4.2.1 An ideal gas is used as the working substance of a cyclic heat engine. The cycle consists of three stages: isochoric pressure reduction from $3 P_{0}$ to $P_{0}$, isobaric density increase from $\rho_{0}$ to $3 \rho_{0}$, and return to the initial state, where the process in the $P / P_{0}, \rho / \rho_{0}$ axes is represented by a quarter circle centered at the point $(1,1)$. Determine the efficiency of this cycle, knowing that it is 8 times less than the maximum possible efficiency for the same minimum and maximum gas temperatures as in this cycle.