# Why does hydrogen fuel produce less horsepower than gasoline in internal combustion engines?

I was looking at hydrogen as alternative fuel and came across an article stating that although $\textrm{H}_2$ produces more energy than conventional fuel, it produces less horsepower in internal combustion engines.

I thought that more energy ($143\ \textrm{MJ/kg}$ for $\textrm{H}_2$ vs $46.4\ \textrm{MJ/kg}$ for gasoline) would mean more heat, more temperature, more pressure and thus more force and horsepower. Why, then, does $\textrm{H}_2$ produce less horsepower than gasoline in internal combustion engines?

• I have very limited knowledge myself, but I am not sure the assertion that more energy yields both more heat AND more pressure is true. The ratio of thermal energy to mechanical energy isn't necessarily a constant across all combustion reactions, or at least I can't see any reason why that would necessarily be so. – wwarriner Feb 13 '16 at 23:49
• Interesting ! so much of the energy of hydrogen is lost as heat transfer rather than useful mechanical work ! – Fennekin Feb 14 '16 at 4:45

• Just to quantify that first point. For stoichiometric conditions the mole fractions of $H_2$ and $C_8H_{18}$ are 0.30 and 0.017. Factor in the molecular weights, and you've got about 3 times as much gasoline by mass in a given volume of fuel-air mixture. That basically balances out the difference in $\Delta H_c$ s.t. both have the same energy. Then, as Chris says, there are practical issues with hydrogen... – Dan Feb 15 '16 at 2:37