The Stirling engine is an external combustion engine that works on the principle of thermal expansion and contraction of a fluid (technically, the term 'fluid' refers to both gases and liquids). It was invented by Rev Robert Stirling, minister of the Church of Scotland, in 1816.

He had noted the death and injury rates caused by working around, or with, steam engines. Steam engines were prone to exploding, and his aim was to devise an engine that could not explode and would produce more power than the then current steam devices.

His initial designs used air as the thermal fluid to drive the engine and, as a consequence, they were known as 'hot air engines' until the 1940s when the use of other fluids (such as helium and hydrogen) started.

How They Work

Understanding how a Stirling engine works is not a simple matter. They are not overly intuitive. A web search will provide links to sites with diagrammatic explanations of the device, which are invaluable to understanding it. The underlying principle is that of thermal expansion and contraction of a fluid due a temperature differential.

A good set of diagrams explaining the operation of the engine can be found at:

• Single Cylinder Stirling engine Diagram
• Two Cylinder Stirling engine Diagram

The basic component of the engine is a sealed cylinder containing the fluid (air, hydrogen etc). One end of the cylinder is heated, the other cooled. Along with the cylinder, the engine will have one power piston and one displacer piston, or two power pistons (these are the basic variants). The engines work on a cycle of Heat - Expand - Cool - Contract.

Beta Type Engine

It should be noted that, for this type, the following applies:

• The displacer piston is not flush fitting; this is to allow the fluid to flow round it. The purpose of this piston is to move the fluid around within the cylinder.

• The heating and cooling processes are applied continually. There is no timed application of either. The heated end is constantly hot; the cooled end is constantly cooled.

To envisage the device, think of a cylinder, capped at one end. Inside the cylinder is a displacer piston, which doesn't quite fit, attached to a fly wheel. Next is the power piston (which does flush fit) which is also attached to the flywheel, but attached 90° further round than the power piston so that the flywheel does not push/pull them together but ¼ turn out of sync. The closed end of the piston is heated and the end near the flywheel is cooled.

1. Heat: At this point the fluid is at the heated end of the cylinder. The heated fluid expands. This drives the pistons towards the other end of the cylinder.

2. Transfer: As the power piston is ¼ turned further round, and is mounted further up the cylinder and as the flywheel turns and the displacer has reached the top of its cycle, the power piston still has some travel left to do. The power piston continues outwards drawing the fluid round the displacer and into the cooled end of the cylinder. As the crank goes round, the displacer is also forced back down the cylinder, driving more fluid into the cooled end of the cylinder. Now the majority of the fluid is in the cooled end.

3. Cool: With the cooled fluid now contracting, the power piston is pulled back into the cylinder. The majority of the gas is still at the cool end, and the displacer is almost at the bottom of its cycle.

4. Transfer: Now with the majority of the gas at the cooled end of the cylinder, and the displacer at the bottom, the crank continues to turn, driving the power piston back into the cylinder (as well as it being pulled back in). Due to the cycle differences between the pistons, and the differing lengths of their strokes, as the power piston descends, the displacer is just starting its way back to the top. This allows the power piston to force the fluid back round the displacement piston and into the heated end.

At this point, the cycle repeats.

Alpha Type Engine

This type uses two power pistons and no displacer. It uses an extra unit called a 'regenerator', also invented by Rev Stirling, which is more commonly known today as a heat exchanger.

Imagine a 90° v-twin conventional engine layout; a common flywheel and crank, with two pistons attached to it. These pistons are at 90° to each other and attached to the crank at the same point, so they are half out of phase. This means when one is at the top of the stroke (Top Dead Centre or TDC¹), the other is halfway through its (down or up, depending) stroke.

One of the two cylinders is heated and the other is cooled. A pipe is added to connect the top of the two cylinders, which allows the space at the top of cylinder A and the space at the top of cylinder B to be linked as one sealed space (sealed by the pistons; in this version there is no flow past the pistons). Halfway along this pipe there is the heat exchanger (or regenerator). Cylinder A is the heated cylinder, cylinder B the cooled cylinder.

The following should be noted during the description below:

• Both pistons are power pistons.

• The pistons fit flush to the cylinder since there is no requirement for the fluid to flow round them.

• The heat and cooling is applied continuously (as per the Beta engine).

• At the start of the cycle, piston B is at the top and piston A is halfway down (ie, on the down stroke) its cylinder. The fluid is cold and entirely in cylinder A and the pipe.

The cycle of the engine is as follows:

1. Heat: The fluid in cylinder A is heated and so expands. As it expands it pushes piston A down, flowing through the pipe and the heat exchanger, into cylinder B - pushing piston B down as well.

2. Transfer: Most of the fluid is still in cylinder A, with piston B halfway down and piston A at Bottom Dead Centre (BDC²). As the crank continues to turn through the next 90°, it pushes piston A back up and pulls piston B to BDC. As it does so, it pushes most of the fluid through the heat exchanger and into cylinder B.

3. Cool: Cylinder B now has most of the fluid in it. Piston B is at BDC and piston A is half way through its upward stroke. Cylinder B is cooled, and the heat exchanger also takes some heat out of the fluid. This causes the fluid to contract, which in turn pulls both pistons up. For piston A this means it is now at TDC, whilst piston B is half way to TDC.

4. Transfer: The final stage of the cycle, with piston A at TDC and piston B halfway to TDC. The fluid is cooled. Now the flywheel and crank turn another 90°, causing Piston A to be drawn back halfway to BDC and piston B to be pushed up to TDC. The fluid is therefore pumped back, through the heat exchanger, into cylinder A (the hot cylinder). As it passes through the heat exchanger it takes back some of the heat so that when it enters cylinder A it is already partially heated. Once in cylinder A it is heated and we go back to step one.

There is another type, the Gamma type engine, which is conceptually the same as the Beta type in that it has a power and a displacer piston. However, in the Gamma type, these two pistons are in their own (linked) cylinders.

The Heat Exchanger, or Regenerator

This plays a key role in efficiency since it both pre-cools the fluid transferred to the cooled cylinder and preheats the fluid transferred to the heated cylinder, thus reducing the work both have to do and reusing some of the energy from each cycle.

Fuels

The Stirling engine can use a variety of fuel systems.

So far solar, biomass, gas (as in natural gas or propane) and various liquid fuels such as paraffin, petrol and diesel have been used. But essentially any fuel can be used since the fuel is only heating the engine, not providing the power of the engine directly.

However, there is an inefficiency here, as the heat (and cooling) is applied through the walls of the cylinder, which means heat and energy is lost to the cylinder wall.

Main Uses

Propulsion

The Stirling engine has lacked development due to the success of the various internal combustion engines. Still, there are examples in use today in a variety of applications. They are all really proprietary models and developments. There are various societies around the world dedicated to these engines and their continued development.

Norwegian maritime engineering firm Kockums, who also make the Stealth battleships, make a military submarine that uses a Stirling engine for silent and underwater running. It greatly increases the submerged time as well, from a couple of days to a couple of weeks. The engine drives a generator which in turn drives the electric motors that drive the submarine.

Similarly, the French research submarine Saga is powered by a Stirling engine.

The engines were also developed by car companies, including Ford, AMC and GM, in the 1970s in the USA. The 1979 AMC Spirit Stirling experimental vehicle was powered by a Stirling engine they christened the 'P-40'.

However, these never became more than experiments as shortly thereafter the price of oil dropped in the 1980s.

The Stirling engine also suffers from a long heat-up time³ which for a car means a delay between switching it on, and moving off.

Generator

There is a company who make a 750W generator/heater/water cooler based on a Stirling engine. They do a fixed and a mobile version, the latter primarily for yachts.

Other Uses

Due to the operating principle of these engines, most can be run in reverse to provide refrigeration services. By connecting a motor to drive the crank, the fluid is compressed and expanded, thus providing a heat transfer mechanism. One cylinder will become heated, the other cooled.

Stirling engines can be found in cryo-cooling systems. If designed correctly, it is possible to get a Stirling engine where the cold end can get as low as 10 Kelvin. One use of this is with very small Stirling engines used to cool the IR chips in some night vision devices.

The temperature differential does not have to be that great. There is at least one company manufacturing low temperature differential Stirling engines as models or demonstrators. These can run off as little heat as having a coffee cup placed on them, or even from body temperature applied via the hand.

The main problem with low temperature differential is that the engine produces very little power.

Energy production. California has contracted a company to provide solar power using Stirling engines. The engines will generate electricity for the state power supply. In this case the stirling engines are even more environmentally friendly since they are powered (ie, heated) using solar reflectors (or dishes).

The Future

Currently, the Stirling engines in production cannot compete with internal combustion engines on a commercial basis. The Stirling engine still needs a lot of development. To get any decent amount of power out of them, they have to be large and heavy. They are difficult to control and they suffer from delays to come to operating temperature.

There are several companies and individuals who are putting in the work to develop a commercial, viable and competitive Stirling engine generator for the market.

Equally, there are various research and educational establishments who are actively refining and perfecting the engine designs.

As it stands, the Stirling engine has a lot of promise, but equally, it does have problems. They are low power, currently, and a 5HP engine can be very large due to the volume that needs to be moved around to get the power.

Equally, heating and cooling through a cylinder wall is not an efficient way of transferring the heat to the fluid. Especially when dealing with large capacity engines.

But, as companies look increasingly to alternative power units, it is entirely possible that the Stirling engine will find its own niche in the marketplace, perhaps as part of a hybrid power plant, or through further development and optimisation.

Further Information

You can find out how to make your own on the SFA Stirling Engine Project page. More useful links can be found on the Stirling Engine and Hot Air Engine homepage.