A heat-engine cycle consists of the following four steps in order:
1. Isothermal expansion of an ideal gas at temperature T1.
2. Adiabatic expansion that lowers the gas temperature to T2.
3. Heating the gas at constant volume by bringing it in contact with a reservoir at temperature T3 such that T3>T2.
4. Adiabatic compression returning the gas to its initial state.
Identify the specific step that makes the overall cycle irreversible and state the reason in one sentence.