Introduction

The Stirling Engine was designed by Rev. Robert Stirling, a Scottish minister in 1816. It is an external combustion engine, with suitable fuels being anything from petrol and wood to sunlight and even human metabolism - there's a toy example which will run from the warmth of your hand. One of Rev. Stirling's design goals was safety. With a fixed quantity of gas inside, the engines do not explode as steam engines of the day were prone to doing.

The engine can be remarkably efficient - up to around 50% of the theoretical maximum - and can even be run on waste heat for truly free power. They are also eerily quiet.

The operation of the engine is based on the Stirling Cycle - that is expansion (by heating), displacement, contraction (by cooling) and displacement again. The basic system requires two zones, one hot and one cold, and a means of transferring gas - which may be air - between them.

The engine is reversible, so that putting mechanical work into the output shaft will warm one cylinder and cool the other. In this capacity, it may be used as a heating or refrigeration unit.

Principles

The volume of a gas alters according to its temperature. When it is warmed, it will expand (or if expansion is impossible, the pressure will rise). When cooled, the gas will contract. A causative system is set up, where the expansion of the gas causes the gas to be moved to the cool zone. The gas then contracts and this causes the gas to move back to the warm zone.

One of the simplest systems to describe contains two pistons and an interconnecting tube. The pistons are placed 90 degrees out of phase on a crankshaft.

     ____________
    |  ________  | 
    | |        | |
  +-+ +-+    +-+ +-+
 A|MMMMM|    |     |B
  |  |  |    |MMMMM|
  |  |  |____|  |  |   I flywheel
  | |-|        ||| |   I
  ==| |========|-|=====I======  output shaft
  | | |        | | |   I
  |________________|   I

Taking the above diagram as stage 1, we can follow the positions of the pistons in each of 4 stages:

Piston        A      B
Position    D   U  U   D   (Down/Up)
-------------------------
Stage 1         *    *
      2       *    *
      3     *        *
      4       *        *
-------------------------

The volume of gas in the system is represented by the distance between pairs of markers, so we can see that in stages 1 and 2, there is a low volume of gas. In stages 3 and 4 there is a large volume.

Thus heating the gas at stage 2 will drive the engine to stage 3. Cooling the gas at stage 4 will drive the engine to stage 1. A flywheel keeps the engine turning on the remaining stages. Between stages 1 and 2, the gas is displaced from cylinder B to A. Likewise the gas is displaced towards cylinder A between stages 3 and 4.

Hopefully, by now, the cycle is clear. If cylinder B is warmed, and cylinder A is cooled, then the displacement combined with heating and cooling will drive the engine with two power stages and two stages running on flywheel intertia.

Real-life Examples

Work from the output shaft can be used to perform many tasks, such as running an air compressor, or an electricity generator.

An interesting real-life example is a water piston solar pump. This uses the Stirling Cycle to pump water vertically. The water is also used as pistons, and so the only moving parts of the sytem are water and two one-way valves. Such a system can be built from readily obtainable parts and is suitable for pumping water in primitive environments. Apart from a little fine-tuning to optimise the pump initially, it requires little to no maintenance.

Stirling Engines are used - in reverse - as refrigerators in the science of Cryogenics. There are examples capable of cooling to below 10 degrees Kelvin.

Drawbacks

With all this talk of efficiency, surely Stirling engines should be in mass-market applications such as automobiles? Well, they're not actually suitable. Due to the method of heating, the engines are not quick to react to changes. As a generator which is started then run at a constant rate for a while, they are good, but fine throttle control is not really feasible.


Sources:
http://www.stirlingengine.com
http://www.howstuffworks.com/stirling-engine.htm

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