The mixture is controlled by controlling fuel rail pressure, and having calibrated fuel injectors for each cylinder. A realtime control system resolves the crankshaft position continuously. Fuel timings and quantities are controlled by when the injector is opened relative to each pistons position in its cycle, and how long it is held open. There are A LOT of other factors involved, but that's it in a nutshell
CNG and propane can work well in diesel engines. If the current boon in natural gas continues and the eco-weenies don't put a clamp on it, it'll be a good direction to go.
You have a fuel rail in an old mechinical system, after that they were controlled by a rotary injector pump, have one of those on my case tractor. electronic enjectors have oil pressure to fire them around 3500+ psi, the same amount of pressure on each injector, the fuel is run into the injector by the fuel pump, the electric switch on the top of the injector controls firing and length of firing. Have had my powerstroke apart several times. And the system is the same on all the new big trucks.