# Energy density and Carnot cycle

For example, diesel has about 40 MJ/kg, what does it mean? Is that if we burn it we will get 40 MJ of energy per kg, but we can not get all of it to mechanical work?

If that is the case, then why can't we get all of the energy to mechanic like in burning?

When you burn diesel the result is heat, but what we actually want is work so we need to find a way to turn heat into work. The theoretical model for this is a heat engine:

$$\eta = 1 - \frac{T_c}{T_h}$$
where $T_h$ is the temperature of our hot source and $T_c$ is the temperature of the cold sink. This efficiency is the maximum for a theoretically ideal engine, and all real heat engines are less efficient than this.