In a vapour cycle heat engine, which is the most common working fluid?

The most common working fluid in a vapor cycle heat engine is steam. This choice is based on the fundamental principles of thermodynamics and the traditional design of many heat engines, particularly in power generation.

Steam is favored because it has a high latent heat of vaporization, which means it can absorb and release large amounts of energy during phase changes between liquid and vapor states. This property is crucial in maximizing the efficiency of the cycle, as it allows the engine to convert heat energy into work effectively.

Additionally, many power plants use steam turbines, where steam is generated from boiling water in a boiler. The steam expands through the turbine, producing mechanical work that can be converted to electrical energy. The widespread use of steam in various thermal systems, from conventional power plants to industrial processes, highlights its role as the standard working fluid in vapor cycle heat engines.

In contrast, the other options—such as air, refrigerants, and natural gas—are used in different types of cycles or applications. Air is typically utilized in gas turbine engines rather than vapor cycles, while refrigerants are used in refrigeration and air conditioning systems. Natural gas, on the other hand, is primarily a fuel source rather than a working fluid in a vapor cycle but may be part