Energies per unit mass

From Academic Kids

The fuel value or relative energy density is the quantity of potential energy in fuel, food or other substance.

Some unit conversions:

  • 1 eV/u = 160.2177 zJ/u = 96.4853 MJ/kg

using:

    • 1 eV = 160.2177 zJ
    • 1 u = 1.66054 yg (or 1 kg = 602.2142 Yu)
  • 1 gray = 1 J/kg, 1 rad = 0.01 J/kg (for an absorbed dose of radiation)

Some values:


For rocket fuel a more relevant quantity is the energy per unit mass including oxidizer. For example, to burn 1 kg of hydrogen, 8 kg of oxygen is needed, so that the high fuel value reduces to 13.3 MJ per kg propellant.

Also relevant is the density: since that is very low for liquid hydrogen, the energy per unit volume is not so high, hence a large and therefore heavy tank is needed.

For projectiles, compare the value for TNT with the energy of a kinetic kill vehicle with a closing speed of e.g. 10 km/s, which is 50 MJ/kg.

The available energy from commercial explosives depends on their composition. The energy yield for ANFO is about 927 Kcal/kg depending on the heat value of fuel oil. Aluminised ones will yield as high as 1470 Kcal/kg (Brady et al, 1985).

NH4NO3 + 2/3 Al  = 2H2O + 1/3Al2O3 + N2 + 1975 (Kcal/kg)
NH4NO3 + 1/3 CH2 = 7/3H2O + 1/3 CO2 + N2 + 986 (Kcal/kg)

Relation with specific fuel consumption

Specific fuel consumption is the amount of fuel needed to do a given amount of work, e.g. a typical value for gasoline engines is 0.5 lb/hp.h = 0.3 kg/kWh, i.e. 1 kg per 12 MJ. The efficiency of the engine is the ratio of this 12 MJ and the 44 MJ mentioned above. In particular, part of the energy goes into heat.

See also

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