The Carnot heat engine is often regarded as the most efficient heat engine because it operates under the idealized Carnot cycle, which consists of two isothermal and two adiabatic processes. The efficiency of a heat engine is defined as the ratio of the useful work output to the heat input.
There are a few reasons why the Carnot heat engine's efficiency is higher than that of other types:
Reversible process: The Carnot cycle is based on reversible processes, which means that it assumes there are no energy losses due to friction, turbulence, or other dissipative effects. In reality, these losses occur in practical engines, reducing their efficiency. The Carnot cycle serves as an idealized benchmark for the maximum possible efficiency.
Isothermal heat transfer: The Carnot cycle involves heat transfer at constant temperature, which is achieved by maintaining perfect thermal contact with a heat source and sink. This allows for maximum heat transfer efficiency, as the temperature difference between the source and sink is always maximized during heat exchange.
Temperature difference optimization: The Carnot cycle works between two heat reservoirs at different temperatures. The efficiency of a heat engine depends on the temperature difference between these reservoirs. The Carnot cycle achieves the maximum temperature difference for a given set of reservoir temperatures, resulting in higher efficiency compared to other cycles.
It's important to note that no real heat engine can achieve the ideal efficiency of the Carnot heat engine in practice. Real engines have various sources of energy losses, such as friction, heat dissipation, and imperfect heat transfer. However, the Carnot cycle provides an important theoretical framework for studying the limits of heat engine efficiency and serves as a reference for comparing the performance of real engines.