A heat engine or heat engine is a machine that can convert part of the heat provided by a heat source into mechanical energy and output it to the outside world. The mechanical energy output by a heat engine is called output work. The working mode of a heat engine can generally be simplified as a thermodynamic cycle model, and the types of heat engines are named according to the different thermodynamic models behind them, such as Carnot heat engine and Dissel heat engine. In addition, according to the heat source or working characteristics, there are also established names, such as diesel engine, gasoline engine, steam engine and so on. The heat engine can be an open system or a closed system. The heat sources can be steam furnaces using coal, combustion chambers of automobile engines, solar steam furnaces, geothermal energy and nuclear reactors. Heat engines are divided into internal combustion engines and external combustion engines.

On the one hand, people use existing heat energy or burning fuel to create heat energy for heat engines, on the other hand, they are wasting a lot of heat energy. For example, many power plants have to use a lot of water for cooling. French engineer Nicolas Carnot introduced Carnot theorem in the study of 1824. This theorem shows that even an ideal heat engine, its efficiency of converting thermal energy into mechanical energy is lower than 100%. This formula is:

Efficiency = useful work/energy injected into the system

For all heat engines

For Carnot heat engine, the formula becomes:

Here is the heat from the high-temperature heat source to the working system, and the heat from the low-temperature heat source to the working system (negative).

Entropy change

Represents the amount of change

The points on the diagram in Carnot heat engine will eventually return to the original point, so

Substitution entropy change formula

Substitute the above formula into the above formula.

Only applicable to Carnot heat engine.

According to Carnot's theorem:

Here sum is the temperature in Calvin, and the equal sign only holds when the engine cycle is reversible.

So we can draw the following conclusions:

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It can be seen from this formula that to get the efficiency of 100%, the low-temperature heat source needs to be at absolute zero, or the high-temperature heat source needs to have an infinite temperature.