So, this weeks preview is a good one. I’ve got to write about my favourite engine, and lucky me, I happen to be one of those special people (read: Steampunk geek) who already had a favourite engine before taking this class. So, prepare to be amazed at the wonders of Victorian era science and engineering, manifest in the Reverend Dr. Robert Stirling’s amazing engine.
What is a Stirling Engine?
Really, the design of the Stirling engine is rather simple: you have a pair of chambers containing gas and a pair of pistons. The chambers are connected to each other, so that gas can flow between the two. If one chamber is heated, it expands, pushing the piston out, and pushing gas into the second chamber. This drives the second piston out, which by linkages push the “hot” cylinder back into its chamber. As this happens, the gas in the “cold” chamber begins to cool, pulling the piston in, and pulling the “hot” piston out again, along with the force of the expanding gas in its chamber. All of this allows the motion of the pistons to follow the natural expansion-contraction cycle of the gas in the two chambers. The gas in the system is totally self-contained, and all that is needed to get it running is a temperature difference between the environment of the two chambers. It all is a bit better explained by this diagram (courtesy of Wikipedia):
So, what does an external combustion engine from the early 19th century have going for it now (other than the fact that it was promoted by the man with the coolest name ever, Fleeming Jenkin)? Well, if you want to beat a heat engine, it is pretty much impossible to beat a well-built Stirling in design alone. An ideal Stirling engine (with perfect insulation, frictionless bearings, etc.) will run at the same efficiency as the Carnot cycle, which is the most efficient possible (ie, as in allowed by the laws of physics!) engine cycle. So, look no further than the old Stirling for an efficient design. It is also completely self-enclosed, so you have no need to worry about refilling water that gets boiled off or any other such silliness. In fact, Stirling engines don’t really involve state changes at all! Just plain old expanding gas. Besides being efficient and durable, Stirling engines can also run on really tiny heat differences (the record is 0.5K!), giving you the ability to draw energy from many different sources of waste heat. As if all that wasn’t enough, Stirling engines can be run in reverse! Rather than using a temperature difference to do work, you can do work to turn a Stirling engine over and pump heat from one end to another. Depending on what end you use, you can use it as either a heater or a cooler (one end heats up, the other cools down). These engines are so versatile, that NASA has evaluated and used them as power sources for robotic space missions, as well as heaters and coolers for instrumentation in space. How many 19th century combustion engines are part of cutting edge space research? In fact, Stirling engines have seen something of a resurgence in the past few decades, as they provide a simple way of efficiently converting heat to work, and can use essentially any heat source (including myriad green power sources, like solar thermal, geothermal, or modern nuclear). How cool is that? (answer: very)