[Terr_]

A sibling comment mentions scrubbing NOx emissions as a significant issue, and it strikes me that the shipping/cruise industry is already a major polluter in that area. (Unlike cars, which must have catalytic converters because of local landlubber laws.)

[toomuchtodo]

High efficiency scrubbing appears to be feasible.

https://pubs.acs.org/doi/10.1021/acs.energyfuels.3c01419

[Terr_]

I guess my point is that cleaner technology is already being avoided by those companies, because they can save a buck by polluting in international waters.

[ForgedEngine]

NOxs are practically zero with HCCI at a stoichiometric ratio.