As a kid, college really, I was intrigued about the idea of a "perpetual" motor. A motor that ran forever. My idea seemed like it should work, but I had a hard time getting someone to explain why it wouldn't. My idea was based on the flywheel of the single engine Briggs & Stratton where a magnet on the flywheel creates the spark for the ignition on each rotation. My idea was to have magnets that attract the flywheel and a reverse magnet to repel the flywheel once it got past. But I had the problem that the flywheel would get attracted and stuck. So I found something called paramegnetic materials, materials that repel both positive and negative magnetic forces. All I needed, then is to have a thin sheet of paramagnetic material pass between the attracting magnets to let the flywheel move on to the repelling magnet. Perfect, a perpetual motor.
I finally got to talk with a Physics professor at USF who explained my small, but subtle issue with the perpetuity of my motor. When you use a magnet, you loose a magnet. It took energy to magnetize a magnet, so the process of using it will deplete it!
For decades, there have been articles about perpetual motors... But generally they have gone the way of "cold fusion".
Here is a very cool article/technology on a motor that runs on air. Liquefied Nitrogen, actually. Very cool. Literally, about -210 C (or -340 F). So, if the internal combustion motor works on the temperature differential before the ignition of fuel and after ignition, the liquid nitrogen concept works in the same way: from really really cold, to cold. Not nearly the same as the 1,000 times differential from gasoline, but still an effective motor. Effective only once you overcome the problem of things freezing up in the process.
So here's the great Wired article by Nicola Twilley about the inventor Peter Dearman: A One-Time Poultry Farmer Invents the Future of Refrigeration: Mechanical cooling revolutionized the global food supply—and accelerated global warming. Peter Dearman’s liquid air engine could change all that.
The thing that Dearman had to overcome is to bring the temp of the super cold nitrogen up enough that it didn't freeze up the works. (Kind of a reverse of the radiator idea to cool the motor down.)
So the motor works, not especially efficient, but it works.
However, your favorite internal combustion engine is very inefficient. Your car is only about 15% efficient. Diesel turbine motors for electricity are generally about 40% efficient, at best... Unless... Unless you need the excess heat. So if you can use the heat, like hot water on a campus environment, then the combined heat and power (CHP) can be very efficient, maybe up to about 70%.
Imagine if you could use the cool from a liquid nitrogen engine? Say, hypothetically, for refrigerated storage or reefer. (No, not a Jimmy Buffet kind of Reefer!:-) A refrigerated reefer truck.
And, wa la. You have a really great method of efficiently transporting and simultaneously cooling perishable products.
The cryogenic reefer truck seems to be really gaining traction (sorry about the pun) within several food chains.
Dearman says the nitrogen solution will result in a 40% improvement over diesel in terms of greenhouse gases. If is the nitrogen is liquefied (chilled) by renewable energy the improvement compared to diesel moves up to 95%.
It also helps to overcome the need for Freon or the replacements for Freon. (Fluorocarbons are a wicked greenhouse gas that blow holes in the ozone layer.)
With 78% of the Earth's atmosphere, nitrogen (N) is readily abundant.
Dearman has several patents related to cryogenics and cryogenic motors.
Interestingly, it would appear that the same Peter (T?) Dearman is also the inventor of respirators and ventilators back in 1990!