First thing guys, I want to apologize to the board for the ongoing argument between myself & Robert. It's counter-productive.
Ok that said, duggyy's original question re: the Cumins 15L spark ignited engine. As I said I believe Westport declined to participate in that venture as they felt it filled too small a niche. That thread went downhill fast. For those that question whether I am correct in saying that the Westport 15L HD requires LNG you simply need to go Westport's site go to the link labeled "products", click on "engines". The first engine at the top of the list will be the 15L Heavy-Duty (NA) followed by the 15L Heavy-Duty (AU). If you click either one of those links you'll find a basic overview of the engines. Where it says "Fuel Type(s)" you will see LNG.
Just LNG......... not LNG or CNG as listed for, as an example, the Cummins-Westport ISX12 G.
(Or check it out on you-tube. look for "Overview of Westport's HD 15 L engine")
So, that would make the ISX12 G is the largest spark ignited Cummins-Westport engine being built. Apparently Cummins sees an opportunity there for a larger engine, that can utilize cheaper CNG & Westport felt that their efforts were better focused elsewhere.