Stirling Engines

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STIRLING ENGINES

The principle that makes Stirling engines possible is quite simple. When air is heated it expands, and when it is cooled it contracts. Stirling engines work by cyclically heating and cooling air (or perhaps another gas such as helium) inside a leak tight container and using the pressure changes to drive a piston. The heating and cooling process works like this: One part of the engine is kept hot while another part is kept cold. A mechanism then moves the air back and forth between the hot side and the cold side. When the air is moved to the hot side, it expands and pushes up on the piston, and when the air is moved back to the cold side, it contracts and pulls down on the piston.

While Stirling engines are conceptually quite simple, understanding how any particular engine design works is often quite difficult because there are hundreds of different mechanical configurations that can achieve the Stirling cycle. Figure 1 shows a schematic of a transparent educational demonstration engine that runs on the top of a cup of hot coffee. This engine uses a piece of foam similar to what would be used as a filter for a window air conditioning unit to "displace" the air between the hot side and the cold side. This foam displacer is carefully mounted so it does not touch the walls of the cylinder. Figure 2 shows how this particular engine achieves the Stirling cycle. In this engine, the air flows through and around the displacer from the hot side then back to the cold side, producing a power pulse during both the hot and cold portion of the cycle. Stirling engines can be mechanically quite simple since they have no valves, and no sparkplugs. This can result in extremely high reliability as there are fewer parts to fail.

It is worthwhile to compare Stirling engines to other more familiar engines and note their similarities as well as their differences. Stirling engines are a type of heat engine. They turn heat into mechanical work and in this sense they perform the same function as other well known heat engines such as gasoline, diesel, and steam engines. Like steam engines, Stirling engines are external combustion engines, since the heat is supplied to the engine from a source outside the cylinder instead of being supplied by a fuel burning inside the cylinder. Because the heat in a Stirling engine comes from outside of the engine, Stirling engines can be designed that will run on any heat source from fossil fuel heat, to geothermal heat, to sunshine. Unlike steam engines, Stirling engines do not use a boiler that might explode if not carefully monitored.

When operating on sunshine, or geothermal heat, Stirling engines obviously produce no pollution at all, but they can be exceedingly low emissions engines even when burning gasoline, diesel, or home heating oil. Unlike gasoline or diesel engines that have many thousands of start stop cycles of combustion each minute, burners in Stirling engines burn fuel continuously. It's much easier to make a continuous combustion engine burn very cleanly than one that has to start and stop. An excellent demonstration of this principle is to strike a match, let it burn for a few seconds, then blow it out. Most of the smoke is produced during the starting and stopping phases of combustion.

A BRIEF HISTORY

In the early days of the industrial revolution, steam engine explosions were a real problem. Metal fatigue was not well understood, and the steam engines of the day would often explode, killing and injuring people nearby. In 1816 the Reverend Robert Stirling, a minister of the Church of Scotland, invented what he called "A New Type of Hot Air Engine with Economiser" as a safe and economical alternative to steam. His engines couldn't explode, used less fuel, and put out more power than the steam engines of the day.

The engines designed by Stirling and those who followed him were very innovative engines, but there was a problem with the material that was used to build them. In a Stirling engine, the hot side of the engine heats up to the average temperature of the flame used to heat it and remains at that temperature. There is no time for the cylinder head to cool off briefly between power pulses. When Stirling built his first engines, cast iron was the only readily available material, and when the hot side of a cast iron Stirling engine was heated to almost red hot, it would oxidize fairly quickly. The result was that quite often a hole would burn through the hot side causing the engine to quit. In spite of the difficulties with materials, tens of thousands of Stirling engines were used to power water pumps, run small machines, and turn fans, from the time of their invention up until about 1915.

As electricity became more widely available in the early 1900s, and as gasoline became readily available as a fuel for automobiles, electric motors and gasoline engines began to replace Stirling engines.

REGENERATION

Robert Stirling's most important invention was probably a feature of his engines that he called an "economiser." Stirling realized that heat engines usually get their power from the force of an expanding gas that pushes up on a piston. The steam engines that he observed dumped all of their waste heat into the environment through their exhaust and the heat was lost forever. Stirling engines changed all that. Robert Stirling invented what he called an "economiser" that saved some heat from one cycle and used it again to preheat the air for the next cycle.

It worked like this: After the hot air had expanded and pushed the piston as far as the connecting rod would allow, the air still had quite a bit of heat energy left in it. Stirling's engines stored some of this waste heat by making the air flow through economiser tubes that absorbed some of the heat from the air. This precooled air was then moved to the cold part of the engine where it cooled very quickly and as it cooled it contracted, pulling down on the piston. Next the air was mechanically moved back through the preheating economiser tubes to the hot side of the engine where it was heated even further, expanding and pushing up on the piston. This type of heat storage is used in many industrial processes and today is called "regeneration." Stirling engines do not have to have regenerators to work, but well designed engines will run faster and put out more power if they have a regenerator.

CONTINUED INTEREST

In spite of the fact that the world offers many competing sources of power there are some very good reasons why interest in Stirling engines has remained strong among scientists, engineers, and public policy makers. Stirling engines can be made to run on any heat source. Every imaginable heat source from fossil fuel heat to solar energy heat can and has been used to power a Stirling engine.

Stirling engines also have the maximum theoretical possible efficiency because their power cycle (their theoretical pressure volume diagram) matches the Carnot cycle. The Carnot cycle, first described by the French physicist Sadi Carnot, determines the maximum theoretical efficiency of any heat engine operating between a hot and a cold reservoir. The Carnot efficiency formula is T(hot) is the temperature on the hot side of the engine. T(cold) is the temperature on the cold side of the engine. These temperatures must be measured in absolute degrees (Kelvin or Rankine).

STIRLING APPLICATIONS

Stirling engines make sense in applications that take advantage of their best features while avoiding their drawbacks. Unfortunately, there have been some extremely dedicated research efforts that apparently overlooked the critical importance of matching the right technology to the right application.

In the 1970s and 1980s a huge amount of research was done on Stirling engines for automobiles by companies such as General Motors, Ford, and Philips Electronics. The difficulty was that Stirling engines have several intrinsic characteristics that make building a good automobile Stirling engine quite difficult. Stirling engines like to run at a constant power setting, which is perfect for pumping water, but is a real challenge for the stop and go driving of an automobile.

Automobile engines need to be able to change power levels very quickly as a driver accelerates from a stop to highway speed. It is easy to design a Stirling engine power control mechanism that will change power levels efficiently, by simply turning up or down the burner. But this is a relatively slow method of changing power levels and probably is not a good way to add the power necessary to accelerate across an intersection. It's also easy to design a simple Stirling engine control device that can change power levels quickly but allows the engine to continue to consume fuel at the full power rate even while producing low amounts of power. However it seems to be quite difficult to design a power control mechanism that can change power levels both quickly and efficiently. A few research Stirling engines have done this, but they all used very complex mechanical methods for achieving their goal.

Stirling engines do not develop power immediately after the heat source is turned on. It can take a minute or longer for the hot side of the engine to get up to operating temperature and make full power available. Automobile drivers are used to having full power available almost instantly after they start their engines.

In spite of these difficulties, there are some automobile Stirling applications that make sense. Hybrid electric cars that include both batteries and a Stirling engine generator would probably be an extremely effective power system. The batteries would give the car the instant acceleration that drivers are used to, while a silent and clean running Stirling engine would give drivers the freedom to make long trips away from battery charging stations. On long trips, the hybrid car could burn either gasoline or diesel, depending on which fuel was cheaper.

To generate electricity for homes and businesses, research Stirling generators fueled by either solar energy or natural gas have been tested. They run on solar power when the sun is shining and automatically convert to clean burning natural gas at night or when the weather is cloudy.

There are no explosions inside Stirling engines, so they can be designed to be extremely quiet. The Swedish defense contractor Kockums has produced Stirling engine powered submarines for the Swedish navy that are said to be the quietest submarines in the world.

Aircraft engines operate in an environment that gets increasingly colder as the aircraft climbs to altitude, so Stirling aircraft engines, unlike any other type of aircraft engine may derive some performance benefit from climbing to altitude. The communities near airports would benefit from the extremely quiet operation that is possible. Stirling engines make sense where these conditions are met:

There is a premium on quiet.
There is a very good cooling source available.
Relatively slow revolutions are desired.
Multiple fuel capacity is desired.
The engine can run at a constant power output.
The engine does not need to change power levels quickly.
A warm-up period of several minutes is acceptable.

LOW TEMPERATURE DIFFERENCE ENGINES

In 1983, Ivo Kolin, a professor at the University of Zagreb in Croatia, demonstrated the first Stirling engine that would run on a heat source cooler than boiling water. After he published his work, James Senft, a mathematics professor at the University of Wisconsin, River Falls built improved engines that would run on increasingly small temperature differences, culminating in an elegant and delicate Stirling engine that would run on a temperature difference smaller than 1°F.

These delicate engines provide value as educational tools, but they immediately inspire curiosity into the possibility of generating power from one of the many sources of low temperature waste heat (less than 100°C) that are available. A quick look at the Carnot formula shows that an engine operating with a hot side at 100°C and a cold side at 23°C will have a maximum Carnot efficiency of [((373 K–296K)/373 K) × 100] about 21 percent. If an engine could be built that achieved 25 percent of the possible 21 percent Carnot efficiency it would have about 5 percent overall Carnot efficiency.

That figure seems quite low until one realizes that calculating Carnot efficiency for an engine that uses a free heat source might not make much sense. For this type of engine it would probably be more worthwhile to first consider what types of engines can be built, then use dollars per watt as the appropriate figure of merit.

Stirling engines that run on low temperature differences tend to be rather large for the amount of power they put out. However, this may not be a significant drawback since these engines can be largely manufactured from lightweight and cheap materials such as plastics. These engines could be used for applications such as irrigation and remote water pumping.

CRYOCOOLERS

It isn't immediately obvious, but Stirling engines are a reversible device. If one end is heated while the other end is cooled, they will produce mechanical work. But if mechanical work is input into the engine by connecting an electric motor to the power output shaft, one end will get hot and the other end will get cold. In a correctly designed Stirling cooler, the cold end will get extremely cold. Stirling coolers have been built for research use that will cool to below 10 K. Cigarette pack sized Stirling coolers have been produced in large numbers for cooling infrared chips down to 80 K. These micro Stirling coolers have been used in high end night vision devices, antiaircraft missile tracking systems, and even some satellite infrared cameras.

Brent H. Van Arsdell

See also: Cogneration Technolgies; Diesel Cycle Engines; Gasoline Engines; Steam Engines; Stiring, Robert.