But remember about 10 years ago when BMW produced a 7 series that could run on either petrol or hydrogen (there was a robotic hydrogen filling station in Southern California around Oxnard, if I recall correctly). It was not optimized for either, so had relatively poor efficiency for each respective fuel type. Nonetheless, it was cool that you swap back and forth on the fly. Internal combustion engines can exist in the zero emissions world. It just may not be particularly practical.
Thanks,
Brian