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Carnot Cycle: Step 4-1: Adiabatic Compression

The cylinder is insulated in step 4-1, making this step adiabatic. When the weights are added to the piston, the pressure rises, but the volume does not decrease sharply because the temperature of the working fluid increases.

An insulated cylinder containing the working fluid.

When the weight is added to the back of the piston and heat is removed from the system, the volume decreases dramatically, but the pressure only increases modestly.

Step 1-2 is an isothermal, reversible expansion that occurs when a weight is removed from the back of the piston and heat flows into the working fluid from the hot reservoir.

This PV diagram shows the location of state 1 at high pressure and low specific volume.

State 3 is represented in this piston-and-cylinder diagram.

In state 2b, insulation is added to the outside of the cylinder so step 2-3 will be adiabatic.

Piston-and-cylinder diagram showing state 2.

Piston-and-cylinder diagram showing the initial state, state 1.

The system is no longer in contact with the heat sink. The cylinder is perfectly insulated.

As the pressure slowly increases, the fluid is slowly compressed and the temperature rises.
This process is a reversible,
adiabatic compression.

Lesson
6E Blog

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Lesson 6E: page 9 of 25

I hope you realized that the last step in this cycle is an adiabatic compression.
To accomplish an adiabatic process, we must first remove the cold reservoir and then perfectly insulate the cylinder, just like we did in step 2-3.
We need to increase the pressure from P₄ to P₁, so we must add weight to the back of the piston.
As the pressure increases, the volume of the gas in the system decreases.
The adiabat is steeper than the path of the isothermal compression because the insulation prevents any heat loss and the energy that cannot escape just increases the rate at which the pressure increases.
That closes the loop on our PV Diagram and so it completes the reversible thermodynamic cycle called the Carnot Cycle.

Carnot Cycle: Step 4-1: Adiabatic Compression

Ch 6, Lesson E, Page 9 - Carnot Cycle: Step 4-1: Adiabatic Compression