical energy again. I'm thinking the amount of energy you need to put in compared to the amount you get out is too great to allow a propane fueled Diesel cycle engine to be very efficient. -- Rudolph would not approve.
I have no idea how much compression you would need to ignite propane, but it's probably considerably less than regular diesel fuel, so it might tend to light off too early in the compression stroke, say when the piston is halfway up the cylinder, for a simple fuel conversion to work. I think you would need to change the valve timing or something to prevent preignition. I'm fairly comfortable in presuming the engine designers have looked at it and decided it's not a viable option.
