Westport injects both diesel and CNG into the cylinder relying on the high compression ratio to self combust both for commercial HD applications. First they inject a little diesel then followed by CNG. NO spark plugs or coil packs required but the injectors are quite complicated. Considering that they have an OEM agreement with cummins means it is a feasible product already in service and production. I would consider hydrogen fuel cell technology a novelty and still in its infancy despite zillions of dollars invested by major OEM's. The Westport solution is a proven product.
In lighter duty applications they offer Ford Superduty line up with gas motor (plugs and coils) which further proves the product.
CNG diesels in the industry have been a novelty up to this point, anybody that works in the trucking industry or truck manufacturing industry will agree. There are less than ~20,000 CNG units on the road in the US and Canada where millions of diesel medium and heavy trucks roam. 20K is a drop in the bucket and a novelty like electric or hydraulic hybrid trucks.
I am not familiar with the entire Westport Cummins history to know how they mixed the diesel/CNG in the early versions but I can tell you the current and future CNG engines based on diesel engines at the genesis of the Green Truck Summit and on the show floor do not run a diesel fuel/CNG mixture and are indeed spark ignited. They are dedicated CNG engines, basically overgrown gas engines based on big bore diesel engine architecture.
Unfortunately, Diesel fuel isn’t consider “Green Technology” and today’s stringent diesel emissions standards are making manufactures take a closer look at other fuels as there is only so much you can do to clean up a diesel engine until you take the “diesel” out of the equation. This does not mean diesel engines running on diesel fuel are completely going away, just plan on seeing more and more CNG dedicated “diesel” engines coming into service as large fleets, municipalities, and manufacturer’s join the Go Green theme and continue to feel the squeeze from emissions regulations as well as seek a reprieve from high cost of diesel fuel.
Here is an excerpt from the ISX12 G (12L) webpage at Cummins Westport:
Featuring proven Stoichiometric cooled Exhaust Gas Recirculation (SEGR) combustion technology, spark
Ignition and simple Three-Way Catalyst (TWC) aftertreatment, the ISX12 G delivers strong pulling power and heavy-duty durability and performance that customers can depend on.
The ISX12 G is a dedicated natural gas engine, based on the Cummins ISX12 diesel engine, that operates on 100 percent clean-burning, low-cost natural gas. The ISX12 G is manufactured on the same assembly line as the Cummins ISX12 engine.
Excerpt from the ISL G webpage:
The ISL G is capable of operating on compressed or liquefied natural gas (CNG, LNG). The ISL G can also
operate on up to 100 percent biomethane – renewable natural gas made from biogas or landfill gas that has been upgraded to pipeline- and vehicle fuel-quality. (no diesel)
The ISX12 G and ISL G are just a couple of many dedicated spark ignited CNG “diesel” engines that will be on the market in short time.
See those 6 round things on the valve cover? Those are coil packs and there are spark plugs about 5 inches below them.