For the latest in Stirling engine development news, see Quiet Revolution Motor Company, LLC.
The Stirling is an external combustion engine, and in that respect is similar to a steam engine. Fuel is not critical, it can run on anything that produces heat. It was invented in 1816 by Dr. Robert Stirling, a Scottish minister, and for many years competed with the steam engine.
When a confined body of gas (air, helium, whatever) is heated, it's pressure rises. This increased pressure can push on a piston and do work. The body of gas is then cooled, pressure drops, and the piston can return. The same cycle repeats over and over, using the same body of gas. That is all there is to it. No ignition, no carburetion, no valve train, no explosions. Many people have a hard time understanding the Stirling because it is so much simpler than conventional internal combustion engines.
First, the Stirling is silent. Aviation needs quiet airplanes. Smooth torque and lack of vibration are good reasons too. General aviation is the last major user of leaded fuel, and we need to find an engine that doesn't cause this pollution. For safety reasons, we also need a fuel that is less explosive. Stirlings will burn turbine fuel, home heating oil, or whatever.
The Stirling is a very fuel efficient cycle. In fact, it comes closest to the Carnot theoretical limit of efficiency, and is better than the diesel or otto or turbine engines. The Stirling has cool exhaust!
Altitude performance is a strong reason to develop the Stirling. Imagine what would happen if we had powerplants that didn't lose power at altitude. If a Bonanza, for example, could hold constant power, it would cruise twice as fast at 40,000 ft as it can at sea level. This is due to reduced airframe drag in thinner air. Since the Stirling operates on the ratio of outside ambient temperature to burner temperature, as OAT drops the power actually increases. So the plane can fly more than twice as fast. We can expect to develop general aviation aircraft that easily fly nonstop coast to coast when we have the Stirling powerplant. Plus, flying above the weather rather than through it has safety advantages, too.
Those are just a few reasons why we need this new powerplant. Most of us are flying with the same basic engine the Wright brothers used in 1903! Isn't it time we moved on?
While millions are being spent on museums and monuments to the past, very little effort is being directed to the future. Aviation seems to be waiting for 1940 to come back. Well, that is not going to happen. Time marches on. One small example: we still use the same magneto ignition used by farm tractors half a century ago. Tractors improved, automobiles improved, but aircraft powerplants haven't. Aviation has the choice of moving forward, or being left behind.
Probably the most common question asked about Stirlings is "If they're so good, why don't we have them already?" There are many answers.
First, a lot of ideas don't take off immediately. It took centuries for most folks to accept that the world was round.
Second, there are Stirlings that touch our daily lives. The Stirling is bidirectional, that is, if temperature difference is applied, rotation is produced. But if rotation is applied, temperature difference is produced. So the Stirling makes a refrigerator. If you go to your local welding supply company and purchase liquefied gas (such as liquefied oxygen or liquefied nitrogen), it was made in a Stirling machine. When you watch the satellite weather pictures, they are courtesy of a tiny Stirling cryocooler, used in the satellite to cool the image sensor to near absolute zero. So the answer is that we do have them already.
The third answer is found in the history of the technology. Steam and Stirling grew up together early in the industrial revolution. Indeed, Rev. Stirling developed the machine in response to the human suffering of steam boiler explosions. But cast iron was the material of the day, and cast Stirlings didn't fare as well as steam engines. Plus, workers were cheap and liability was nil. The fuel efficiency of the Stirling didn't matter when coal was a few cents per ton. So steam won out. If Bessemer had come along sooner with his steel, we might have enjoyed the Stirling age rather than the age of steam.
Once a technology is established and has a constituency, it's difficult to displace. We see that today in the effort to interest the aviation community in a new powerplant. Spark plugs are available, piston rings are available, exhaust valves are available, crankshaft grinding equipment is available, there are experts in every tiny engine component. Mass production makes most of the parts reasonable in price, and the technology is well understood.
Most of the Stirling R&D of the past 25 years has been directed toward the automotive field. This is the ultimate example of established technology versus the newcomer. There is an advantage in Stirlings, but not enough advantage to outweigh the present investment in doing engines the way they're done. (And Stirlings don't make very good car engines, anyway.)
But aviation is different. We make very few engines, they are hand assembled, and prices are outrageous. Plus, present engines don't do what we need. As we climb, they put out less and less. Although the plane wants to fly faster due to decreased drag, the engine is losing power faster than the airframe is losing drag. So we reach a ceiling. We've always been limited by a powerplant ceiling so we accept it. Likewise, airplanes make a lot of noise because they've always made a lot of noise. But it doesn't need to be that way.
Why don't we have them already? Perhaps we've lost the sense of adventure. Perhaps we believe everything has been invented. Or perhaps in a few years we will have Stirling powered aircraft. Then the question will no longer exist.
For the latest in Stirling engine development news, see Quiet Revolution Motor Company, LLC.