The Carnot efficiency represents an idealised, unreachable limit in thermodynamics. Its status as a theoretical, rather than practically attainable, maximum creates a significant learning challenge for students, who must bridge this theoretical concept with real-world engine performance. This pedagogical review addresses this gap by analysing five progressively complex configurations of work extractors interacting with thermal reservoirs. We systematically derive generalised efficiency formulae. These formulae incorporate irreversibilities from internal dissipation and reservoir imperfections, as well as finite heat transfer and radiative exchange. Rather than proposing a single superior model, we demonstrate how these formulae are complementary to one another. Each provides a relevant upper bound for different physical constraints. The primary outcome is a clear, comparative framework for educators to enhance the teaching of thermodynamic limits. By unifying these derivations and linking them to applications in power plants and concentrated solar power systems, this review aims to solidify student understanding of both fundamental principles and their practical engineering implications.