Physics trivia

# Stirling engine

External combustion engines that run on the Stirling cycle are the most efficient ever invented and can operate on different types of fuels.

The Stirling engine was invented in 1816 by Robert Stirling with the help of his brother. They sought to replace steam engines, which caused several accidents among their employees. This engine is an external combustion engine – combustion takes place outside the engine – and uses only the air heated by combustion as a working fluid .

It is a very simple engine, as it basically consists of two chambers with different temperatures. The expansion of air in these chambers moves a piston (or plunger), generating mechanical work . As there are no emissions of pollutants from the working substance (gases or atmospheric air), Stirling engines are considered closed-cycle .

Stirling engines have very high efficiency compared to internal combustion engines (such as those that run gasoline cars), reaching up to 45% energy efficiency, far beyond the 20% to 30% achieved by other types of engines. , such as diesel or gasoline powered engines.

The efficiency of Stirling engines can be calculated in the same way as for the Carnot cycle:

Being:

• R : Engine efficiency;
• f : Temperature of the lowest temperature chamber (in Kelvin);
• Tq : Temperature of the highest temperature camera (in Kelvin).

Furthermore, virtually any source of heat can be used to run these engines: the flame of a candle and even the heat transferred by the palm of a hand. It is enough that there is a significant difference between the temperatures of the hot and cold parts of the engine.

Stirling engine cycle

1. Isothermal expansion – process in which the air present in the engine undergoes an approximately isothermal expansion, absorbing heat from external sources (burning coal, candles, etc.);
2. Isovolumetric cooling  the air present in the engine transfers heat to the external environment, maintaining a constant volume;
3. Isothermal compression – process in which the air contained within the engine cylinder is contracted and its pressure increases greatly, in a process that occurs at constant temperature;
4. Isovolumetric heating – the latter process occurs at constant volume and involves heat transfer from the hot source to the air contained within the engine cylinder.

In the following figure, the PV (Pressure x Volume) diagram represents the operation of the Stirling cycle:

Stirling cycle engines have the following advantages :

• They cause little pollution ;
• They are very quiet and generate little vibration;
• They can use virtually any heat source as fuel: gasoline, diesel, LPG, solar energy, geothermal heat, etc.;
• They are high-performance engines – their efficiency depends exclusively on the temperature difference between their hot and cold chambers , as does the Carnot Cycle.

We can list as disadvantages of Stirling engines:

• Because they are not very popular, their production cost is high ;
• The gas sealing system used in the chambers is difficult to control ;
• Changing the rotation speed of this type of motor is a complex process.