OK. How efficient is hydrogen, really? Shout out to all of the chemistry majors out there who might answer this.
One of the reasons that fuels work, from my understanding, is that you start with a small number of molecule, combust them, and get a larger number of molecules with more heat. The heat increases the pressure, and the increase in the number of molecules increases the pressure.
Example: combustion of alcohol:
C2H6O +3O2 --> 3H2O + 2CO2
We start with four molecules on the left, and get five molecules on the right. Even if the reaction was not exothermic, we would still get a pressure increase good for pushing a piston.
Now, when we burn hydrogen, we get a decrease in the number of molecules (goes from three down to two):
2H2 + O2 --> 2H2O
So, yes, we get increased pressure due to heat production, but we get decreased pressure due to fewer molecules.
So, I guess that my question is: when burning a fuel, how much pressure created is due to the typical increase in molecules, and how much pressure is due to heat?