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?

  • $\begingroup$ 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. $\endgroup$
    – wwarriner
    Feb 13 '16 at 23:49
  • $\begingroup$ Interesting ! so much of the energy of hydrogen is lost as heat transfer rather than useful mechanical work ! $\endgroup$
    – Fennekin
    Feb 14 '16 at 4:45

One consideration is the relative densities of hydrocarbon fuels compared to hydrogen. If we assume that the fuels are completely vapourised in the combustion chamber then the mass of fuel depends on the volume, temperature and pressure. So energy density per kg doesn't necessarily tell you much about how many molecules of fuel you can get in one charge or indeed the chemical energy available per molecule.

There are also issues in IC engines relating to the combustion properties of hydrogen. One particular problem is that hydrogen will detonate at a broad range of air/fuel ratios which has knock on effects for ignition timing and compression ratios which in turn affect how well the chemical energy available from combustion can be converted into useful mechanical power. Premature detonation under compression (especially in hot cylinders) is a particular problem with hydrogen.

Hydrogen is often associated with low compression ratio engines like Wankel (and other rotary) engines for exactly this reason.

Hydrogen also burns hot compared to other fuel gases, so even when overall energies are comparable hydrogen engines can produce localised hot spots in the combustion chamber.

Gasoline is a mixture of fairly short chain hydrocarbons which can be approximated to C8 but the specific mixture of different molecules has a significant effect on its combustion behavior and there is quite a lot of scope for adjusting a particular fuel formulation to get the best performance. This is much more difficult with hydrogen.

  • 3
    $\begingroup$ 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... $\endgroup$
    – Dan
    Feb 15 '16 at 2:37
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    $\begingroup$ Remember also that power of an engine has something to do with pressure work. Gasoline combustion produces many more moles of gas than the precombustion state. $\endgroup$
    – tillmas
    Apr 9 '16 at 16:06

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