Modern Engineering Thermodynamics

2.6 Thermodynamic Processes 39
WHAT IS THERMODYNAMIC EQUILIBRIUM?
A system is said to be in thermodynamic equilibrium if it does not spontaneously change its state after it has been isolated.
2.6 THERMODYNAMIC PROCESSES
Engineering thermodynamics is primarily concerned with systems that undergo thermodynamic processes.
A system subjected to a thermodynamic process normally experiences a change in its thermodynamic state.
Consequently, we define a thermodynamic process as in the following box.
Neither the initial, final, nor any intermediate states need be in thermodynamic equilibrium during a thermodynamic
process. A process can change a system from one nonequilibrium state to another nonequilibrium state
via a path of nonequilibrium states. Figure 2.3 illustrates several process paths that change a system from an
initial state A to a final state B.
If a process path closes back on itself so that it is repeated periodically in time, then the thermodynamic process
is called a thermodynamic cycle. Figure 2.4 illustrates the definition of a thermodynamic cycle.
There is a difference between a thermodynamic cycle and a mechanical cycle. In a mechanical cycle, all the
mechanical components begin and end in the same geometrical configuration. For example, the engine of an
automobile goes through a mechanical cycle once per two crankshaft rotations for a four-stroke engine, but it
does not go through a thermodynamic cycle. For an automobile engine to go through a thermodynamic cycle
the engine’s exhaust would have to be converted back into air and fuel (the initial state).
Understanding the process path is extremely important in thermodynamic analysis because it often determines the
final state of the system. In most thermodynamic textbook problem statements, the process path is only vaguely
alluded to or else is hidden in one or more of the technical terms used. Therefore, in addition to deciding on the
type of system to use in the analysis of a problem and preparing a system sketch, you must also determine the
type of process that is occurring.
B
Thermodynamic
state
A
Process 1
Process 2
Process 3
Thermodynamic
state
Initial state
Final state
Start
Process variable
Finish
Process variable
FIGURE 2.3
Three process paths that change the state of the system from A to B.
FIGURE 2.4
A thermodynamic cycle.
WHAT IS A THERMODYNAMIC PROCESS?
A thermodynamic process is the succession of thermodynamic states that a system passes through as it goes from an initial state
to a final state.
WHAT IS A THERMODYNAMIC CYCLE?
A system process is said to go through a thermodynamic cycle when the final state of the process is the same as the initial state
of the process.