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Diesel doesn't solve any ecological, political or financial issues for the world. It's just another way of refining oil.

Well, higher efficiency does mean a bit lower CO2 emissions and marginally less reliance on oil imported from volatile parts of the world. Also, as diesel engines can run on 'bio-diesel', that might come handy for countries with excess agricultural production potential (and the fuel is renewable). Of course, contribution to solving the world's energy problems is marginal at best (not even on par with wind, 'direct' solar, geothermal and similar sources).

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Hydrogen is probably the near-term "answer", but you need to make electricity to make hydrogen, which, at least in North America, is over 90% coal generated. If we can ever figure out how to make non-hydrocarbon electricity on a large scale (hyrdo doesn't do this), then hydrogen represents a great way of transporting the engergy.

Why do you think that hydro is not the answer (not everywhere, of course, but, say, in Canada and Norway)? I don't see anything but nuclear energy (hopefully fusion relatively soon, but fission for now) as the only long-term solution.

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Airplanes are a bigger issue. The energy density of hydrogen is quite low, so what do the airplanes run when the oil is gone?

Hydrogen transport and storage is difficult enough for ground vehicles... Saving fuel for airplanes is good reason to try to preserve oil, but its value as raw material for fertilizers and plastics is much more critical. It would also make sense to conserve coal as petrochemical raw material. (Hm, I have a vague feeling of deja vu; we must have discussed this before )
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