The Stirling engine is the principle of operation. Low-temperature Stirling engine (photo)

The Stirling engine, the principle of operation of which is qualitatively different from the usual engine for all ICEs, once was the last worthy competition. However, they forgot about it for a while. How this motor is used today, what is the principle of its operation (in the article you can find also the drawings of the Stirling engine, which clearly demonstrate its work), and what are the prospects for future use, read below.

History

In 1816, in Scotland, Robert Stirling, a thermal machine was patented , named today in honor of its inventor. The first engines of hot air were invented even before it. But Stirling added a cleaner to the device, which in the technical literature is called a regenerator, or a heat exchanger. Thanks to it, the motor's performance increased when the unit was kept warm.

The engine was recognized as the most durable steam engine available at that time, since it never exploded. Prior to it on other motors such a problem arose frequently. Despite its rapid success, at the beginning of the twentieth century its development was abandoned, as it became less economical, compared with the then appeared other internal combustion engines and electric motors. However, Stirling continued to be used in some industries.

External combustion engine

The principle of operation of all thermal motors is that greater mechanical forces are required to produce gas in the expanded state than when compressing the cold one. For a visual demonstration of this, you can conduct an experiment with two pots filled with cold and hot water, as well as a bottle. The latter is lowered into cold water, plugged with a stopper, then transferred to a hot one. At the same time, the gas in the bottle starts to perform mechanical work and pushes out the cork. The first external combustion engine was based on this process completely. However, later the inventor realized that some of the heat can be used for heating. Thus, the productivity has increased significantly. But even this did not help the engine become common.

Later, Ericson, an engineer from Sweden, improved the design by proposing to cool and heat the gas at constant pressure instead of volume. As a result, many copies were used to work in mines, on ships and in printing houses. But for the crews they were too heavy.

External combustion engines from Philips

Such motors are of the following types:

• steam;
• Steam turbine;
• Stirling.

The latter did not develop due to low reliability and the remaining not the highest performance in comparison with other types of aggregates that have appeared. However, in 1938, Philips resumed its work. The engines began to serve for the drives of generators in non-electrified areas. In 1945, the engineers of the company found them the opposite application: if the shaft is untwisted by an electric motor, then the cooling of the cylinder head reaches minus one hundred and ninety degrees Celsius. Then it was decided to use in the refrigeration units an improved Stirling engine.

Principle of operation

The action of the motor is to work on thermodynamic cycles in which compression and expansion take place at different temperatures. In this case, the flow control of the working fluid is realized due to the changing volume (or pressure - depending on the model). This is the principle of operation of most such machines, which can have different functions and constructive schemes. The motors can be piston or rotary. Machines with their plants work as heat pumps, refrigerators, pressure generators and so on.

In addition, there are open cycle motors where flow control is realized by means of valves. They are called Erickson's engines, except for the common name of Stirling's name. In ICE, useful work is carried out after preliminary compression of air, injection of fuel, heating the resulting mixture interspersed with combustion and expansion.

The Stirling engine has the same operating principle: at low temperature compression takes place, and at high temperature - expansion. But differently, heating is carried out: heat is supplied through the wall of the cylinder from the outside. Therefore, it was called the external combustion engine. Stirling used a periodic temperature change with a displacing piston. The latter moves the gas from one cylinder cavity to another. On the one hand, the temperature is constantly low, and on the other - high. When the piston moves upwards, the gas moves from the hot to the cold cavity, and downwards - it returns to the hot one. First, the gas gives a lot of heat to the refrigerator, and then from the heater gets as much as it gave. Between the heater and the refrigerator there is a regenerator - a cavity filled with material to which the gas gives off heat. With the reverse flow, the regenerator returns it.

The propellant system is connected to a working piston that compresses the gas in the cold and allows it to expand in the heat. Due to compression in a lower temperature, useful work is performed. The whole system goes through four cycles with intermittent movements. The crank mechanism ensures continuity. Therefore, sharp boundaries between the stages of the cycle are not observed, and the efficiency of the Stirling engine is not reduced.

Given all of the above, the conclusion suggests that this engine is a piston machine with an external heat supply, where the working fluid does not leave the enclosed space and is not replaced. The drawings of the Stirling engine well illustrate the device and the principle of its operation.

Work Details

The sun, electricity, nuclear energy or any other heat source can supply energy to the Stirling engine. The principle of his body's work is the use of helium, hydrogen or air. The ideal cycle has a thermal maximum possible efficiency, equal to thirty to forty percent. But with an efficient regenerator it will be able to work with a higher efficiency. Regeneration, heating and cooling are provided by the built-in heat exchangers operating without oils. It should be noted that the engine needs very little lubrication. The average pressure in the cylinder is usually 10 to 20 MPa. Therefore, it requires an excellent sealing system and the possibility of oil entering the working cavities.

Comparative characteristics

In most of today's engines of this kind, liquid fuel is used. At the same time, continuous pressure is easy to control, which helps to reduce emissions. The absence of valves ensures quiet operation. Power with a mass comparable to the engines with a turbo, and the specific power received at the output is equal to the indicator of the diesel unit. Speed and torque do not depend on each other.

The cost of producing the engine is much higher than on the ICE. But in operation, the opposite is obtained.

Benefits

Any Stirling engine model has many advantages:

• Efficiency in modern design can reach up to seventy percent.
• The engine does not have a high-voltage ignition system, a camshaft and valves. It will not need to be adjusted during the entire service life.
• In Stirling there is no explosion, as in ICE, which heavily loads crankshaft, bearings and connecting rods.
• They do not have that effect when they say that the engine has died down.
• Due to the simplicity of the device it can be used for a long time.
• It can work both on firewood, and with nuclear and any other type of fuel.
• The combustion takes place outside the motor.

disadvantages

• The main disadvantage of the design is its material consumption.
• The working body needs to be cooled, because of which the dimensions significantly increase.
• To obtain equal characteristics with ICE, it is necessary to use high pressure.
• To the working body, heat is supplied through the walls of heat exchangers, in which a limited thermal conductivity.
• To change the engine power, change the volume of the buffer capacity, the average pressure of the working fluid, the phase angle between the displacer and the piston.

Application

Currently, the Stirling engine with the generator is used in many areas. This is a universal source of electrical energy in refrigerators, pumps, submarines and solar power plants. It is thanks to the use of various types of fuel that it is possible to use it widely.

Revival

These engines began to develop again thanks to Philips. In the middle of the twentieth century, she entered into a contract with General Motors. She led the development for the application of Stirling in space and underwater devices, on ships and cars. Following them, another company from Sweden, United Stirling, began to develop them, including the possible use on passenger cars.

Today, the linear Stirling engine is used in installations of underwater, space and solar devices. Great interest in it is due to the urgency of the issues of deterioration of the environmental situation, as well as the fight against noise. In Canada and the United States, Germany and France, as well as Japan, there are active searches for the development and improvement of its use.

Future

The obvious advantages of the Stirling piston and rotary engine , which consist in a large service life, the use of different fuels, noiselessness and low toxicity, make it very promising against the background of an internal combustion engine. However, given the fact that the ICE has been improved over the entire time, it can not be easily shifted. One way or another, it is this engine that currently occupies a leading position, and it is not intended to hand them over in the near future.