Stirling Engine types
Engines have become the source of power since the era of industrial revolution. Over the years, the engines have become more powerful and more efficient. One of the simplest and commonly used engines of modern world is the Stirling engine (previously known as economizer), developed and patented by Rev. Robert Stirling in Edinburgh, Scotland in 1816.
It was developed as a safer version of the steam engine. Earlier version of Stirling engine was further developed in the mid 1800’s by Professor McQuorne Rankine. Its early use was limited to low power domestic applications for a century. Stirling engines were over shadowed by other engines such as steam engines and internal combustion engines.
As many scientists have predicted an end to our dependence on fossil fuels, Stirling engines has been in limelight. Today, Stirling engines can run on various fuel and heat sources and provide an output far superior to modern day engines.
Stirling engine is a heat engine, where there is cyclic compression and expansion of air or other gas at different temperatures, such that heat energy is converted to mechanical energy. They are highly efficient compared to steam engines with 50% efficiency. They can operate without any noise generation and can work on any heat source. As Stirling engines are compatible with alternative and renewable energy sources, it is gaining its significance in the recent years amidst concerns over rising oil prices, climatic changes and depletion of fossil fuels.
Types of Stirling Engines
Modern engineers classify Stirling engines into three major types. The Alpha type uses multiple cylinders while the Beta and the Gamma types use only one cylinder.
It contains two pistons in two separate cylinders, one hot piston and one cold piston. The hot piston cylinder is placed inside the higher temperature heat exchanger while the cold piston cylinder is placed inside the low temperature heat exchanger. High temperature of the hot piston raises some technical issues. Another distinct disadvantage of this model is the durability of the seals. However, this engine has very high power- to- volume ratio.
It has only a single displacer piston to move the working gas back and forth between heat and cold exchangers in the same cylinder. The displacer piston does not use any power from the expanding gas produced and it pushes the gas from high heat exchanger to cold exchanger. As the working gas is pushed to the hot end of the cylinder, the gas expands and helps to move the piston. This type does not exhibit any technical issues with the seals.
It is a mechanically simpler version which produces a low compression ratio. Almost similar to the Beta version, but the power piston is mounted on a cylinder next to the displacer piston cylinder. Due to convenient positioning of the cylinders connected to the same flywheel, along with the convenient piston and displacer assemblies, it is highly preferred among professionals.
In Stirling engines, the gas moves from the hot side of the engine where it expands, to the cold side where it contracts. Heating, expansion, cooling and contraction are the four main events that occur in a Stirling engine cyclically. It is a closed cycle and has a fixed mass of gas known as working gas. Air, hydrogen or helium is used as the working gas. The Stirling engine works on the temperature difference between the hot end and cold end, converting thermal energy into mechanical power. Greater the temperature difference, greater the efficiency of the cycle.
New varieties of Stirling engine are being developed. Thermo acoustic Stirling engine is a recent version of this engine without displacer and it uses high amplitude sound waves to pump heat. Stirling Radioisotope Generator is currently used for space exploration. Stirling engines are also used in automotive industry, for portable refrigeration and in solar power generation. In recent years, it is widely used to generate electrical energy.
Video explanations of Stirling Engines and how they work