# Energies per unit mass

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.

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