What is a Stirling engine? How does the Stirling engine work? How was the Stirling engine discovered? In what areas is it used? How is heat energy converted into motion energy? Details about Stirling engines are in our article.
What is Stirling Engine?
A Stirling engine is a machine that converts the energy generated by the external heating of a closed chamber into mechanical energy. Also known as a hot air engine. As the heated air expands and compresses, the engine starts to move. It was invented in 1816 by the Scottish priest, Reverent Robert Stirling. The engine was developed by his brother, James Stirling. In the time of the inventors, steam-powered machines were used and they were quite dangerous. They set out to find a more reliable alternative. What they wanted was to convert heat energy directly into motion energy.
What's in the Stirling Engine?
- Power piston (displacer): It serves to move the gas in the closed chamber. It is generally used in beta type and alpha type engines.
- Piston: It helps to convert heat energy into mechanical energy by moving in the cylinders in the engine.
- Flywheel: It is the structure to which the pistons are attached. The task of this structure is to transfer the generated mechanical energy to the moving parts.
- Cooler: It helps to cool the gas in the closed chamber. It helps the engine to be used for longer periods.
- Heater: It is the most important part of the engine. It is used to heat the gas in the closed chamber to convert the heat energy into motion energy.
In addition, in some engine types, it can be used in different components other than these. This is entirely at the discretion of the developers.
Working Principle of Stirling Engine
A Stirling engine operates by repeated heating and cooling of an insulated amount of working gas (usually air or gases such as helium, hydrogen).
The gas exhibits behavior defined by the gas laws (relative to pressure, temperature, and volume). When the gas is heated, because it is in an insulated space, its pressure rises and affects the power piston, producing a power stroke. When the gas is cooled, the pressure drops and as a result the piston uses some of the work done on its return stroke to recompress the gas. The resulting net work creates force on the spindle. The working gas periodically flows between the hot and cold heat exchangers. The working gas is sealed within the piston cylinders. So there is no exhaust gas here. Unlike other types of piston engines, valves are not needed.
Some Stirling engines use a splitter piston to move the working gas back and forth between cold and hot tanks. The working gas moves by keeping the cylinders at different temperatures, thanks to the interconnection of the power pistons of the multiple cylinders.
In real Stirling engines, a regenerator is placed between the tanks. This heat is transferred from the regenerator as gas cycle occurs between the hot and cold side. In some designs, the separator piston is the regenerator itself. This regenerator contributes to the efficiency of the Stirling cycle. The structure referred to here as the regenerator is actually a solid structure that will not prevent some air from passing through it. For example, steel balls can be used for this job. As the air moves between a cold room and a warm room, it passes through this regenerator. Before the hot air reaches the cold part, it leaves some heat energy on these balls. As the cold air passes to the hot side, it warms up a little with the heat energy released before. In other words, it increases the efficiency of the engine by pre-heating the air before entering the hot part and pre-cooling before entering the cold part.
An ideal Stirling engine cycle has the same theoretical efficiency as a Carnot heat engine for the same inlet and outlet temperatures. Its thermodynamic efficiency is higher than steam engines. (or some simple internal combustion and diesel engines)
Any heat source can power the Stirling engine. External combustion engine, combustion in the expression is often misunderstood. The heat source can be generated by combustion, but can also be solar energy, geothermal energy or nuclear energy. Likewise, the cold source used to create a temperature difference can be different materials below the ambient temperature. Cooling can be achieved with the use of cold water or a refrigerant. However, since the temperature difference to be obtained from the cold source will be low, it will require working with larger masses, and the power loss that will occur in the pumping will reduce the efficiency of the cycle. Combustion products do not come into contact with the internal parts of the engine. Lubricating oil life in Stirling engine is longer than in internal combustion engines.
Stirling Engine Types
There are 3 main types of stirling engines. Other engine types are improved versions of 3 engines.
- Alpha type stirling engine:
It consists of two pistons, a flywheel, a closed gas chamber with the pistons, heat exchangers, a heat generator, and a flywheel. It is aimed to activate the gas in it by heating the area of the piston placed at the top with a heat source. The heated gas starts to push the piston back and forth, the other connected piston starts to move, so that the hot and cold gas are displaced in the chamber. The energy generated is transferred with the help of the flywheel to which these two pistons are connected.
- Beta type stirling engine:
There are 2 pistons on the same shaft. These two pistons are connected to each other. By heating the chamber with the piston at the bottom, the gas in the closed chamber is heated and activated. In this way, the piston starts its upward movement. The other connected piston also helps the cold gas to move in the chamber. The flywheel, to which the pistons are attached, transfers the generated energy.
- Gamma type stirling engine:
There are two separate pistons. The chamber with the larger piston is heated and the gas in it is activated. In this way, the pistons connected to each other with the flywheel start to move.
Advantages of Stirling Engines
- Since the heat is applied externally, we can accurately control the fuel and air mixture.
- Since a continuous heat source is used to provide heat, the amount of unburned fuel is very small.
- This type of engine requires less maintenance and lubrication than engine types at their power level.
- They are quite simple in structure compared to internal combustion engines.
- They can work even at low pressure, they are safer than steam source machines.
- Low pressure allows the use of lighter and more durable cylinders.
Disadvantages of Stirling Engines
- The cost is high in terms of fuel economy, as the necessary heat is required in the first start of the engine.
- It is quite difficult to take his power to a different level.
- Some stirling engines cannot start quickly. They need sufficient warmth.
- Generally, hydrogen gas is used in a closed chamber. However, when the molecules of this gas are quite small, it is difficult to keep it in the chamber. Therefore, we face additional costs.
- The cooler part must absorb enough heat. If there is too much heat loss, the efficiency of the engine will decrease.
Stirling Engines Application Areas
Stirling engines are used in low power aviation engines, marine engines, heat pumps, combined heat and power systems. Today, it is mostly used to generate electricity in solar panel fields.