Monday, August 24, 2009

The Reverend Stirling's Engine



Robert Stirling's Unique Engine
2nd post on the Reverend Stirling's Engine

Beautiful German model of a gamma Stirling engine
clearly showing the 2 piston configuration

(note the cooling fins on the power piston)

It is almost 200 years since a minister of the Church of Scotland invented a unique type of engine. His engine employed radically different techniques to anything before and much since. It took science some time to explain the principles at work, even today there are doctorates being written on aspects of the Reverend Stirling's engine.

The Stirling Engine was not Popular

The engine never really 'caught-on'. It was used mainly to drive water pumps and for some industrial purposes but compared to the steam engine, it remained little more than a curiosity.

There are two main reasons for this:

1. The engine does not produce the massive power that steam engines can produce.
2. The engine uses a direct external heat source and does not have the store of power like steam built up in a boiler. Because of this, it cannot easily or quickly accelerate, nor can it slow down quickly, due to heat lag. The Stirling engine prefers to run at a steady speed and was therefore not great for automotive purposes.

Has Stirling's Time come?

It would appear that the time have arrived come for a second coming of Robert Stirling's engine. Today we need engines to drive electricity generators and for this purpose the Stirling engine is just the biz because with electrical generator what you need is an economical motor with a steady speed and output, there is no need for acceleration.

The Stirling Design Concept

Reverend Stirling's engine directly converts heat into mechanical motion, and it does this very efficiently. Given the times 1818, almost 200 years ago, Robert Stirling's engine was very special indeed in both its concept and design.

The main advantages of the engine were (a) economy, (b) quietness of operation, (c) the ability to use just about any fuel or heat source, and (d) safety.

Wikipedia says of the Stirling Engine:

"The Stirling engine is noted for its high efficiency, quiet operation, and the ease with which it can utilize almost any heat source. This compatibility with alternative and renewable energy sources has become increasingly significant as the price of conventional fuels rises, and also in light of concerns such as peak oil and climate change. This engine is currently exciting interest as the core component of micro combined heat and power (CHP) units, in which it is more efficient and safer than a comparable steam engine. Widespread adoption of CHP could have a significant effect upon worldwide energy utilization."

In the next post I will have a go at understanding how the Stirling engine works and a look at some of the main variations of the design.


1 comment:

brutontom said...

Hi Tony, great to read your article about the Sterling engine.
The Wikipedia text is misleading. I would have said a stirling engine was noted for its relatively low efficiencies rather than its high efficiency (10 or 12% electrical vs. >30% for a standard internal combustion gas engine). This is the single biggest drawback to a stirling engine.