2 - Background

2.2 - Specific examples

2.2.1 - Thermodynamics of heat engines

2.2.1.1 - The laws of thermodynamics

Thermodynamics is based on the following laws:

- energy cannot be created or destroyed;

- energy can only be changed from one form to another: fuel into heat and heat into mechanical energy;

- heat can never be fully transformed into mechanical work, due to the friction generated by contact between parts in movement;

- none of the natural and technical energy transformation processes can be reversed. They all travel in the most likely direction, heat only passes freely from a hotter body to a colder body;

- the reverse effect where cold passes to heat can only be achieved with an input of energy.

2.2.1.2 - Types of transformations

Thermodynamic transformations are defined according to the conditions under which they take place. Thus, a transformation

- at constant pressure is an isobaric transformation;

- at constant volume is an isochore transformation;

- at constant temperature is an isothermal transformation;

- with no heat exchange is an adiabatic transformation;

- with no heat exchange and no friction is an isentropic transformation;

- with a general change in state is a polytropic transformation.

When studying the theoretical cycles of ideal gases, we can express the laws governing these changes using the constant developed by L. J. Gay Lussac (1778 - 1850) : PV = RT. He developed the constant: PV = R (267 + t), which is close to PV = R (273,15 + t).

A few years later, a combination of the Gay-Lussac law and the Boyle-Mariotte law gave the “ideal gas” law whose equation of state is PV = nRT or PV = NkT, and forms the basis for simple modeling of gases in thermodynamic systems.

2.2.1.3 - Operating principle

Chemical energy in the fuel is transformed into heat by combustion which requires oxygen and this thermal energy is converted into work with the help of mechanical components forming an assembly known as a heat engine.

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