A thermodynamic system is a defined region chosen for study, separated from its surroundings by a boundary. The classification of this system—open, closed, or isolated—depends on what is permitted to cross this boundary. Understanding whether a piston-cylinder device qualifies as a closed system is central to engineering thermodynamics. This assembly is a common model used to analyze energy transformations.
Understanding System Boundaries
Thermodynamic systems are categorized based on their ability to exchange mass and energy with their surroundings. An open system freely exchanges both matter and energy, such as an unsealed pot of boiling water. A closed system strictly contains matter, permitting the transfer of energy, such as heat or work, but no mass can enter or leave.
The constant amount of mass within the boundaries is the defining characteristic of a closed system. This containment allows engineers to focus solely on the energy transformations occurring inside. The third classification, an isolated system, is the most restrictive, as it exchanges neither mass nor energy with the environment.
The Piston-Cylinder Device
A piston-cylinder device is a mechanical assembly consisting of a cylindrical chamber, a working fluid (like a gas or vapor) confined within it, and a movable piston. The cylinder walls and the piston itself form the boundary of the system. This setup is fundamental to converting thermal energy into mechanical work, as seen in engines, or performing work on a fluid, as in compressors.
Because the piston is not fixed, its movement allows the volume of the system to change. This movable boundary enables the transfer of mechanical energy, or work, between the working fluid and the surroundings. Despite this movement, the cylinder walls and the piston rings are designed to maintain a sealed environment.
Determining Mass Containment
A standard, ideal piston-cylinder device, one without open inlet or outlet valves, is correctly classified as a closed system. The piston and the cylinder walls form an impermeable barrier that physically prevents the working fluid from escaping. Even as the piston moves up or down during expansion or compression, the total mass of the gas or vapor inside remains unchanged.
This principle is directly applied in the analysis of engine cycles, where the gas inside the cylinder is treated as a closed system during the compression and power strokes. During these phases, the valves are sealed, ensuring the mass of the air-fuel mixture or combustion products stays constant while the piston performs work. The closed system classification is only valid for the specific time frame when the boundary is sealed, distinguishing it from a continuously operating engine that is an open system during its intake and exhaust phases.
The Difference Between Closed and Isolated
Although a piston-cylinder device contains a fixed amount of mass, it is not an isolated system because it readily exchanges energy with its surroundings. The movement of the piston itself is a mechanism for work transfer, where work is done either by the system on the surroundings (expansion) or on the system by the surroundings (compression). This boundary work is a direct form of energy exchange that prevents the system from being isolated.
The cylinder walls are not perfectly insulated and permit heat transfer to occur. Heat can be added to the system to cause expansion or removed to cause contraction, affecting the internal energy of the working fluid. Because both work and heat can cross the system boundary, the piston-cylinder device fulfills the definition of a closed system, not an isolated system.