The price of diesel is in the $4/gal range and may stay there quite a while.
Around here, CNG sells for about $2/gal (gasoline equivalent) so it is worth a thought. The EPA is still making it difficult for one to do CNG conversions commercially, but that may change in the next year or so.
Diesel engines are naturals for natural gas because they have high compression ratios. Compression = efficiency.
Diesel conversions today are what anybody who has ever been in the oil patch calls "dual-fuel." The octane rating on natural gas is so high (~120 motor octane) that the stuff simply will not compression-ignite. It need something hot to make it burn. A "dual fuel" engine uses the diesel injector as a spark plug. Just a small squirt of diesel compression-ignites and blow-torches the natural gas into combustion.
Diesel CNG conversions use either a "fumigation" system that has a single point of natural gas injected into the intake manifold (technically an "air box") with what looks a lot like the throttle body injection used by older gasoline engines. The "electronic carburetor" if you will. The other alternative is the "port injection" option. Here, a port injection hole is drilled and tapped into the intake manifold and the injector is aimed at the intake valve. Just like with port injection gas engines.
In either case, the diesel injectors are reduced in output to a minimum volume squirt per combustion event and the engine output is controlled by varying natural gas flow. In either case, a throttle is not needed because the diesel "spark" is hot enough to fire the leanest or richest mixture of natural gas and air, so the engine always operates under an "excess air" situation.
In the short term, this is the conversion we will see if the EPA ever gets out of the way.
But for new engines, maybe you don't want to have the expense and complication of two fuel systems.
You could make a diesel into a very high compression spark-ignition engine by pulling out the diesel injection system and replacing it with a strong spark plug. But now you need to control air:fuel mixture to assure combustion unless you have a spark with the power of a lightning bolt. Most big diesel engine manufacturers have natural gas engines that are exactly that: Diesel engines with spark plugs.
The diesel racing community has some sharp cookies. I'd bet they could come up with a spark plug top replace injectors.
Another thing holding up CNG in diesel trucks is tankage. CNG tanks are necessarily pressure vessels. They are pretty much identical to SCUBA tanks. Heavy, bulky, and expensive.
I looked into CNG tanks for my truck. Because i drive so much I really need a minimum driving range of 500 miles. Using the "dead cat
space" in my short bed, I could piece in the 24 gallons of tankage I needed but it rapidly got to be a $2,000 project. If CNG is gonna take off this has to be improved on.