The ideal Carnot engine operates reversibly — zero entropy production, maximum efficiency η_C = 1−T_c/T_h — but delivers zero power (infinite cycle time). Real engines operate at finite speed: entropy is produced irreversibly, reducing efficiency below Carnot. The Curzon–Ahlborn result gives efficiency at maximum power: η_CA = 1−√(T_c/T_h). The fundamental trade-off between efficiency and power is a thermodynamic law.