A.3.14—Fuel energy density

Energy per volume

For the current IB Physics definition, fuel energy density $u$ is the transferable energy per unit volume:

$$u=\frac{E}{V}$$

Its SI unit is $\mathrm{J\,m^{-3}}$. This lets fuels be compared when storage volume is the constraint.

Connect it to a fuel flow

If fuel flows at volume rate $\dot V$, its input power is $P_{in}=u\dot V$. Apply efficiency only after finding the input energy or power.

Worked example from local Question Bank row 36970

An engine produces $20\,\mathrm{kW}$ useful power at $50\%$ efficiency while consuming $1.0\times10^{-5}\,\mathrm{m^3\,s^{-1}}$ of fuel.

$$P_{in}=\frac{20\,\mathrm{kW}}{0.50}=40\,\mathrm{kW}$$
$$u=\frac{P_{in}}{\dot V}=\frac{4.0\times10^4}{1.0\times10^{-5}}=4.0\times10^9\,\mathrm{J\,m^{-3}}=4.0\,\mathrm{GJ\,m^{-3}}$$

Common trap

Specific energy is energy per unit mass, measured in $\mathrm{J\,kg^{-1}}$. Some sources use the words loosely, so let the stated definition and units determine whether to divide by volume or mass.