Air engines require a heat source and a method of giving up the heat in the medium (cooling). No explosive gas mixtures are required such as in the IC engine. Because of this the concept has always had great appeal.
The low power outputs have always held back the idea of the air engine especially in the face of the tremendous power achieved with the oil guzzling IC engines. However in the last 50 years or so, there has been a resurgence of interest in this technology. The ability to run clean is a draw no doubt, in this carbon adverse age. As early as the turn of the 19th century, hot air engines were built that ran on the concentrated rays of the sun. And of course most model builders are familiar with the model that runs on the heat from a cup of coffee.
If you have always wanted to build one or are just interested in the tech., here is a great book written by T. Finkelstein and A.J. Organ in 2001. The subject is well covered from Stirling's development, through the 18 and 1900's, to the modern post-revival. If your interested this is good stuff.
To download "Air Engines" go to my Books - Free Downloads page. # 55 - 4 MB - pdf
So a little update on our delayed spring. I can't remember the last time we still had winter snow on the ground on May 2, it's been a while. Before I did the aggressive landscaping, about half of the lane way had land sitting higher than the surface of the lane way, on both sides. This resulted in the lane way remaining soft for a long time, during spring melt, (my old Cavalier sank to the axles one year). After terracing the north side of the lane way, I dug a drainage ditch between the terrace and the lane way, and cut away the south side of the lane way to a good slope. I haven't had a problem since. I was away for a while, but it looks like the lane way drains and dries as quickly as the snow melts.