LPG fumigation has been succesfully adopted to Diesel engines.
It works like CNG, the kit is connected to the intake manifold, which injects LPG into the air sucked in by the engine, and when compressed it creates more heat, and so ignition can be achieved with quite less diesel being injected in the cylinder. This can also be used as a performance mod.
You cannot control combustion let alone a diesel engine on CNG/LPG only.