Thus, there results a non-linear relationship between the effort and the generalized position:
q = q0*(ln(e/e0) + 1);
In out case, the effort is an enthalpy, whereas the generalized position is a mass, i.e.
m = m0*(ln(h/h0) + 1);
This equation can also be solved to compute the enthalpy:
h = h0*exp(m/m0 - 1);
which is the form that is being used in the model.