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A Closed, Gas Power Cycle

We will use a circle, usually a gray one, to represent all of the steps and processes that make up a cycle.

We will represent a system that goes through a

thermodynamic cycle

by:

This arrow indicates the transition from state 1 to state 2.

State 4 is a piston-and-cylinder device containing vapor at low pressure.

State 3 is a piston-and-cylinder device containing vapor at high pressure.

State 2 is a piston-and-cylinder device containing liquid at high pressure.

State 1 is a piston-and-cylinder device containing liquid at low pressure.

Win

Wout

Qin

Qout

Heat transfer into the system at step 2-3.

Heat transfer out of the system at step 4-1.

Here is an example of a system that goes through a closed cycle. The fluid inside the system is called the

working fluid

Lesson
4F Blog

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Lesson 4F: page 3 of 14

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Ch 4, Lesson F, Page 3 - A Closed, Gas Power Cycle

The cycle shown here is a gas power cycle operating on a closed system.
A power cycle receives a net input of heat and delivers a net output of work.
In step 1-2, the gas is compressed adiabatically. So, W12 < 0 and Q12 = 0.
The gas is heated isochorically in step 2-3. W23 = 0 and Q23 > 0.
In step 34, the gas expands adiabatically. W34 > 0 and Q34 = 0.
The cycle is completed by step 4-1, in which the gas is cooled isochorically. W41 = 0 and Q41 < 0.
Qnet = Q23 + Q41 and Qnet > 0. So, there is a net heat input during the cycle.
Wnet = W12 + W34 and Wnet > 0.
Although the change in volume is the same for steps 12 and 34, the pressure is always higher in step 34 than in step 12. Boundary work is the integral of P dV. Therefore more work is output in step 34 than is input in step 12. So, there is a net output of work during the cycle.
Don’t worry if this seems a bit fuzzy at this point, you will learn MUCH more about the behavior of power cycles in Chapters 6, 7 and 8.
For now, accept that power cycles have a net heat input and a net work output.
Refrigeration cycles and heat pumps have a net work input and a net heat output.
It is often helpful to represent a thermodynamic cycle in a simplified drawing.
All of the equipment and processes that make up the cycle shown here within the big gray circle are represented by a small gray circle like the one shown in the lower right corner of this page.
When a power cycle is drawn in this simplified way, it is known as a heat engine.
Flip the page to see how we use simplified diagrams to help us analyze the performance of heat engines.

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