Nothing but hot air - Stirling engines

I found this chap and his demonstration models of Stirling engines in the model tent at the Boconnoc Steam Fair. I'm afraid I forgot to ask him his name but I had a long chat with him about the Stirling engine and the principles involved. I'd heard of Stirling engines many years ago but had discounted from my sphere of interest because they didn't have any automotive application. Stirling engines operate simply on temperature differentials. Low friction materials have given a new lease of life to this very old concept, named after Rev Dr Robert Stirling who patented the idea in 1816.

Boyle's Law states that as the temperature of a gas in a sealed container increases so does its pressure and the Stirling engine exploits this principle by have a heat source and a heat sink, such as air fins. When gas is allowed in contact with the cold and heat alternately, it can move a piston, displacer or diaphragm, which in turn can drive a crankshaft.

Stirling engines operate on a closed cycle, so contain a fixed mass of gas called the "working fluid", most commonly air, hydrogen or helium. No valves are required, unlike other types of piston engines, and instead of the ignition, compression, expansion and exhaust cycle of the internal combustion engine, Stirling engines operate through cooling, compression, heating and expansion. There are no external emissions and the thermal efficiency increases with the temperature difference between the hot and cold sources.

Very low-power engines have been built which will run on a temperature difference of as little as 7 °C, or between room temperature and melting water ice. This guy had his model engines running on little hot plates powered by small bulbs. He also showed me how one can run from the heat of your palm. The heat sink in this case was the atmosphere.

After years in the doldrums, Sterling engines are now being examined by Powerdrive (of Subaru works rally car fame) for domestic CHPs or Combined Heat and Power units. By using domestic temperature differentials, such as waste hot water or cold air outside, Stirling engines could provide power for households and even enable them to generate electricity for sale back to the grid when it's not required by the householder.

NASA have been developing Stirling engines for use in space missions and the Swedish Navy have used them to power their Gotland and Sodermanland class submarines, although I suspect their quite running has more to do with this than environmental reasons.

So could you use a Stirling engine in a car? Probably not.

Stirling engines take a while to get going and, once running, operate best at a constant speed. It's a fairly low speed at that. To overcome this, a fancy transmission would be necessary and this complication outweighs the Sterling engine's virtue of simplicity. Heat exchanger design can also get a little involved and the power to weight ratio is poor. But what an elegant concept!

NASA converted a 1986 Chevy Celebrity to Stirling power and Ford built a prototype that used electricity to overcome the start up time, which NASA got down to 30 seconds. Norwegian company Think Nordic are experimenting with Stirling engines as a hybrid power source to supplement their range of wholly electric vehicles. That seems to be the most likely avenue of automotive development to me.

On the whole, though, Stirling engines must have a bright future. I just can't see them running around on wheels much.

Wouldn't mind on in a shed next to my house, though, especially if I can sell the excess electricity to the grid.