Name: adiabatic compression in an engine cylinder
Uploaded: 2025-05-22T09:05:49.000Z
Duration: 1 min 20 s
Channel: JoVE

K12 Standards

Florida Department of Education

High School

Physical Science

SC.912.P.10

SC.912.P.10.2

Work Done in an Adiabatic Process

Considérons un ballon météorologique rempli d’hélium gazeux ayant une pression initiale et un volume augmentant et se dilatant adiabatiquement jusqu’à un volume final sous pression atmosphérique décroissante. Quel est le travail effectué par le gaz ?
Si les températures initiale et finale sont connues, alors, pour un processus adiabatique, le travail effectué est obtenu à partir de son énergie interne, exprimée en termes de changement de température.
Une autre méthode consiste à utiliser la condition pour le processus adiabatique, à partir duquel les pressions et les volumes initiaux et finaux du gaz peuvent être liés. Les termes sont réorganisés pour simplifier.
Rappelez le travail pendant le changement de volume et remplacez-y l’expression de pression obtenue.
En intégrant cette expression au changement de volume, le travail effectué au cours d’un processus adiabatique peut être déterminé.
Si la pression initiale du gaz dans le ballon d'hélium est de 1 atm et que le volume du ballon passe de 25 m3 à 100 m3, alors en substituant les quantités connues dans l'expression dérivée, le travail effectué par le gaz peut être calculé.

<ol>
	<li>Young, H.D. et Freedman, R.A. (2012). La physique universitaire avec la physique moderne. San Francisco, Californie : Pearson. p. 641 et 642</li>
	<li>OpenStax. (2019). Physique universitaire Vol. 2. [Version Web]. Récupéré de <a href="https://openstax.org/books/university-physics-volume-2/pages/3-6-adiabatic-processes-for-an-ideal-gas">https://openstax.org/books/university-physics-volume-2/pages/3-6-adiabatic-processes-for-an-ideal-gas</a></li>
</ol>

travail effectué dans un processus adiabatique

Considérez la compression adiabatique d'un gaz idéal dans le cylindre d'un moteur diesel automobile. La vapeur d'essence est injectée dans le cylindre d'un moteur automobile lorsque le piston est en position étendue. La température, la pression et le volume du mélange air-gaz résultant sont respectivement de 20 °C, 1,00 x 105 N/m2 et 240 cm3. Ensuite, le mélange est comprimé adiabatiquement jusqu'à un volume de 40 cm3. Notez que, dans le fonctionnement réel d'un moteur automobile, la compression n'est pas quasi-statique, bien que nous supposions cela ici. Quelles sont la pression et la température du mélange après la compression ? Quel est le travail effectué pendant la compression ?

Solution :

Pour un processus de compression adiabatique, l'expression de la pression est donnée par

<img alt="Equation1" src="/files/ftp_upload/13689/13689_Equation_1.svg" style="text-align: center; height: 34px;" />

En utilisant le rapport des capacités calorifiques molaires de l'air égal à 1,4 et en substituant les quantités connues dans l'expression (1), la pression du mélange après la compression est calculée.

<img alt="Equation2" src="/files/ftp_upload/13689/13689_Equation_2.svg" style="text-align: center; height: 34px;" />

à partir de la loi des gaz parfaits et en utilisant la valeur de pression obtenue, la température du mélange après la compression est également calculée.

<img alt="Equation3" src="/files/ftp_upload/13689/13689_Equation_3.svg" style="text-align: center; height: 34px;" />

<img alt="Equation4" src="/files/ftp_upload/13689/13689_Equation_4.svg" style="text-align: center; height: 34px;" />

<img alt="Equation5" src="/files/ftp_upload/13689/13689_Equation_5.svg" style="text-align: center; height: 10px;" />

Ainsi, la pression et la température du mélange après la compression à l'intérieur du cylindre d'un moteur diesel automobile sont déterminées.

Si le processus de compression est isotherme, alors la valeur de pression change par rapport à la valeur obtenue. Cela est dû au fait que, dans un processus de compression adiabatique, si la température augmente, la pression devient plus grande. Ainsi, en injectant le carburant dans le cylindre près de la course de compression, le mélange d'air atteint une température plus élevée, ce qui entraîne une ignition spontanée du carburant sans besoin de bougies d'allumage.

En connaissant la pression initiale et finale, le travail effectué pendant la compression peut être obtenu en utilisant

<img alt="Equation6" src="/files/ftp_upload/13689/13689_Equation_6.svg" style="text-align: center; height: 34px;" />

En substituant les quantités connues et obtenues dans l'expression ci-dessus, le travail effectué est égal à -1,63 J. Ici, le signe négatif indique que le travail est effectué par le piston sur le mélange air-gaz.

Travail effectué dans un processus adiabatique

Considérons la compression adiabatique d’un gaz parfait dans le cylindre d’un moteur diesel automobile. La vapeur d’essence est injectée dans le cylindre d’un ...

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Unit 1

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The First Law of Thermodynamics

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adiabatic compression in an engine cylinder

Adiabatic compression in an engine cylinder changes the pressure, temperature, and volume of a gas-air mixture. In this setup, gasoline vapor is injected into the cylinder when the piston is in its expanded position. The mixture starts at 20 °C, 1.00 x 10^5 N/m^2, and 240 cm^3.

The mixture is then compressed adiabatically to 40 cm^3. Adiabatic means no heat is exchanged with the surroundings during the process. For this calculation, the compression is treated as quasi-static, even though real automobile engine compression is not quasi-static.

Using the adiabatic pressure relation and the molar heat capacity ratio for air, 1.4, the final pressure is found from the initial conditions and the volume change. The ideal gas law is then used with that pressure to calculate the final temperature of the mixture. The result shows that the compressed gas reaches a higher pressure and temperature than it had before.

If the compression were isothermal, the pressure would be different from the value found here. In an adiabatic compression, the temperature rises, so the pressure becomes greater. This helps explain why fuel injected near the compression stroke can ignite on its own in a diesel engine, without spark plugs.

The work done during compression is found from the initial and final pressure values. The calculated work is -1.63 J. The negative sign means the piston does work on the air-gas mixture during compression.

Adiabatic Compression in an Engine Cylinder

adiabatic-compression-in-an-engine-cylinder

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Adiabatic compression in an engine cylinder changes the pressure, temperature, and volume of a gas-air mixture. In this setup, gasoline vapor is injected into the cylinder when the piston is in its expanded position. The mixture starts at 20 °C, 1.00 x 10^5 N/m^2, and 240 cm^3. The mixture is then compressed adiabatically to 40 cm^3. Adiabatic means no heat is exchanged with the surroundings during the process. For this calculation, the compression is treated as quasi-static, even though real...

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See how adiabatic compression changes pressure, temperature, and work in an engine cylinder, using ideal gas relations and a diesel engine example.

Adiabatic Compression in an Engine Cylinder | JoVE K12

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SC.912.P.10.3

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New York State Science Learning Standards

HS. Energy

HS-PS3-3.

Energy Basics

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First Law of Thermodynamics

Energy Conservation

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Kinetic Energy - I

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Differentiate among the various forms of energy and recognize that they can be transformed from one form to others.

SC.912.P.10.1

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First Law Of Thermodynamics: Problem-Solving

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Cyclic Processes And Isolated Systems

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In the case of a ...

Isothermal Processes

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Isochoric and Isobaric Processes

A thermodynamic process that occurs at constant volume is called an isochoric process. According to the first law of thermodynamics, heat supplied or ...

Consider the adiabatic compression of an ideal gas in the cylinder of an automobile diesel engine. The gasoline vapor is injected into the cylinder of an ...

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Maxwell's Thermodynamic Relations

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Energy and Work

Source: Ketron Mitchell-Wynne, PhD,&#160;Asantha Cooray, PhD, Department of Physics &#38; Astronomy, School of Physical Sciences, University of ...

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Compare and contrast work and power qualitatively and quantitatively.

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SC.912.P.10.4

Kinetic Theory of an Ideal Gas

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Behavior of Gas Molecules: Molecular Diffusion, Mean Free Path, and Effusion

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Relate temperature to the average molecular kinetic energy.

SC.912.P.10.5

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SC.912.P.10.6

A. Energy is involved in all physical and chemical processes. It is conserved, and can be transformed from one form to another and into work. At the atomic and nuclear levels energy is not continuous but exists in discrete amounts. Energy and mass are related through Einstein's equation E=mc2.

B. The properties of atomic nuclei are responsible for energy-related phenomena such as radioactivity, fission and fusion.

C. Changes in entropy and energy that accompany chemical reactions influence reaction paths. Chemical reactions result in the release or absorption of energy.

D. The theory of electromagnetism explains that electricity and magnetism are closely related. Electric charges are the source of electric fields. Moving charges generate magnetic fields.

E. Waves are the propagation of a disturbance. They transport energy and momentum but do not transport matter.

Adiabatic Compression in an Engine Cylinder

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