Its not exactly suppressed when you can buy model kits based on the principles ...
http://www.en.boehm-stirling.com/a1.html
here is another interesting principle
http://www.en.boehm-stirling.com/hb24.html
If you are interested in some really mad paranoid conspiracy theories about suppressed impossible technology look online at the crazy stuff about Bruce De Palma (brother of Brian De Palma the famous movie director) and his homo-polar "N machine". I met De Palma on a few occasions before he died and he was definitely a few feathers short of a full duck and the machine definitely does not work.
http://depalma.pair.com/