Stirling Engine – Enormous Potential Or Lame Duck?

The inventor of the Stirling Engine, also often referred to as the Sterling Engine, was Robert Stirling, a Church of Scotland Minister. Born in 1790, Stirling was the third of eight children and apart from studying Divinity had a keen interest in engineering.

In 1816, at the tender age of 26, Stirling invented and patented his “heat economizer” later known as the Stirling Engine.

A Stirling Engine is a closed cycle heat engine which obtains its heat externally, in contrast to the internal combustion engine. It has this feature in common with a steam engine.

The gas used in the cylinders can be air, helium or hydrogen but the external heat source can be a big range of substances such as solar, salad oil, coal, peat and straw. This feature makes it an attractive proposition for research in the current climate of declining fossil fuels.

How Does A Stirling Engine Work?

The engine operates through a temperature differential process where the gas (air) is moved from the hot side to the cold side of a sealed cylinder.

There are two basic types – the two-piston engine and the displacer engine.

In a displacer engine the operation is illustrated below:

diagram of displacer Stirling engine operation

The heat is applied from the bottom and the top is kept cool. The displacer piston pushes the air to the top and the power piston drives it back down again and so the cycle repeats itself. The crankshaft also drives a flywheel.

In a 2-piston type the operation is illustrated below.

diagram of 2-piston Stirling engine operation

Graphics courtesy of American Stirling Company

In this case the cycle occurs as the piston on one side is kept continually warm, while the other piston is kept cool.

Small models can be constructed that work with the heat of the hand as the heat source.

The engines have the following features:

  • Very quiet operation
  • Require minimum fuel usage
  • Much more efficient than an internal combustion engine
  • No harmful emissions
  • The cycle is reversible

What Are The Uses Of A Stirling Engine

As a result of its obvious advantages, considerable research has been done in order to come up with a model that has widespread usage capabilities. For example, in the 1970's, car makers invested heavily in research and development to produce a Stirling engine to power motor vehicles. Unfortunately they couldn't overcome the problem of quick starting and at the time the production ceased.

One of the main issues is that engines running on small temperature differentials tend to be pretty large for the amount of power they put out. Any decrease in the temperature differential means an even bigger engine for same power output.

Today the Stirling engine is used in submarines and power or heating for yachts where the low noise factor is an important consideration. The reversible feature finds a use in cryogenics where substances are subjected to very low temperatures and their behavior is monitored and studied at those temperatures.

The research is back on the agenda again these days as the search for more efficient and non-polluting engines heats up. it is definitely not a lame duck - it has great potential.

It will be a great breakthrough if a Stirling engine in the range 5-25 Kw can be commercially produced. The applications would be enormous as far as powering our homes is concerned.

There is quite a substantial market for small scale working models, beautifully engineered, that are used in educational settings and also snapped up by people who are fascinated by them and keep them in a prominent place at home or work.

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