Stirling external combustion engine efficiency. External combustion engine - principle of operation and advantages

- a heat engine in which a liquid or gaseous working fluid moves in a closed volume, a kind of external combustion engine. It is based on periodic heating and cooling of the working fluid with the extraction of energy from the resulting change in the volume of the working fluid. It can work not only from fuel combustion, but also from any heat source.

You can observe the chronology of events associated with the development of engines of the 18th century in an interesting article - "The History of the Invention of Steam Engines". And this article is dedicated to the great inventor Robert Stirling and his brainchild.

History of creation...

The patent for the invention of the Stirling engine, oddly enough, belongs to the Scottish priest Robert Stirling. He received it on September 27, 1816. The first "hot air engines" became known to the world at the end of the 17th century, long before Stirling. One of the important achievements of Stirling is the addition of a purifier, nicknamed by him "housekeeper".

In modern scientific literature, this cleaner has a completely different name - "recuperator". Thanks to him, the performance of the engine increases, since the cleaner retains heat in the warm part of the engine, and at the same time the working fluid is cooled. Through this process, the efficiency of the system is greatly increased. The recuperator is a chamber filled with wire, granules, corrugated foil (corrugations go along the direction of the gas flow). The gas passes through the recuperator filler in one direction, gives (or acquires) heat, and when moving in the other direction, takes (gives) it away. The recuperator can be external in relation to the cylinders and can be placed on the displacer piston in beta and gamma configurations. The dimensions and weight of the machine in this case are less. To some extent, the role of the recuperator is played by the gap between the displacer and the cylinder walls (if the cylinder is long, then there is no need for such a device at all, but significant losses appear due to the viscosity of the gas). In alpha stirling, the heat exchanger can only be external. It is mounted in series with a heat exchanger, in which the working fluid is heated from the side of the cold piston.

In 1843, James Stirling used this engine in a factory where he worked as an engineer at the time. In 1938, Philips invested in a Stirling engine with a capacity of more than two hundred horsepower and a return of more than 30%. Because the Stirling's engine has many advantages, it was widespread in the era of steam engines.

Flaws.

Material consumption is the main drawback of the engine. For external combustion engines in general, and the Stirling engine in particular, the working fluid must be cooled, and this leads to a significant increase in the weight and dimensions of the power plant due to enlarged radiators.

To obtain characteristics comparable to those of an internal combustion engine, it is necessary to apply high pressures (over 100 atm) and special types of working fluid - hydrogen, helium.

Heat is not supplied to the working fluid directly, but only through the walls of the heat exchangers. The walls have limited thermal conductivity, due to which the efficiency is lower than expected. The hot heat exchanger operates under very stressful heat transfer conditions and at very high pressures, which requires the use of high quality and expensive materials. Creating a heat exchanger that would satisfy conflicting requirements is very difficult. The higher the heat exchange area, the lower the heat loss. At the same time, the size of the heat exchanger and the volume of the working fluid that is not involved in the work increase. Since the heat source is located outside, the engine responds slowly to changes in the heat flux supplied to the cylinder, and may not immediately produce the desired power at start-up.

To quickly change the engine power, methods are used that are different from those used in internal combustion engines: a variable volume buffer tank, a change in the average pressure of the working fluid in the chambers, a change in the phase angle between the working piston and the displacer. In the latter case, the reaction of the engine to the control action of the driver is almost instantaneous.

Advantages.

However, the Stirling engine has advantages that force it to be developed.

The “omnivorousness” of the engine - like all external combustion engines (or rather, external heat supply), the Stirling engine can operate from almost any temperature difference: for example, between different layers in the ocean, from the sun, from a nuclear or isotope heater, coal or wood stove and etc.

Simplicity of design - the design of the engine is very simple, it does not require additional systems, such as a gas distribution mechanism. It starts on its own and does not need a starter. Its characteristics allow you to get rid of the gearbox. However, as noted above, it has a greater material consumption.

Increased resource - simplicity of design, the absence of many "delicate" units allows Stirling to provide an unprecedented resource for other engines of tens and hundreds of thousands of hours of continuous operation.

Profitability - in the case of converting solar energy into electricity, stirlings sometimes give greater efficiency (up to 31.25%) than steam heat engines.

Noiselessness of the engine - Stirling has no exhaust, which means it does not make noise. Beta Stirling with a rhombic mechanism is a perfectly balanced device and, with a fairly high quality of workmanship, does not even have vibrations (vibration amplitude is less than 0.0038 mm).

Environmentally friendly - Stirling itself does not have any parts or processes that can contribute to environmental pollution. It does not consume the working fluid. The environmental friendliness of the engine is primarily due to the environmental friendliness of the heat source. It should also be noted that it is easier to ensure the completeness of fuel combustion in an external combustion engine than in an internal combustion engine.

Alternative to steam engines.

In the 19th century, engineers tried to create a safe alternative to the steam engines of the time, due to the fact that the boilers of engines already invented often exploded, unable to withstand the high pressure of steam and materials that were not at all suitable for their manufacture and construction. Stirling's engine became a good alternative because it could convert any temperature difference into work. This is the basic principle of the Stirling engine. The constant alternation of heating and cooling of the working fluid in a closed cylinder sets the piston in motion. Usually air acts as a working fluid, but hydrogen and helium are also used. But experiments were also carried out with water. The main feature of the Stirling engine with a liquid working fluid is its small size, high operating pressures and high power density. There is also a Stirling with a two-phase working fluid. The specific power and working pressure in it are also quite high.

Perhaps you remember from a physics course that when a gas is heated, its volume increases, and when it is cooled, it decreases. It is this property of gases that underlies the operation of the Stirling engine. Stirling's engine uses the Stirling cycle, which is not inferior to the Carnot cycle in terms of thermodynamic efficiency, and in some way even has an advantage. The Carnot cycle consists of slightly different isotherms and adiabats. The practical implementation of such a cycle is complex and unpromising. The Stirling cycle made it possible to obtain a practically working engine in acceptable dimensions.

In total, there are four phases in the Stirling cycle, separated by two transitional phases: heating, expansion, transition to a cold source, cooling, compression, and transition to a heat source. When moving from a warm source to a cold source, the gas in the cylinder expands and contracts. During this process, the pressure changes and useful work can be obtained. Useful work is produced only by processes that take place at a constant temperature, that is, it depends on the temperature difference between the heater and cooler, as in the Carnot cycle.

Configurations.

Engineers classify Stirling engines into three different types:

Preview - Click to enlarge.

Contains two separate power pistons in separate cylinders. One piston is hot, the other is cold. The cylinder with the hot piston is in the heat exchanger with a higher temperature, and the cylinder with the cold piston is in the colder heat exchanger. The ratio of power to volume is quite large, but the high temperature of the "hot" piston creates certain technical problems.

Beta Stirling- one cylinder, hot at one end and cold at the other. A piston (from which power is removed) and a “displacer” move inside the cylinder, changing the volume of the hot cavity. The gas is pumped from the cold part of the cylinder to the hot part through the regenerator. The regenerator may be external, as part of a heat exchanger, or may be combined with a displacing piston.

There is a piston and a “displacer”, but at the same time there are two cylinders - one cold (the piston moves there, from which power is removed), and the second is hot from one end and cold from the other (the “displacer” moves there). The regenerator can be external, in which case it connects the hot part of the second cylinder with the cold one and simultaneously with the first (cold) cylinder. The internal regenerator is part of the displacer.

It replaced other types of power plants, however, work aimed at abandoning the use of these units suggests an imminent change in leading positions.

Since the beginning of technological progress, when the use of engines that burn fuel inside was just beginning, their superiority was not obvious. The steam engine, as a competitor, contains a lot of advantages: along with traction parameters, it is silent, omnivorous, easy to control and configure. But lightness, reliability and efficiency allowed the internal combustion engine to take over the steam.

Today, issues of ecology, economy and safety are at the forefront. This forces engineers to throw their forces on serial units operating on renewable fuel sources. In the year 16 of the nineteenth century, Robert Stirling registered an engine powered by external heat sources. Engineers believe that this unit is able to change the modern leader. The Stirling engine combines efficiency, reliability, runs quietly, on any fuel, this makes the product a player in the automotive market.

Robert Stirling (1790-1878):

Stirling engine history

Initially, the installation was developed with the aim of replacing the steam-powered machine. Boilers of steam mechanisms exploded when the pressure exceeded the permissible norms. From this point of view, Stirling is much safer, functioning using a temperature difference.

The principle of operation of the Stirling engine is to alternately supply or remove heat from the substance on which work is performed. The substance itself is enclosed in a closed volume. The role of the working substance is performed by gases or liquids. There are substances that perform the role of two components, the gas is transformed into a liquid and vice versa. The liquid-piston Stirling engine has: small dimensions, powerful, generates high pressure.

The decrease and increase in the volume of gas during cooling or heating, respectively, is confirmed by the law of thermodynamics, according to which all components: the degree of heating, the amount of space occupied by the substance, the force acting per unit area, are related and described by the formula:

P*V=n*R*T

• P is the force of the gas in the engine per unit area;
• V is the quantitative value occupied by gas in the engine space;
• n is the molar amount of gas in the engine;
• R is the gas constant;
• T is the degree of gas heating in the engine K,

Stirling engine model:

Due to the unpretentiousness of the installations, the engines are divided into: solid fuel, liquid fuel, solar energy, chemical reaction and other types of heating.

Cycle

The Stirling external combustion engine uses a set of phenomena of the same name. The effect of the ongoing action in the mechanism is high. Thanks to this, it is possible to design an engine with good characteristics within normal dimensions.

It should be taken into account that the design of the mechanism provides for a heater, a refrigerator and a regenerator, a device for removing heat from the substance and returning heat at the right time.

Ideal Stirling cycle, (diagram "temperature-volume"):

Ideal circular phenomena:

• 1-2 Change in the linear dimensions of a substance with a constant temperature;
• 2-3 Removal of heat from the substance to the heat exchanger, the space occupied by the substance is constant;
• 3-4 Forced reduction of the space occupied by the substance, the temperature is constant, heat is removed to the cooler;
• 4-1 Forced increase in the temperature of the substance, the occupied space is constant, the heat is supplied from the heat exchanger.

The ideal Stirling cycle, (pressure-volume diagram):

From the calculation (mol) of a substance:

Heat input:

Heat received by the cooler:

The heat exchanger receives heat (process 2-3), the heat exchanger gives off heat (process 4-1):

R – Universal gas constant;

CV - the ability of an ideal gas to retain heat with a constant amount of space occupied.

Due to the use of a regenerator, part of the heat remains, as the energy of the mechanism, which does not change during the passing circular phenomena. The refrigerator receives less heat, so the heat exchanger saves the heat of the heater. This increases the efficiency of the installation.

Efficiency of circular phenomenon:

ɳ =

It is noteworthy that without a heat exchanger, the set of Stirling processes is feasible, but its efficiency will be much lower. Running the set of processes backwards leads to a description of the cooling mechanism. In this case, the presence of a regenerator is a mandatory condition, since when passing (3-2) it is impossible to heat the substance from the cooler, the temperature of which is much lower. It is also impossible to give heat to the heater (1-4), the temperature of which is higher.

The principle of the engine

In order to understand how the Stirling engine works, let's look at the device and the frequency of the phenomena of the unit. The mechanism converts the heat received from the heater located outside the product into a force on the body. The whole process occurs due to the temperature difference, in the working substance, which is in a closed circuit.

The principle of operation of the mechanism is based on expansion due to heat. Immediately prior to expansion, the substance in the closed circuit heats up. Accordingly, before being compressed, the substance is cooled. The cylinder itself (1) is wrapped in a water jacket (3), heat is supplied to the bottom. The piston that does the work (4) is placed in a sleeve and sealed with rings. Between the piston and the bottom there is a displacement mechanism (2), which has significant gaps and moves freely. The substance in a closed circuit moves through the volume of the chamber due to the displacer. The movement of matter is limited to two directions: the bottom of the piston, the bottom of the cylinder. The movement of the displacer is provided by a rod (5) which passes through the piston and is operated by an eccentric 90° late compared to the piston drive.

• Position "A":

The piston is located in the lowest position, the substance is cooled by the walls.

• Position "B":

The displacer occupies the upper position, moving, passes the substance through the end slots to the bottom, and cools itself. The piston is stationary.

• Position "C":

The substance receives heat, under the action of heat it increases in volume and raises the expander with the piston up. Work is done, after which the displacer sinks to the bottom, pushing out the substance and cooling.

• Position "D":

The piston goes down, compresses the cooled substance, useful work is done. The flywheel serves as an energy accumulator in the design.

The considered model is without a regenerator, so the efficiency of the mechanism is not high. The heat of the substance after work is removed into the coolant using the walls. The temperature does not have time to decrease by the required amount, so the cooling time is extended, the motor speed is low.

Types of engines

Structurally, there are several options using the Stirling principle, the main types are:

The design uses two different pistons placed in different contours. The first circuit is used for heating, the second circuit is used for cooling. Accordingly, each piston has its own regenerator (hot and cold). The device has a good power to volume ratio. The disadvantage is that the temperature of the hot regenerator creates design difficulties.

• Engine "β - Stirling":

The design uses one closed circuit, with different temperatures at the ends (cold, hot). A piston with a displacer is located in the cavity. The displacer divides the space into cold and hot zones. The exchange of cold and heat occurs by pumping a substance through a heat exchanger. Structurally, the heat exchanger is made in two versions: external, combined with a displacer.

• Engine "γ - Stirling":

The piston mechanism provides for the use of two closed circuits: cold and with a displacer. Power is taken off a cold piston. The displacer piston is hot on one side and cold on the other. The heat exchanger is located both inside and outside the structure.

Some power plants are not similar to the main types of engines:

• Rotary Stirling engine.

Structurally, the invention with two rotors on the shaft. The part performs rotational movements in a closed cylindrical space. A synergistic approach to the implementation of the cycle has been laid. The body contains radial slots. Blades with a certain profile are inserted into the recesses. The plates are put on the rotor and can move along the axis when the mechanism rotates. All the details create changing volumes with phenomena taking place in them. The volumes of the various rotors are connected by channels. Channel arrangements are offset by 90° to each other. The shift of the rotors relative to each other is 180°.

• Thermoacoustic Stirling engine.

The engine uses acoustic resonance to carry out processes. The principle is based on the movement of matter between a hot and a cold cavity. The circuit reduces the number of moving parts, the difficulty in removing the received power and maintaining resonance. The design refers to the free-piston type of motor.

DIY Stirling engine

Today, quite often in the online store you can find souvenirs made in the form of the engine in question. Structurally and technologically, the mechanisms are quite simple; if desired, the Stirling engine is easy to construct with your own hands from improvised means. On the Internet you can find a large number of materials: videos, drawings, calculations and other information on this topic.

Low temperature Stirling engine:

• Consider the simplest version of the wave engine, for which you will need a tin can, soft polyurethane foam, a disk, bolts and paper clips. All these materials are easy to find at home, it remains to perform the following steps:
• Take a soft polyurethane foam, cut a circle two millimeters smaller than the inner diameter of the can. The height of the foam is two millimeters more than half the height of the can. Foam rubber plays the role of a displacer in the engine;
• Take the lid of the jar, make a hole in the middle, two millimeters in diameter. Solder a hollow rod to the hole, which will act as a guide for the engine connecting rod;
• Take a circle cut out of foam, insert a screw into the middle of the circle and lock it on both sides. Solder a pre-straightened paperclip to the washer;
• Drill a hole two centimeters from the center, three millimeters in diameter, thread the displacer through the central hole of the lid, solder the lid to the jar;
• Make a small cylinder out of tin, one and a half centimeters in diameter, solder it to the lid of the can in such a way that the side hole of the lid is clearly centered inside the engine cylinder;
• Make an engine crankshaft out of a paper clip. The calculation is carried out in such a way that the spacing of the knees is 90 °;
• Make a stand for the crankshaft of the engine. From a plastic film, make an elastic membrane, put the film on the cylinder, push it through, fix it;

• Make an engine connecting rod yourself, bend one end of the straightened product in the shape of a circle, insert the other end into a piece of eraser. The length is adjusted in such a way that at the lowest point of the shaft the membrane is retracted, at the extreme upper point, the membrane is maximally extended. Adjust the other connecting rod in the same way;
• Glue the engine connecting rod with a rubber tip to the membrane. Mount the connecting rod without a rubber tip on the displacer;
• Put a flywheel from the disk on the crank mechanism of the engine. Attach legs to the jar so as not to hold the product in your hands. The height of the legs allows you to place a candle under the jar.

After we managed to make a Stirling engine at home, the engine is started. To do this, a lighted candle is placed under the jar, and after the jar has warmed up, they give impetus to the flywheel.

The considered installation option can be quickly assembled at home, as a visual aid. If you set a goal and a desire to make the Stirling engine as close as possible to factory counterparts, there are drawings of all the details in the public domain. Step-by-step execution of each node will allow you to create a working layout that is no worse than commercial versions.

Advantages

The Stirling engine has the following advantages:

• A temperature difference is necessary for the operation of the engine, which fuel causes heating is not important;
• There is no need to use attachments and auxiliary equipment, the engine design is simple and reliable;
• The resource of the engine, due to the design features, is 100,000 hours of operation;
• The operation of the engine does not create extraneous noise, since there is no detonation;
• The process of engine operation is not accompanied by the emission of waste substances;
• Engine operation is accompanied by minimal vibration;
• Processes in the plant cylinders are environmentally friendly. Using the right heat source keeps the engine clean.

Flaws

The disadvantages of the Stirling engine include:

• It is difficult to establish mass production, since the engine design requires the use of a large amount of materials;
• High weight and large dimensions of the engine, since a large radiator must be used for effective cooling;
• To increase efficiency, the engine is boosted using complex substances (hydrogen, helium) as a working fluid, which makes the operation of the unit dangerous;
• The high temperature resistance of steel alloys and their thermal conductivity complicate the engine manufacturing process. Significant heat losses in the heat exchanger reduce the efficiency of the unit, and the use of specific materials makes the manufacture of the engine expensive;
• To adjust and switch the engine from mode to mode, special control devices must be used.

Usage

The Stirling engine has found its niche and is actively used where dimensions and omnivorousness are an important criterion:

• Stirling engine-generator.

A mechanism for converting heat into electrical energy. Often there are products used as portable tourist generators, installations for the use of solar energy.

• The engine is like a pump (electric).

The engine is used for installation in the circuit of heating systems, saving on electrical energy.

• The engine is like a pump (heater).

In countries with a warm climate, the engine is used as a space heater.

Stirling engine on a submarine:

• The engine is like a pump (cooler).

Almost all refrigerators use heat pumps in their design, installing a Stirling engine saves resources.

• The engine is like a pump that creates ultra-low heat levels.

The device is used as a refrigerator. To do this, the process is started in the opposite direction. The units liquefy gas, cool measuring elements in precise mechanisms.

• Underwater engine.

The submarines of Sweden and Japan work thanks to the engine.

Stirling engine as a solar installation:

• The engine is like a battery of energy.

Fuel in such units, salt melts, the engine is used as an energy source. In terms of energy reserves, the motor is ahead of chemical elements.

• solar engine.

Convert the sun's energy into electricity. The substance in this case is hydrogen or helium. The engine is placed in the focus of the maximum concentration of the energy of the sun, created using a parabolic antenna.

Despite its high performance, the modern internal combustion engine is beginning to become obsolete. Its efficiency has probably reached its limit. Noise, vibration, gases poisoning the air and other inherent shortcomings force scientists to look for new solutions, to reconsider the possibilities of long-forgotten cycles. One of the "revived" engines is the Stirling.

Back in 1816, the Scottish priest and scientist Robert Stirling patented an engine in which the fuel and air entering the combustion zone never get inside the cylinder. When burned, they only heat the working gas in it. This gave reason to call Stirling's invention an external combustion engine.

Robert Stirling built several engines; the last of them had a capacity of 45 liters. With. and worked at a mine in England for more than three years (until 1847). These engines were very heavy, took up a lot of space and looked like steam engines.

For navigation, external combustion engines were first used in 1851 by the Swede John Erickson. The ship "Erickson" built by him safely crossed the Atlantic Ocean from America to England with a power plant consisting of four external combustion engines. In the age of steam engines, this was a sensation. However, Erickson's power plant developed only 300 hp. s., not 1000 as expected. The engines were huge (cylinder diameter 4.2 m, piston stroke 1.8 m). The consumption of coal turned out to be no less than that of steam engines. When the ship arrived in England, it turned out that the engines were not suitable for further operation, as their cylinder bottoms burned out. To return to America, they had to replace the engines with a conventional steam engine. On the way back, the ship had an accident and sank with all the crew.

Low-power external combustion engines at the end of the last century were used in houses for pumping water, in printing houses, at industrial enterprises, including the St. Petersburg Nobel plant (now Russian Diesel), they were also installed on small ships. Stirlings were produced in many countries, including Russia, where they were called "warmth and strength." They were valued for their noiselessness and safety of work, which made them compare favorably with steam engines.

With the development of internal combustion engines, Stirlings were forgotten. In the Encyclopedic Dictionary of Brockhaue and Efron, the following is written about them: “Safety from explosions is the main advantageous side of caloric machines, thanks to which they can again come into use if they find new materials for their construction and lubrication that can better withstand high temperatures.”

The point was, however, not only in the absence of relevant materials. The modern principles of thermodynamics, in particular the equivalence of heat and work, were still unknown, without which it was impossible to determine the most advantageous ratios of the main elements of the engine. Heat exchangers were made with a small surface, due to which the engines operated at prohibitively high temperatures and quickly failed.

Attempts to improve Stirling were made after the Second World War. The most significant of them consisted in the fact that the working gas began to be used compressed to 100 atm and not air was used, but hydrogen, which has a higher thermal conductivity coefficient, low viscosity and, moreover, does not oxidize lubricants.

The device of an external combustion engine in its modern form is schematically shown in fig. 1. There are two pistons in a cylinder closed on one side. The upper one - the piston - in the displacer serves to accelerate the process of periodic heating and cooling of the working gas. It is a hollow closed stainless steel cylinder, which conducts heat poorly, and moves under the action of a rod connected to a crank mechanism.

The lower piston is a working one (shown in section in the figure). It transmits force to the crank mechanism through a hollow rod, inside which the displacer rod passes. The working piston is equipped with sealing rings.

Under the working piston there is a buffer tank that forms a cushion that acts as a flywheel - to smooth out the unevenness of the torque due to the selection of part of the energy during the working stroke and its return to the engine shaft during the compression stroke. To isolate the volume of the cylinder from the surrounding space, seals of the “wrapped stocking” type are used. These are rubber tubes attached at one end to the stem and the other to the body.

The upper part of the cylinder is in contact with the heater, and the lower part is in contact with the refrigerator. Accordingly, “hot” and “cold” volumes are distinguished in it, freely communicating with each other through a pipeline in which the regenerator (heat exchanger) is located. The regenerator is filled with a tangle of small-diameter wire (0.2 mm) and has a high heat capacity (for example, the efficiency of Filipe regenerators exceeds 95%).

The working process of the Stirling engine can be carried out without a displacer, based on the use of a spool distributor of the working charge.

At the bottom of the engine is a crank mechanism that serves to convert the reciprocating motion of the piston into rotational motion of the shaft. A feature of this mechanism is the presence of two crankshafts connected by two gears with helical teeth rotating towards each other. The displacer rod is connected to the crankshafts by means of a lower rocker arm and trailed connecting rods. The working piston rod is connected to the crankshafts through the upper rocker arm and trailer connecting rods. A system of identical connecting rods forms a movable deformable rhombus, hence the name of this gear - rhombic. Rhombic transmission provides the necessary phase shift when the pistons move. It is completely balanced, there are no lateral forces on the piston rods.

In the space limited by the working piston, there is a working gas - hydrogen or helium. The total volume of gas in the cylinder does not depend on the position of the displacer. Volume changes associated with compression and expansion of the working gas occur due to the movement of the working piston.

During engine operation, the upper part of the cylinder is constantly heated, for example, from the combustion chamber into which liquid fuel is injected. The bottom of the cylinder is constantly cooled, for example by cold water pumped through a water jacket surrounding the cylinder. The closed Stirling cycle consists of four cycles shown in Fig. 2.

Stroke I - cooling. The working piston is in its lowest position, the displacer moves up. In this case, the working gas flows from the "hot" volume above the displacer into the "cold" volume below it. Passing along the way through the regenerator, the working gas gives it some of its heat, and then cools in the "cold" volume.

Bar II - Compression. The displacer remains in the upper position, the working piston moves up, compressing the working gas at low temperature.

Stroke III - heating. The working piston is in the upper position, the displacer moves down. In this case, the compressed cold working gas rushes from under the displacer into the vacant space above it. On the way, the working gas passes through the regenerator, where it is preheated, enters the “hot” cylinder cavity and heats up even more.

Stroke IV - expansion (work stroke). When heated, the working gas expands, while moving the displacer and with it the working piston down. Useful work is being done.

Stirling has a closed cylinder. On fig. 3a shows a theoretical cycle diagram (diagram V - P). The abscissa shows the volumes of the cylinder, and the ordinate shows the pressures in the cylinder. The first stroke is isothermal I-II, the second occurs at a constant volume II-III, the third - isothermal III-IV, the fourth - at a constant volume IV-I. Since the pressure during the expansion of the hot gas (III-IV) is greater than the pressure during the compression of the cold gas (I-II), the work of expansion is greater than the work of compression. The useful work of the cycle can be graphically depicted as a curvilinear quadrilateral I-II-III-IV.

In the actual process, the piston and displacer move continuously, since they are connected with the crank mechanism, so the diagram of the actual cycle is rounded (Fig. 3, b).

The theoretical efficiency of the Stirling engine is 70%. Studies have shown that in practice it is possible to obtain an efficiency equal to 50%. This is significantly more than the best gas turbines (28%), gasoline engines (30%) and diesel engines (40%).

Stirling can run on gasoline, kerosene, diesel, gaseous and even solid fuels. Compared to other engines, it has a softer and almost silent running. This is explained by a low compression ratio (1.3 ÷ 1.5), in addition, the pressure in the cylinder rises smoothly, and not with an explosion. The products of combustion are also emitted without noise, as the combustion takes place continuously. There are relatively few toxic components in them, because fuel combustion occurs continuously and with a constant excess of oxygen (α=1.3).

Stirling with a rhombic gear is completely balanced, vibration does not occur in it. This quality, in particular, was taken into account by American engineers who installed a single-cylinder stirling on an artificial Earth satellite, where even a slight vibration and imbalance can lead to a loss of orientation.

Cooling remains one of the problematic issues. Stirling with exhaust gases removes only 9% of the heat received from the fuel, so, for example, when installing it on a car, you would have to make a radiator about 2.5 times larger than when using a gasoline engine of the same power. The problem is solved more simply on ship installations, where effective cooling is provided by an unlimited amount of outboard water.

On fig. 4 shows a cross-section of a 115 hp Philips two-cylinder boat engine. With. at 3000 rpm with a horizontal arrangement of cylinders. The total working volume of each cylinder is 263 cm 3 . Opposite pistons are connected to two traverses, which made it possible to completely balance the gas forces and dispense with buffer volumes. The heater is made of tubes surrounding the combustion chamber through which the working gas passes. The cooler is a tubular cooler through which sea water is pumped. The engine has two crankshafts connected to the propeller shaft by means of worm gears. The engine height is only 500 mm, which allows it to be installed under the deck and thus reduce the size of the engine room.

Stirling power is controlled mainly by changing the pressure of the working gas. At the same time, in order to keep the temperature of the heater constant, the fuel supply is also regulated. Almost any heat source is suitable for an external combustion engine. It is important that it can convert low-temperature energy into useful work, which internal combustion engines are not capable of. From the curve in Fig. Figure 5 shows that at a heater temperature of only 350°C, the Stirling efficiency is still ≈ 20%.

Stirling is economical - its specific fuel consumption is only 150 g / l. With. hour. In the “stirling engine-heat accumulator” power plant used on American Earth satellites, lithium hydrite serves as a heat accumulator, which absorbs heat during the “illumination” period and gives it to the stirling when the satellite is on the shadow side of the Earth. On the satellite, the engine is used to drive a generator with a power of 3 kW at 2400 rpm.

An experimental scooter with Stirling and a heat accumulator has been created. The use of a heat accumulator and stirling on a submarine allows it to go several times longer in a submerged position.

Literature

• 1. Smirnov GV External combustion engines. "Knowledge", M., 1967.
• 2.Dr. Ir. R. I. Meijer. Der Philips - Stirlingmotor, MTZ, N 7, 1968.
• 3 Curtis Anthony Hot air and the wind of change. The Stirling engine and its revival. Motor (Engl.), 1969, (135), N 3488.

Only about a hundred years ago, internal combustion engines had to win the place they occupy in modern automotive industry in a fierce competition. Then their superiority was by no means as obvious as it is today. Indeed, the steam engine - the main rival of the gasoline engine - had enormous advantages in comparison with it: noiselessness, ease of power control, excellent traction characteristics and amazing "omnivorousness" that allows it to work on any type of fuel from wood to gasoline. But in the end, the efficiency, lightness and reliability of internal combustion engines prevailed and made us come to terms with their shortcomings as inevitable.
In the 1950s, with the advent of gas turbines and rotary engines, an assault began on the monopoly position occupied by internal combustion engines in the automotive industry, an assault that has not yet been crowned with success. Approximately in the same years, attempts were made to bring to the scene a new engine, which amazingly combines the efficiency and reliability of a gasoline engine with the noiselessness and "omnivorous" steam installation. This is the famous external combustion engine that the Scottish priest Robert Stirling patented on September 27, 1816 (English Patent No. 4081).

Process physics

The principle of operation of all heat engines, without exception, is based on the fact that when a heated gas expands, more mechanical work is performed than is required to compress a cold one. To demonstrate this, a bottle and two pots of hot and cold water are sufficient. First, the bottle is dipped into ice water, and when the air in it cools, the neck is plugged with a cork and quickly transferred to hot water. After a few seconds, a pop is heard and the gas heated in the bottle pushes the cork out, doing mechanical work. The bottle can be returned to the ice water again - the cycle will repeat.
the cylinders, pistons, and intricate levers of the first Stirling machine reproduced this process almost exactly, until the inventor realized that part of the heat taken from the gas during cooling could be used for partial heating. All that is needed is some kind of container in which it would be possible to store the heat taken from the gas during cooling, and give it back to it when heated.
But, alas, even this very important improvement did not save the Stirling engine. By 1885, the results achieved here were very mediocre: 5-7 percent efficiency, 2 liters. With. power, 4 tons of weight and 21 cubic meters of occupied space.
External combustion engines were not saved even by the success of another design developed by the Swedish engineer Erickson. Unlike Stirling, he proposed heating and cooling the gas not at a constant volume, but at a constant pressure. In 1887, several thousand small Erickson engines worked perfectly in printing houses, in houses, in mines, on ships. They filled the water tanks, powered the elevators. Erickson even tried to adapt them to drive crews, but they turned out to be too heavy. In Russia, before the revolution, a large number of such engines were produced under the name "Heat and Power".

Ecology of consumption. Science and technology: The Stirling motor is most often used in situations where a device for converting thermal energy is required, which is simple and efficient.

Less than a hundred years ago, internal combustion engines tried to win their rightful place in the competition among other machines and moving mechanisms available. At the same time, in those days, the superiority of the gasoline engine was not so obvious. Existing steam-powered machines were distinguished by their quietness, excellent power characteristics for that time, ease of maintenance, and the ability to use various types of fuel. In the further struggle for the market, internal combustion engines prevailed due to their efficiency, reliability and simplicity.

The further race to improve the units and driving mechanisms, which gas turbines and rotary engines entered in the middle of the 20th century, led to the fact that, despite the supremacy of the gasoline engine, attempts were made to introduce a completely new type of engine onto the “playing field” - thermal, for the first time invented back in 1861 by a Scottish priest named Robert Stirling. The engine was named after its creator.

STIRLING ENGINE: THE PHYSICAL SIDE OF THE ISSUE

To understand how a Stirling benchtop power plant works, you need to understand the basics of how heat engines work. Physically, the principle of operation is to use mechanical energy, which is obtained by expanding the gas during heating and its subsequent compression during cooling. To demonstrate the principle of operation, an example can be given based on an ordinary plastic bottle and two pots, one of which contains cold water, the other hot.

When lowering the bottle into cold water, the temperature of which is close to the temperature of ice formation, with sufficient cooling of the air inside the plastic container, it should be closed with a cork. Further, when the bottle is placed in boiling water, after a while the cork “shoots” with force, since in this case the work done by the heated air is many times greater than that done during cooling. When the experiment is repeated many times, the result does not change.

The first machines that were built using the Stirling engine faithfully reproduced the process demonstrated in the experiment. Naturally, the mechanism required improvement, consisting in the use of part of the heat that was lost by the gas during cooling for further heating, allowing heat to be returned to the gas to accelerate heating.

But even the application of this innovation could not save the situation, since the first Stirlings were large in size with low power output. In the future, more than once attempts were made to modernize the design to achieve a power of 250 hp. led to the fact that in the presence of a cylinder with a diameter of 4.2 meters, the real output power that the Stirling power plant produced at 183 kW was actually only 73 kW.

All Stirling engines operate on the principle of the Stirling cycle, which includes four main phases and two intermediate ones. The main ones are heating, expansion, cooling and compression. As the transition stage, the transition to the cold generator and the transition to the heating element are considered. The useful work done by the engine is based solely on the temperature difference between the heating and cooling parts.

MODERN STIRLING CONFIGURATIONS

Modern engineering distinguishes three main types of such engines:

• alpha stirling, the difference of which is in two active pistons located in independent cylinders. Of all three options, this model has the highest power, having the highest temperature of the heated piston;
• beta stirling, based on one cylinder, one part of which is hot and the other is cold;
• gamma-stirling, which, in addition to the piston, also has a displacer.

The production of the power plant at Stirling will depend on the choice of engine model, which will take into account all the positive and negative aspects of such a project.

ADVANTAGES AND DISADVANTAGES

Due to their design features, these engines have a number of advantages, but they are not without drawbacks.

Stirling's desktop power station, which cannot be bought in a store, but only from amateurs who independently collect such devices, includes:

• large dimensions, which are caused by the need for constant cooling of the working piston;
• the use of high pressure, which is required to improve engine performance and power;
• heat loss, which occurs due to the fact that the generated heat is transferred not to the working fluid itself, but through a system of heat exchangers, whose heating leads to a loss in efficiency;
• a sharp reduction in power requires the application of special principles that differ from those traditional for gasoline engines.

Along with the disadvantages, power plants operating on Stirling units have undeniable advantages:

• any type of fuel, since, like any engines that use heat energy, this engine is able to function at a temperature difference in any environment;
• economy. These devices can be an excellent replacement for steam units in cases where it is necessary to process solar energy, giving out an efficiency of 30% higher;
• environmental Safety. Since the kW tabletop power plant does not generate an exhaust moment, it does not produce noise or emit harmful substances into the atmosphere. Ordinary heat acts as a source of power, and the fuel burns out almost completely;
• constructive simplicity. For his work, Stirling will not require additional parts or fixtures. It is able to start independently without the use of a starter;
• increased resource of working capacity. Due to its simplicity, the engine can provide more than one hundred hours of continuous operation.

STIRLING ENGINE APPLICATIONS

The Stirling motor is most often used in situations where an apparatus for converting thermal energy is required, which is simple, while the efficiency of other types of thermal units is significantly lower under similar conditions. Very often, such units are used in the power supply of pumping equipment, refrigerators, submarines, batteries that store energy.

One of the promising areas for the use of Stirling engines is solar power plants, since this unit can be successfully used to convert the energy of sunlight into electrical energy. To carry out this process, the engine is placed in the focus of a mirror that accumulates the sun's rays, which provides permanent illumination of the area requiring heating. This allows you to focus solar energy on a small area. The fuel for the engine in this case is helium or hydrogen. published