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By the end of this section, you will be able to:
  • Describe the function and components of a heat engine
  • Explain the efficiency of an engine
  • Calculate the efficiency of an engine for a given cycle of an ideal gas

A heat engine    is a device used to extract heat from a source and then convert it into mechanical work that is used for all sorts of applications. For example, a steam engine on an old-style train can produce the work needed for driving the train. Several questions emerge from the construction and application of heat engines. For example, what is the maximum percentage of the heat extracted that can be used to do work? This turns out to be a question that can only be answered through the second law of thermodynamics.

The second law of thermodynamics can be formally stated in several ways. One statement presented so far is about the direction of spontaneous heat flow, known as the Clausius statement. A couple of other statements are based on heat engines. Whenever we consider heat engines and associated devices such as refrigerators and heat pumps, we do not use the normal sign convention for heat and work . For convenience, we assume that the symbols Q h , Q c , and W represent only the amounts of heat transferred and work delivered, regardless what the givers or receivers are. Whether heat is entering or leaving a system and work is done to or by a system are indicated by proper signs in front of the symbols and by the directions of arrows in diagrams.

It turns out that we need more than one heat source/sink to construct a heat engine. We will come back to this point later in the chapter, when we compare different statements of the second law of thermodynamics. For the moment, we assume that a heat engine is constructed between a heat source (high-temperature reservoir or hot reservoir) and a heat sink (low-temperature reservoir or cold reservoir), represented schematically in [link] . The engine absorbs heat Q h from a heat source ( hot reservoir    ) of Kelvin temperature T h , uses some of that energy to produce useful work W , and then discards the remaining energy as heat Q c into a heat sink ( cold reservoir    ) of Kelvin temperature T c . Power plants and internal combustion engines are examples of heat engines. Power plants use steam produced at high temperature to drive electric generators, while exhausting heat to the atmosphere or a nearby body of water in the role of the heat sink. In an internal combustion engine , a hot gas-air mixture is used to push a piston, and heat is exhausted to the nearby atmosphere in a similar manner.

The figure shows schematic of an engine with a downward arrow Q subscript h at T subscript h. When this goes through the engine, the arrow splits with a downward arrow Q subscript c at T subscript c and a left arrow W.
A schematic representation of a heat engine. Energy flows from the hot reservoir to the cold reservoir while doing work.

Actual heat engines have many different designs. Examples include internal combustion engines, such as those used in most cars today, and external combustion engines, such as the steam engines used in old steam-engine trains. [link] shows a photo of a nuclear power plant in operation. The atmosphere around the reactors acts as the cold reservoir, and the heat generated from the nuclear reaction provides the heat from the hot reservoir.

Questions & Answers

what is electrostatics
Hero Reply
A proton initially at rest falls through a p.d of 25000V. what speed does it gain?
Minister Reply
what is the reaction of heat on magnet
what is a physical significant of electric dipole moment .
A dipole moment it's a mechanical electrical effect used in nature
what is the uses of carbon brushes in generator
Malik Reply
at what temperature is the degree Fahrenheit equal to degree Celsius
Grace Reply
Celsius and Faharaneith are different, never equal
find their liners express of n=a+b/T² ( plot graph n against T)
Donsmart Reply
Radio Stations often advertis "instant news,,if that meens you can hear the news the instant the radio announcer speaks it is the claim true? what approximate time interval is required for a message to travel from Cairo to Aswan by radio waves (500km) (Assume the waves Casbe detected at this range )
mahmod Reply
what is growth and decay
Pawan Reply
Oladejo Reply
Can someone please predict the trajectory of a point charge in a uniform electric field????
erlinda Reply
what is deference between strong force and coulomb force
zahid Reply
so basically the electric lines or the field lines
zach Reply
it's the measure of the flow of an electric field through a certain area
zach Reply
What is electric flux
Pranju Reply
Practice Key Terms 4

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Source:  OpenStax, University physics volume 2. OpenStax CNX. Oct 06, 2016 Download for free at http://cnx.org/content/col12074/1.3
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