External combustion engines that run on the Stirling cycle are the most efficient ever invented and can operate on different types of fuels.
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:
- R : Engine efficiency;
- T 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
- 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.);
- Isovolumetric cooling – the air present in the engine transfers heat to the external environment, maintaining a constant volume;
- 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;
- 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:
Advantages and disadvantages of the Stirling engine
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.