Except that the going back and forth between administrations will also kill any clear directive to the EPA on actually permitting diesel engines more efficiently. Currently, it's not the engine and emissions system that needs to be certified, but every model that has that engine for light duty vehicles. Separately. Only VW and Mercedes bothered (except one BMW model IIRC).
So basically, the market for EU-style diesel engines was killed by government "inaction" because of the opinion that "domestic" manufacturers had nothing to compete with that. Which also mallarkey, since both Ford and GM's European divisions have excellent diesel models of the vehicles sold here.
Contrary to popular belief, hydrogen burned in a conventional internal combustion engines produces more pollutants that actually matter than gasoline. Why? Because combustion temperatures are so high that oxides of nitrogen are off the charts. That's the stuff that gives cities their lovely orange haze, causes asthma, etc.
Elevated oxides of "other stuff" are also a problem in CNG and LPG vehicles.
Then there are issues with fuel density and storage. You need a few thousand psi to compress hydrogen to be liquid, and even so, it has a lower energy density than other fuels. So you need more volume, for which you need larger pressure vessels, and so on. So now 30% of your vehicle weight is hydrogen tanks. CNG has the same problem, and it's a deal breaker for most.
That leaves LPG. Here today, and engines built for it last nearly as long as diesels. And we have lots of it.
