I'm trying to understand Stirling Cycle that consists of two isochoric processes and two isothermal processes. The heat transfer are given as
$$Q_1 = \nu RT_1 \mathrm{ln} \frac{V_1}{V_2}$$ $$Q_2 = C_V (T_2 - T_1)$$ $$Q_3 = \nu RT_2 \mathrm{ln} \frac{V_2}{V_1}$$ $$Q_4 = C_V (T_1 - T_2)$$
Where $T_1$ is the temperature of the hot heat reservoir, $T_2$ is the temperature of the cold heat reservoir, and $V_1$ is the higher volume, $V_2$ is the lower volume. $Q_1$ is the heat absorbed in the isothermal inflation process, $Q_2$ is the heat released in the isochoric cooling process, $Q_3$ is the heat released in the isothermal compression process, $Q_4$ is the heat absorbed in the isochoric heating process.
And the efficiency of the cycle is
$$\eta = \frac{W}{Q} = \frac{Q_1 - Q_3}{Q_1} = 1 - \frac{T_2}{T_1} $$
I am confused why $Q_2$ and $Q_4$ don't count towards the calculation of $\eta$ as they're absorbed and released by the working ingredient (gas).