Question

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Please explain why hydrogen fuel is much efficient using in a fuel cell than used in an internal combustion engine. If the efficiency is assume as e=Work/DH, can the efficiency of a fuel cell higher than 100%, if so, under what circumstance the efficiency of a fuel cell can be higher than 100%?

Answer 1

The primary reason for using hydrogen in internal combustion engines is that they already exist and are comparatively inexpensive. Since hydrogen combusts fairly readily, it doesn't take much in the way of modifications, mainly new fuel injectors and a storage system, to make hydrogen work in an ICE. This is, of course, a bit of an oversimplification. While the basics are the same, the combustion properties of hydrogen are very different from gasoline or diesel. It burns much faster than those fuels, so getting the most out of hydrogen in an ICE requires optimizing the shape of the combustion chamber and calibrating the timing of the spark in order to avoid damaging knock.
Contrary to our normal expectations, hydrogen can exhibit optical efficiencies well above 100%--but only if you re-define "input power" in a unique way.