# Motivation for and History of the Carnot Cycle

• Imagine a process that involves no friction.
• Any expansions and compressions occur at infinitesimally slow rates.
• Heat transfer only occurs between bodies with infinitesimal temperature difference.
• This process is completely reversible, internally and externally.
• It is a cyclic process that has the highest efficiency of any cyclic process operating between the same hot and cold reservoirs.
In 1824, a French engineer named Nicolas Leonard Sadi Carnot (1796-1832) proposed such a process. This cycle is called the

## Carnot Cycle

and its efficiency is the standard against which the efficiency of all real cycles is compared.
In this lesson, we will define and interpret the Carnot Cycle for both closed and open systems.
We will discuss Carnot Power Cycles (HE's), Carnot Refrigerators and Carnot Heat Pumps. We will also present the Carnot Principles and how they relate to the Clausius and Kelvin-Planck Statements of the 2nd Law.
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### Ch 6, Lesson E, Page 1 - Motivation for and History of the Carnot Cycle

• In 1824, Carnot proposed a completely reversible cycle.
• This cycle has the highest efficiency of any cyclic process that operates between the same two thermal reservoirs.
• We will show why this is true at the tend of this lesson.
• The efficiency of the Carnot Cycle is the standard against which the efficiency of all real cycles is compared.
• We will begin this lesson by describing each step in a Carnot gas power cycle operating in a closed system.
• We will plot the process path on a PV Diagram and explain the graphical interpretation of the work produced by the cycle.
• We will then explain what happens when the processes that make up this cycle are executed in reverse order.  The result is a Carnot Refrigeration or Heat Pump Cycle.
• Then, we move on to a discussion of Carnot Cycles in open systems.  This is the part of the lesson that will be used most in the lessons and chapters ahead.
• Finally we will discuss the two most important attributes of Carnot Cycles.  These are called the Carnot Principles.
• Before we dive into the Carnot Cycle though, let’s take a quick look at the nature of reversible compressions and expansions.