You Got to Be Kidding Me!

Dean Kamen Has Another Idea That’s Not As Good As He Thinks It Is

Posted in Engineering by Stacy McMahon on October 29, 2008

Remember when the Segway was “IT”, the miracle something-or-other that Jeff Bezos embarassingly predicted was going to change human society forever? This time IT’s a hybrid car powered by a Stirling engine.

The internal combustion engine, as we’ve known it for over a century, is what’s known as an open cycle device. That is, the so-called working fluid flows into and out of the engine and is constantly circulated. A closed cycle engine keeps the same working fluid contained within the device and heat is generated externally rather than from combustion inside the cylinder. The best known example of the latter is the stirling cycle engine.

Well actually the best-known closed-cycle reciprocating engine would be the triple-expansion steam engine that powered most of the world’s powerplants and steam ships until the late 1940s. But anyway…

The theoretical thermodynamic efficiency of the stirling cycle is 100 percent although creating a 100 percent efficient mechanism has proved elusive.

Of course being 100% efficient would make it a perpetual motion machine, but don’t worry, there’s no doubt the inventor of the Segway can repeal the laws of physics.

What makes the Stirling engine cool is that it can work ‘backwards’ i.e. you can make it run by feeding it a cold material just as well as a hot one. Perfect for geothermal power, or vehicles that travel into hostile environments where the ability to use whatever resources are available is more important than ultimate horsepower. Good for a hybrid car? Maybe, but as a general rule external combustion isn’t as efficient as internal combustion – that’s why steam disappeared off the scene rather quickly between 1925 and 1945.


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