What is a Stirling Engine?

Q: Who invented the Stirling engine?

In 1816 , Robert Stirling invented the Stirling engine .

External combustion (EC) engines are most widely used all over the world in different vehicles. There are multiple types of engines according to application needs. A Stirling engine is the most famous type of EC engine. It is a steam engine. It is the most commonly used engine. This article explains the Stirling engine working, parts, and applications.

A Stirling engine is a reciprocating engine that changes the thermal energy of the fuel into mechanical power by heating and cooling the working fluid trapped inside the engine cylinder. Because it is a reciprocating engine, therefore, it uses a piston instead of a rotary rotor like the Wankel engine.

In 1816, Robert Stirling invented the Stirling engine. The Stirling cycle has almost equal thermal efficiency to the Carnot cycle.

This external combustion engine has more efficiency than an IC engine (such as a diesel engine or gasoline engine). Stirling engines are most commonly used in different applications, such as combined heat and power (CHP) systems, solar power generation, and even in some spacecraft for generating electricity from radioisotope thermoelectric generators (RTGs).

Working of Stirling Engine

The Stirling engine works on the base of the Stirling cycle.

Stirling Engine Working

The Stirling engine works in the following way:

As the operator turns on the engine starter, the external heat source also starts. The operator controls the force through the mechanism provided.
When the external heat source is activated, the heat starts transforming from the heat source to the hot end of the cylinder. This process of heat transfer increases the temperature of the gas molecules trapped in the cylinder’s hot end. As the temperature of the gas molecules rises, a disturbance starts producing between them and gas molecules start expanding inside the cylinder.
The gas expansion increases the pressure on the piston surface, pushes the piston away, and generates useful work.
A displacer piston is coupled with a crankshaft. The movement of the crankshaft causes the displacer piston to move between the cold end and the hot end of the cylinder.
The displacer piston’s movement causes a gas exchange from the cold end to the hot end and the hot end to the cold end of the cylinder.
The hot end gas provides the power to the displacer piston, which transfers the gas into the cold end of the cylinder.
As the hot gas is entered into the cold end, the cooling device extracts the heat of the hot gas and cools it.
After cooling the gas, the piston compresses the gas in the cold end of the cylinder. A cooling device removes the excess heat from the gas.
After the compression process, the displacer piston sends the compressed gas back into the hot end of the cylinder, where the cycle repeats.

This is the working principle of the Stirling engine. As you can see above that, in this cycle, there is no exhaust stroke-like IC engine. Therefore, a Stirling engine has more efficiency than an internal combustion (IC) engine.

Read Also: Working of 4-stroke Engine

Types of Stirling Engines

The Stirling engine has the following major types:

Alpha Engine
Beta Engine
Gamma Engine
Double-acting Engine
Rotary Stirling Engine

1) Alpha Stirling Engine

The Alpha Stirling engine has two cylinders:

Hot or Expansion Cylinder
Cold or Compression Cylinder

Alpha Stirling Engine

The hot cylinder is connected to an external heat source. This heat source delivers heat to the hot cylinder’s gas. In contrast, the cold cylinder has a cooling device. This cooling device uses to extract the heat of the gas received by the hot cylinder. These two cylinders connect with a common pipe or tube from where the hot gas transfers from the hot cylinder to the cold cylinder and vice versa.

These engines have low power. Therefore, these are used for light load purposes.
The alpha engine has two power pistons.

2) Beta Stirling Engine

The beta engine has only one cylinder. One end of this cylinder is connected to a cooling device, while the other end is connected to a heat source.

Beta Stirling Engine

The cylinder of the beta stirling engine has two pistons:

Power piston
Displacer piston

The power piston assists the engine to control the fluid and runs the engine, while the displacer piston is installed between the cold and hot ends of the cylinder. The displacer piston uses to transfer the hot gas from the hot end to the cold end and the cold gas from the cold end to the hot end of the cylinder. The crankshaft controls the movement of the displacer inside the cylinder.

The power piston links with a flywheel, and it reciprocates between the cold and hot sides of the cylinder. This piston has the responsibility to deliver the output power.

Beta Stirling engine has higher power. Therefore, it uses for high-load applications.

Read Also: Different Types of Engines

3) Gamma Engine

This type of Stirling engine has a power piston and a displacer that are connected to two separate cylinders.

The gas from the two cylinders flows freely between them and remains integrated. Due to the large volume of connection between the two cylinders, this structure has a lower compression ratio but has a simple design and is typically used in a multi-cylinder Stirling engine.

In addition, a certain expansion takes place in the compression area during the expansion process, which leads to reduced specific power.

4) Rotary Stirling Engine

These engines pursue to transform the power from the Stirling cycle in the torque.

5) Double-acting Engine

The piston of this type of engine uses its both end (top and bottom) for pressurizing the mixture.

Read Also: Different Types of IC Engines

PV Diagram of Stirling Cycle

A Stirling engine works on the Stirling cycle. A Stirling cycle works in the following way:

Isothermal Expansion
Isochoric Heat removal
Isothermal Compression
Isochoric heat addition

Stirling Cycle

1) Isothermal expansion (line 1 to 2): –

In the isothermal (constant temperature) expansion process, the energy is provided from the external heat source to the gas trapped in the cylinder.

As the gas molecules get energy, they start to expand. Due to the expansion of gas molecules, the gas pressure increases, which forces the piston to move from a hot cylinder to a cold cylinder.

2) Isochoric heat removal (line 2 to 3): –

In the isochoric ((constant volume) heat removal process of the Stirling cycle, the displacer transfers the hot gas from the hot cylinder into the compression or cold cylinder.

A cooling device extracts the heat energy from the hot gas and converts it into cold gas. The main objective of this cooling process is to decrease the hot gas pressure so that it can be compressed easily.

3) Isothermal compression (line 3 to 4): –

In this step, the gas is compressed at a constant low temperature. The heat released during the compression process is delivered to the cold sink.

During this compression process, the piston increases the gas pressure. This increased pressure drives the power piston, which further moves the flywheel.

4) Isochoric heating (line 4 to 1): –

In the isochoric heat addition process, the piston again transfers cold gas from the cold cylinder into the hot cylinder, where it heats up again through an external heat source, and the whole Stirling cycle repeats.

Components of the Stirling Engine

The Stirling engine has the following parts:

Cylinder
Piston
Flywheel
Connecting rod
Crankshaft
Gas
External heat source

1) Cylinder

The different types of Stirling engines use a different number of cylinders:

The alpha type uses two cylinders that are:

A hot wall cylinder: This cylinder is connected to an external heat source. This heat source delivers heat to the cylinder gas; due to that, the gas becomes hot, and gas expands.
A cold wall cylinder: The main objective of the cold is to transform the hot gas into a cold gas. This cylinder has a cooling device that removes the heat from the hot gas so that this gas can be used again.

The beta type has only one cylinder, which has two ends that are:

Cold end: The cold end has a cooling device that transforms hot gas into cold form.
Hot end: The hot end connects with a heating source which transfers heat to the gas inside the cylinder.

2) Piston

The piston is a device that reciprocates from the cold cylinder to the hot cylinder and vice versa. The piston has the responsibility to deliver the final output power to the engine so that it can move the vehicle.

The pistons used for the Stirling engine have two types that are:

Power piston: It is a smaller piston located at the engine This piston has a tight seal that compresses the cold gas in the cold cylinder.
Displacer Piston: It is a large piston. It is a loosely fitted piston in the cylinder. The displacer piston transfers the gas from the cold cylinder to the hot cylinder and vice versa.

The number of pistons for an engine depends on the type of the Stirling engine, such as:

The beta-type engine has two pistons (i.e., displacer piston and power piston).
The alpha engine also has two pistons, but both are power pistons.
The gamma-type engine has two pistons (i.e., power piston and displacer piston) that are connected with two various cylinders (hot and cold cylinders).

3) Gas

The gas is used in a Stirling engine as a working medium. The working and circulation of the gas vary according to the engine types, such as:

In the case of the alpha type, firstly, the gas is trapped in the hot cylinder. As the external heat source delivers heat to the gas, it starts expanding. Due to the gas expansion, the piston moves upward and transfers gas from the hot cylinder into the cold cylinder.
In the case of the beta Stirling engine, the displacer transfers the gas between the hot end to the cold end and the cold end to the hot end of the cylinder.

4) External Heat Source

The source used to deliver the heat to the cylinder’s hot end (Beta SE) or hot cylinder wall (Alpha SE) is known as an external heat source.

As the heat source delivers heat to the hot cylinder gas, the potential energy of the gas starts increasing due to that it expands. This expansion moves the piston inside the cylinder.

Read Also: Working of 2-stroke Engine

5) Cooling System

The Stirling engine has a cooling system connected with the cold cylinder (Beta type engine) or the cylinder’s cold end (Alpha type).

The purpose of the cooling system is to remove the heat from the hot gas and converts it into the cold form so that the piston can compress it easily.

6) Crankshaft

The crankshaft uses to transfer the piston motion to the flywheel.

The crankshaft transforms the piston’s reciprocating movement into rotary motion and delivers it to the flywheel.
The crankshaft links with the piston via a crank pin.

Read More: Working of Crankshaft

7) Flywheel

The flywheel is connected with the crankshaft. It receives mechanical power (rotary motion) from the crankshaft and stores it for further transmission.

What is the Stirling Engine Efficiency?

The efficiency of a Stirling engine is very dependent on the temperature variance between the hot cylinder and the cold cylinder. The lower the temperature variance between the cold cylinder and hot cylinder, the lower the efficiency of the engine. A Stirling engine can attain an energy conversion efficiency of up to 40% in a solar thermal application.

How to Increase the Efficiency of a Stirling Engine

The efficiency of a Stirling engine can be increased through the below-given methods:

Increase the power in the first stage: In the first stage of the cycle, the pressure produced by the hot gas pushes the piston to perform work. If the pressure during this stage will be higher, the engine’s output power will also be higher. One method to increase this pressure is to increase the gas temperature.
Reduction of power consumption in the 3^rd stage: In the 3^rd stage of the Stirling cycle, the piston compresses the gas by utilizing some part of the energy generated in the 1^st Reducing the pressure during the 3^rd stage of the cycle can reduce the power consumed at this stage of the cycle (it can effectively increase engine power). Another method to reduce the pressure is to reduce the gas temperature by cooling it.
Make a large Temperature difference: The large temperature between the hot and cold cylinders helps to increase the efficiency of the Stirling engine.

Advantages and Disadvantages of Stirling Engine

The Stirling engine has the following advantages and disadvantages:

Advantages of Stirling Engines

These types of external combustion engines have high flexibility.
The Stirling engines start quickly and are more efficient in cold weather.
The piston of this engine doesn’t require an explosion for movement.
They have quiet operation.
These engines can use any available heat source such as biological, geothermal, and nuclear sources.
They have easy maintenance and high fuel versatility.

Disadvantages of Stirling Engines

These engines have a complex design.
They require an additional external heat source to heat up the working fluid (gas).
These are expensive engines.
Stirling engines have a high weight due to an additional heat source.
They have very expansive heat exchangers.
They can’t start quickly and run efficiently in hot weather.

Applications of Stirling Engines

A Stirling engine is an energy transformation equipment that can use as a heat pump, refrigerating engine, and prime mover.
Stirling engines are used at the water pump stations.
They are employed for solar power production.
Stirling engines can be used as a propulsion system in submarines or other underwater vehicles.
They are used in nuclear power plants.
These engines are used for aircraft.

Difference between the Stirling Engine and IC Engine

The main difference between the Stirling engine and the internal combustion engine is given below:

Stirling Engine	IC Engine
The Stirling engine needs an external source for heating the working fluid (gas).	IC engine doesn’t use any external heat source for heating the working fluid.
There is no limitation of heat source because Stirling engines can run on any accessible fuel such as biological, geothermal, and nuclear sources, etc.	It can’t run at any available fuel. This engine needs a specific fuel for its working, such as petrol or diesel.
It utilizes a single-stage working fluid.	It uses a two-stage working fluid.
These engines run quietly.	These engines produce high noise.
They have a quick start-up and efficient working in cold weather.	They run efficiently in cold weather.
The Stirling engines have more flexibility than IC engines.	These have lower flexibility.
They have a high cost.	They have a low cost.
These engines have a high weight compared to IC engines.	They are light.

FAQ Section

Why are Stirling engines not used?

All over the world, the maximum electricity is generated via steam turbines. This is because steam turbines have high flexibility and can operate by using various heat sources like heat from a nuclear reactor, biomass, or coal.

The Stirling engines are infrequently depleted to generate electricity. This is because they require very exotic alloys in their heat exchangers. In theory, these engines have high efficiency, but they have high cost and large size. When it comes to generating electricity, Stirling engines cannot match the long-term efficiency, performance, and reliability of turbines. Due to these reasons, Stirling engines are not popular.

What type of engine is a Stirling engine?

The engine which uses the Stirling cycle is known as a Stirling engine. The Stirling engine has an additional external heat source to heat up the working fluid (gas).

Why aren't Stirling engines most common?

They have a large size.
They have a high cost.
These engines need complex heat exchangers.