Main features of the new Stirling technology

The New Stirling Technology (worldwide patent pending), which is presented here, resolves the existing technical problems by implementing the following measures:
  • The components of the standard machine are used as their own, separate units or devices and various configurations of a system can be put together through flexible combinations. Instead of talking about a Stirling Motor, it is more appropriate to talk about a Stirling Plant. It is best to equip the system with several pairs of working and compression cylinders.
  • The number of cylinders is increased. The more the better.
  • The system has only one regenerator based on the counterflow principle, which is used simultaneously and continuously by all cylinders (i.e. any number) and is dimensioned so large that the heat exchange is maximized.
  • The system has separate pipes for cold and hot gas flows. There is only one line for the compressed gas and one for the expanded gas on each side of the regenerator, which are used by all cylinders successively.
  • The regenerator serves as a working gas reservoir, which continuously holds sufficient gas quantities with a suitable pressure and temperature level in front of the cylinders.
  • The system has double-acting working and compression cylinders, which have the same temperatures in each of the two chambers of a cylinder.
  • The pistons of the cylinders are so flat that they only have one sealing ring and are not used to guide the pistons or to seal against the outside.
  • The cylinders are filled and emptied successively and discontinuously in the high and low pressure lines without complex valve control devices, so that expansion and compression cannot propagate in the pipelines and in the regenerator.
  • Therefore, there are no dead spaces and a maximum compression ratio can be maintained.

Main components of the New Stirling Installation

  • A reciprocating piston machine with one or more double-acting working cylinders. Cross-head guides, swash platesopen in new window or other linear guide systems (preferably a Hypocycloid straight line guidanceopen in new window) or Twin-Crankshaft-Driveopen in new window) prevent lateral forces on the piston rods of the reciprocating machine.
    Each cylinder contains a piston which symmetrically divides the cylinder space into two separate chambers. Both chambers of these double-acting cylinders do not form a pair of work and compression space as in conventional Stirling machines, but they have the same temperature (2 hot rooms or 2 cold rooms per cylinder) and the same task within the overall system. For each working cylinder with two chambers, a compression cylinder with two chambers is therefore required.
  • An equal number of double-acting compression cylinders which are mechanically coupled via a crankshaft or another device.
  • High pressure pipeline system between the outlet sides of the compression cylinders and the inlet sides of the working cylinders.
  • Low pressure piping system between the outlet sides of the working cylinders and the inlet sides of the compression cylinders.
  • Heating device in the high pressure piping system and on the expansion cylinders.
  • Cooling device in the low-pressure piping system and on the compression cylinders.
  • A central heat source. The heat transfer from the source to the heater and to the walls of the working cylinders takes place via heat pipes.
  • A counterflow heat exchanger as a regenerator / recuperator that is used simultaneously, continuously and jointly by all cylinders.
  • A system for the discontinuous filling of the working cylinder and a system for the discontinuous emptying of the compression cylinder which makes it possible to combine the gas flows of the cylinders and successively feed or remove them from a common pipe.
  • A heat recovery system that pre-heats the combustion air.

The Configuration of the New Stirling Installation

Configuration of the New Stirling Installation

Basic description of the functionality of the New Stirling Installation

The Cylinders

The expansion of the hot working gas in the working cylinders of the reciprocating piston engine drives the engine. The compression cylinders are connected to this via a crankshaft, Hypocycloid straight line guidanceopen in new window, linear generator, Twin-Crankshaft-Driveopen in new window, swash plateopen in new window or some other mechanical device and are thereby driven. The cold gas is compressed there at the same time.
While the gas expands in one chamber of the hot cylinder (working cylinder), it is simultaneously expelled in the other chamber of the same cylinder and vice versa. While the gas is compressed in one chamber of the cold cylinder (compression cylinder), it is sucked in in the other chamber. At the end of these cycles, i.e. after half a crankshaft revolution, the two chambers of a cylinder swap their function. All four cycles thus take place twice simultaneously during a complete crankshaft revolution in a cylinder pair.

The Flow pathes

The gas flows of the individual working cylinders are collected in a pipe, the low-pressure pipe. The low pressure line connects the outlet sides of the hot working cylinders with the inlet sides of the cold compression cylinders.

The hot, expanded working gas then reaches the shell space of the regenerator (counterflow recuperator) to the cooler ➜ through the pipes ➜ to the cold compression cylinders, where it is distributed to their inlet sides where it is available for suction into the cylinder. During this way the gas is cooled down and it loses pressure.

The cold, compressed gas amounts from the single compression cylinders are combined in a similar way in a pipeline, the high-pressure pipeline. The high pressure line connects the outlet sides of the cold compression cylinders with the inlet sides of the hot working cylinders.

After being pushed out of the compression cylinders, the cold, compressed working gas is brought together in the piping system and moves in the opposite direction ➜ from the pipe space of the regenerator to the heater ➜ to the pipes ➜ the hot working cylinder, where it is distributed on their inlet sides and is available for feeding into the cylinder. In this way, the gas is heated and its pressure is further increased.

Discontinuous filling and emptying

In contrast to the standard machine, the working cylinders are only filled with the required amount of compressed, hot working gas at the reversal points of the pistons (dead points), but they are continuously emptied during the return stroke.
In contrast to the standard machine, the compression cylinders are emptied exclusively at the reversal points of the pistons (dead points), i.e. after the gas has been fully compressed in the corresponding cylinder chamber, but filled continuously during the return stroke (intake phase).

The intermittent filling or emptying of the cylinder chambers is controlled by controlled closing or opening of the chambers at the end of the work cycle. This is achieved without using a complex valve control. Instead, a newly developed (free) piston control ("switchover device") is used, which is located in the hollow piston rods of the cylinders (details explained below).

The free piston control proposed here ensures that the working gas in the cylinder is sealed off during the expansion phase; it is separated from the working gas in the regenerator. The same applies to the working gas in the compression cylinder during the compression process. At the end of these working cycles, the switching device opens again and establishes a connection to the regenerator or to the pipeline systems.

Continuous flow

In the manner described above, after each working cycle of one of the chambers of a working cylinder, an amount of gas corresponding to the cylinder chamber size is successively fed to the low-pressure pipeline system and simultaneously withdrawn from the compression cylinders on the cold side. Analogously to this, after each compression stroke, gas is fed to the high-pressure pipeline system into one of the chambers of a compression cylinder and taken from the hot side, near the working cylinders.

Since gas quantities are constantly being supplied from the cylinders and withdrawn on the other side, the working gas moves practically continuously through the pipes. In this way, a quasi-continuous flow through the regenerator is created.

Animation Stirling plant with flow pathes

Stirling plant with flow pathes


The regenerator is designed as a counterflow tube bundle heat exchanger (recuperator). The flows of the low-pressure and high-pressure lines meet in the regenerator in separate areas (shell space and pipe space) without mixing there, and exchange heat there. In the following, the terms “regenerator” and “recuperator” are therefore used synonymously. Due to the countercurrent principle, the heat exchanger has a permanently hot and a permanently cold side with a stationary temperature profile.

Regenerator der Stirling-Anlage

In the classic machine, each individual pair of cylinder chambers (hot chamber and cold chamber) has its own regenerator, which can absorb the amount of heat corresponding to the filling quantity of a cylinder chamber.

The New Stirling Technology uses only one common recuperator for all cylinder pairs and two piping systems for all cold and hot cylinder chambers together.

This is made possible by the fact that the cylinder chambers are filled and emptied discontinuously and the connections between the regenerator and cylinder chambers are constantly switched over.

Gas can no longer escape from the cylinders into the regenerator which no longer represents a dead space and can therefore be enlarged as required. The volume of the pipelines and the regenerator / recuperator therefore far exceeds the volume of a cylinder chamber in contrast to the classic machines. If the regenerator / recuperator is made sufficiently large, the gas in a cylinder chamber needs several working cycles to flow through it completely.

In this way, the regenerator functions as a working gas reservoir, so that working gas is permanently available in a suitable condition on the cold and hot side in front of the cylinders.

As shown in the diagram above, the regenerator is very large compared to the classic Stirling engines, it is by far the largest element in the overall system. Calculations have shown that its efficiency can easily reach more than 95%. Money invested in a large regenerator is recovered through a smaller heater, a smaller cooler, and most importantly, greater efficiency.

Working steps of the cylinders

Working cylinder

The following detailed steps for a chamber of a working cylinder for one crankshaft revolution result from the described mode of operation:
  1. The piston in the working cylinder has reached dead center after the gas has expanded. The outlet channel is now opened.
  2. During the return stroke, the expanded gas is pushed out the entire way. It leaves the cylinder chamber through the hollow piston rod
  3. The exhaust port closes when the other dead center is approached.
  4. The switching device ensures that the inlet duct is opened.
  5. While the piston is close to dead center, fresh, hot, compressed gas flows into the cylinder chamber. Since the pressure increase in the cylinder chamber, the compressed gas also acts as a buffer and reduces the inertia forces of the piston movement when the direction is reversed.
  6. The inlet channel is closed again.
  7. The trapped gas expands and drives the piston back in the other direction . In contrast to the standard machine, the expansion does not affect dead spaces such as the connecting pipes in the direction of the regenerator.
  8. The piston reaches the other dead center.

Compression cylinder

For one chamber of a compression cylinder, the following steps result for one crankshaft revolution:
  1. After compressing the gas, the piston is just before dead center.
  2. The outlet channel is now opened.
  3. The piston travels a short distance to dead center and pushes the compressed gas into the subsequent pipelines.
  4. The outlet channel closes, the inlet channel opens at the same time.
  5. The piston moves back in the other direction and cold, expanded gas is sucked in during the entire path.
  6. The piston reaches dead center and the inlet port (= intake port) closes.
  7. The piston changes direction and the trapped cold gas is compressed. In contrast to the standard machine, the compression does not propagate into dead spaces such as the connecting pipes in the direction of the regenerator.

Cylinder design

Piston Piston Rod Cylinder Head left Cylinder Head right Anular high pressure chamber left Anular high pressure chamber right Heat Transfer Elements Heat Transfer Elements Heat Insulation Heat Insulation Anular Ring Elements (Casing) Anular Ring Elements (Casing) Expansion bolts Expansion bolts Guiding Sleeve (Low pressure pipe side) Heat Piping Piston Rod Piston Rod Piston Rod Guiding Sleeve (Low pressure pipe side) Guiding Sleeve (Low pressure pipe side) Guiding Sleeve (Crankshaft side) Guiding Sleeve (Crankshaft side) Cylinder Wall (exchangeable liner) Cylinder Wall (exchangeable liner) Heat Transfer Element Heat Transfer Element Heat Transfer Element Heat Transfer Element High Pressure Inlet (Left Chamber) High Pressure Inlet (Right Chamber) Left Cylinder Chamber Left Cylinder Chamber Right Cylinder Chamber Right Cylinder Chamber

The sketch demontrates the main layout of the cylinders:

A flat symetrical double acting piston devides the cylinder in two chambers of the same displacement volume. The hollow piston rod is extended beyond the cylinder heads and is guided in sleeves. One side of each piston rod is connected to the crank shaft. The other side guides to the subsequent low pressure piping system. The hollow piston rod contains inside the switching device, a free piston ("control piston"), which follows the movement of the piston rod and furthermore can move inside the piston rod between two final positions.

The machine has to be designed in such a way that the piston rods can move easily in the sleeves, that there is only a minimum of friction between the piston rings and the inner walls of the cylinder, and that the gap dimensions are designed so that only a minimum of working gas can escape or overflow.

1 = Expansion cylinder, 2 = Crankshaft with axial guidance of the piston rods, 3 = Compression cylinder

Details of the mechanical structure of the cylinder

High pressure chambers

There are annular chambers on the cylinder heads which are connected to the high pressure piping lines. In the case of the working cylinders they are used for filling, in the case of the compression cylinders they are used for emptying the cylinder chambers.

Piston Rod

The hollow piston rod is part of the piping system. In the case of the working cylinders they are used for emptying, in the case of the compression cylinders they are used for filling.

The piston rod contains the switching device inside, which results in a comparatively large diameter. This is also necessary because of the flow velocities occurring inside.

On the surface of the piston rod there are longitudinal grooves on both sides of the piston ("ports"). When one side of the piston rod reaches the dead point, these grooves form a flow connection between the annular high pressure chamber and the cylinder chamber.

Furthermore, there are also radial bores near the piston. These openings establish a flow connection from the cylinder chamber ➜ to the interior of the hollow piston rod ➜ to the subsequent low pressure lines. In the case of the working cylinders they are used for emptying, in the case of the compression cylinders they are used for filling the cylinder chambers.

The switching device inside the hollow piston rod opens and closes these flow pathes. In the case of the working cylinders they are maintained open during the discharge phase and closed during the expanding phase. In the case of the compression cylinders these flow pathes are maintained open during the suction phase and closed during the expansion phase.


The piston preferably is disc-shaped, it is made of flat sheet steel plates. Common Stirling engines have a dome (hemispherical attachment) on the piston as protection against high temperatures. It could be provided that the pistons have a curved, i.e. an elliptical cross-section instead of a dome, which could eliminate static and thermal problems. The purpose is that the pistons have linear contact (ring contact) with the cylinder wall.

Cylinder Housing and Heating Devices

The cylinder heads are made of flat steel plates / flanges. They are held together outside by expansion bolts. The bolts absorb all tensile stresses between the cylinder heads,

The inner wall of the cylinder consist of an exchangeable sleeve, preferably made of ceramic materials with high thermal conductivity and high abrasion resistance, such as silicon carbide (Thermal conductivity: 0.1 ... 0.14 kW/(m·K)). Materials that can withstand high pressures, such as nickel- or copper-based alloys, are also possible.

Heating devices on the outer walls of the sleeve of the working cylinders and cooling devices on the sleeve of the compression cylinders have the task of keeping the temperature of the gas in the cylinders constant during the stroke. The heat transfer elements on the working and compression cylinders consist of single ring elements around the cylinder sleeve. The heating resp. cooling on the cylinder outer walls takes place along the entire stroke length. The connections from heat source to the heating/cooling devices are designed as heat pipesopen in new window. The effective thermal conductivity depends on material, length and the diameter of the pipe and can be up to 100 kW/(m·K) (for comparison copper: 0.4 kW/(m·K)).

Ring elements around the heating/cooling elements enable high internal pressure inside the cylinders. The ring elements form the outer casing of the cylinder and should be thermally isolated from the heating elemens, for example by calciumsilicate liners. (Thermal conductivity: 0.00031 ... 0.00035 kW/(m·K)). Thus, reducing heat loss via the cylinder surface.

Overall, the cylinder has a sandwich-like structure: the inside of the cylinder chamber is followed by a liner (silicon carbide) ➜ heat transfer elements ➜ thermal insulation (calcium silicate) ➜ outer shell. Only the liner has a completely closed design, the other sleeves are made up of ring elements.

Lateral guidance

When using a crankshaft, a crosshead guide or comparable devices such as roller bearings, Hypocycloid straight line guidanceopen in new window), prevent lateral forces on the pistons.

The central guidance of the piston is only ensured by sliding rings, stuffing boxes / packings or similar installed at the piston rod (small diameter). The piston ring(s) (big diameter, if installed) do not have the task of axial guidance along the piston stroke or of sealing the cylinder chambers from the outside. A gas loss from one chamber can only occur by flowing over into the other chamber of the same cylinder. The pistons can be made very flat. Defined gap dimensions / fits reduce the piston friction.

Switching Device

Each cylinder contains two chambers, one is closed during the stroke (to expand or compress the gas) while the other is open (to expel or suck the gas). The corresponding inlet and outlet channels are alternately opened and closed by a switching device inside the hollow piston rod.

The switching device consists of a movable control piston which, by its position inside the piston rod, either opens or blocks flow channels from the cylinder chamber to the center of the hollow piston rod.

As soon as the piston rod reaches a dead center, the control piston changes to the other position and the flow paths are switched. The cylinder chambers exchange their functions.

cross section view of expansion cylinder with gas flow through control piston
Cross section view of expansion cylinder
with gas flow through control piston.

Expansion takes place in the right chamber and moves the piston to the left side.
Expanded gas is expelled from the left chamber

1 = Exhaust flow of expanded gas, 2 = Control piston in left position, 3 = Expanded gas (here: in left cylinder chamber), 4 = Open flow channel, 5 = Expanding gas (here: in right cylinder chamber), 6 = Piston rod

Switching device - Versions

The switching device can be constructed according to three different principles:
  • Pneumatic Actuation:

    The position of the control piston within the piston rod is determined due to the pressure differences at its two ends, so that it can move between two end positions, left and right. The pressure ratios are maintained throughout the stroke, which is why the control piston remains in its position inside the piston rod during the entire stroke.

    In this position, the radial bores within the piston rod on a cylinder chamber are aligned with the radial bores within the control piston. Thus, creating a flow passage ➜ to the inside of the piston rod and further ➜ to the low pressure piping system. The respective other cylinder chamber is closed.

    When the dead center is reached, the ring channel on the other side is pressurized. The control piston moves axially to the other direction. When switching over, the radial bores in the piston rod and control piston are shifted against each other and the other outlet channel is activated.

  • Mechanical actuation:

    When the end positions (dead centers) are reached, the control piston is not displaced in the axial direction, but is mechanically rotated through an angle α, so that other axial bores in the piston and in the piston rod are aligned or are displaced in relation to one another. For details see below.
  • Actuation by magnetic forces:

    Magnets are embedded in the control piston and in the piston rod, which are slightly offset from one another in axial direction. As soon as the piston rod approaches dead center, magnets in the control piston and in the piston rod approach and turn the control piston by an angle α. This option could also be combined with the mechanically actuated solution.
In summary, the movement of the control piston can be designed according to two different principles:
  • Alternate axial displacement from left to right between two end positions
  • Alternate rotation by a given angle α between two end positions

The graphics below show the principle of pneumatic actuation. In order to understand the mode of action in detail, the following points must be observed:
  • The movements of the control pistons are superimpositions of two independent actions:
    1. The movement of the piston rods
    2. The movement of the control pistons within the piston rods
  • The control pistons remain in their position within the piston rod throughout the stroke and change to their other position when the piston rod reaches its reverse position. The cylinder chambers are therefore closed during an upstroke, but are open at the same position on the return stroke, and vice versa.

Comparison of expansion and compression cylinders
with pneumatically operated control piston

Switching device comparison Expansion Cylinder Compression Cylinder
Dimensions Short control piston Long control piston
Function of the axial bores in the control piston exhaust channel inlet channel
Start of stroke at first dead center Control piston is held pneumatically in the end position in the direction of movement (due to pressure differences at the two ends) and carried along by the piston rod. Control piston is mechanically taken along by the piston rod.
From the middle of the stroke (maximum speed of the piston rod) to the other dead center Control piston is held in the direction of movement by inertial forces in the mechanical end position
and held by pressure of the cylinder chamber.
Control piston is held pneumatically in the original end position.
Switching process at the other dead center Control piston is shifted pneumatically in the direction of the beginning return stroke. Forces of inertia push control piston into the new end position, the control piston is decelerated pneumatically by the existing pressure in the ring chamber,
Then pressure drop in the ring chamber,
Then mechanical stopping in the new end position
Then new holding pressure in the opposite ring chamber.
Holding pressure in the ring chamber Acts continuously, but decreases as the stroke progresses.
From the middle of the piston stroke ( decreasing piston speed inertial forces on the control piston) holding pressure is no longer required.
Pressure is locked in, pressure reduction only at the end of the stroke through overflow channels to the inside of the piston rod

Switching device (pneumatic actuation)

Expansion cylinder:

Animation - Piston of Stirling Engine

Compression Cylinder:
Animation of compression cylinder of Stirling engine

FAQ - Pneumatically activated control pistons

  • [Question]: Why not just use pressure relief valves to drain the compression cylinder chambers?
    • Compression and extension are clearly separated.
    • Pressure relief valves can only relieve peak pressures into a lower pressure space. The overall pressure level in the compression cylinders is lower than in the working cylinders! It may therefore be pushed out into a room with higher pressure, because the highest pressure is only present after heating in the heater.
    • It must be ensured that a defined quantity of gas with a defined pressure is released at a specific position of the piston, regardless of the conditions in the downstream pipelines. The New Stirling Engine Technology ensures that the cylinder chambers are emptied as completely as possible. During the return stroke, the cylinder chamber should be refilled completely, not just partially.
    • The small amount of gas remaining in the cylinder after unloading is further compressed and serves as a gas buffer to reduce the inertial forces when changing direction at dead center.
    • This small remaining volume of gas experiences the most compression, hence the most heating. The additional cooling of the cylinder heads allows this to be cooled down effectively (large area, small thickness). When the cylinder is reversed, this gas expands again and leads to an even lower temperature in the compression cylinder. This effect can be further optimized if the inside of the cylinder head is lined with a heat-storing material.
    • Since the compression cylinders are coupled to the expansion cylinders, it is optimal if the stroke length and compression ratios are identical. Pressure relief valves would create additional dead space and affect the finely tuned compression ratio.
    • Relief valves are mostly used for maintaining pressure, not for high frequency switching
    • Pressure relief valves are potential sources of error and wearing parts, especially at the high temperatures required.
    • The achievable standard cross-sections with the free-piston solution through the ports are also larger than with poppet valves. The flowing gases are not impeded by a flow-unfavorable valve.
    • The straight gas flow and the short flow paths result in only slight turbulence. This benefits a significantly better filling / emptying within a defined short distance / period of the stroke and thus a higher engine performance. Further advantages arise depending on the version.

  • [Question]: What about the typical disadvantages of sliders like binding or galling versus large gaps?
    • The control piston should have adequate clearance within the piston rod and could be made of ceramic material. Appropriate gap dimensions between the inner diameter of the piston rod and the outer diameter of the control piston create an air bearing (as in microturbines), which eliminates friction and lubrication.
    • The spool may be reduced in diameter at many locations along its length.
    • The gas losses through annular gaps remain minimal, since the flow path to the subsequent low-pressure line is relatively long and the pressure resistance is high.
    • The use of ring seals or roller bearings at certain positions may be considered. Ceramic roller bearings can eliminate both sealing and longitudinal guidance problems.
    • A material expansion with strong temperature changes does not take place due to the constant temperature.

  • [Question] Why not just use pressure relief valves to fill the expansion cylinder chambers?
    [Answer] It must be ensured that no further gas flows in after the start of the expansion process.

Switching Device
(Mechanically controlled version - Expansion cylinder)

An inner sleeve has the function of a guide template. It is secured against axial movement and rotation within the piston rod and has at least one cut-out representing the freedom of movement of a guide pin.

During the strokes of the piston rod, the pin mounted on the control piston is driven within the recess. This movement is essentially a back and forth wandering along two long parallel straight lines (according to the two strokes). Shortly before the dead center is reached, the pin is deflected to the side by guiding the sleeve and thus rotates the control piston (including the flow openings) by the angle α. This corresponds to the rotation of the spool to the other end position so that the flow paths are switched as described above. During the return stroke, the pin moves along the other straight line. Overall, it follows a hysteresis-like path.

Animation of a cylinder of a Stirlingengine with machanical actuating

Guide sleeve for forced reversal:

Forced reverse guide sleeve
Guide sleeve for forced reversal

Flow paths in axial view:

The control piston rotates through a defined angle α and thus opens or blocks flow paths. This method applies to mechanical or magnetic actuation.

Flow paths in axial view
Flow paths in axial view

What is the best working gas for a Stirling engine system?

The modern "classic" Stirling engines mostly use helium as the working gas. Air, hydrogen or nitrogen would also be conceivable.

Can better efficiencies be achieved with helium or hydrogen? The existing knowledge on this is based on the non-generalizable experiences of the machines for the common small capacities. Hydrogen offers the most favorable properties from a thermodynamic point of view. However, since hydrogen always poses an explosion hazard, makes steel brittle and diffuses through many materials, hydrogen is excluded here. The question is therefore: air or helium as the working gas?

   Density  Specific
 Heat capacity cp  
 Heat capacity cv  
 Air (dry)    1.29 kg/m3 1.005 kJ/(kg·K) 0.72 kJ/(kg·K)
 Helium  0.179 kg/m3    5.193 kJ/(kg·K)   3.22 kJ/(kg·K)

Air is around 7 times heavier (denser) than helium, but has only around 5 times the specific heat capacity. This means that a machine with a certain volumetric flow rate cannot transfer more heat with helium than with air.

The heat capacity cv of polyatomic gases such as air increases with increasing temperature, the isentropic exponent κ (the ratio cp/cv) thus decreases. At normal pressure, the value of κ for helium is consistently 1.67. In the case of air, on the other hand, the value for κ = 1.4 at 0°C decreases with increasing temperatures and approaches κ = 1.3 in the temperature ranges of around 800°C which is relevant for Stirling engines.

If the ideal case of an isothermal expansion (p·Vn = const., with n=1) can not be achieved in the hot cylinder, but approaches rather the isentropic case (n=κ), there are losses in efficiency . A smaller value for κ is advantageous here.
With other words: Air disposes about a more favourable κ.

Comparison of adiabatic and isothermal change of state
Comparison of adiabatic and isothermal change of state

Due to these circumstances, the actual advantages of helium over air are partially offset.

Advantages of helium:

  • Very good thermodynamic properties
  • Smaller heat exchangers required
  • Less friction/flow losses

Disadvantages of helium:

  • High cost
  • High effort for sealing
  • Need of an automatic refilling device

Power of a Stirling engine as a function of the gas throughput
Power of a Stirling engine as a function of the gas throughput
(based on: Tmax=800°C, Tmin=150°C, pmax=75 bar, pmin=3 bar)

Calculations have shown that the advantage of helium over air in relation to the required regenerator size (counterflow tube bundle heat exchanger) mainly apply at low outputs.

Required regenerator exchange area depending on the gas flow rate
Required regenerator exchange area depending on the gas flow rate
(based on: Tmax=800°C, Tmin=150°C, pmax=75 bar, pmin=3 bar)

If you want to optimize the equipment, you should definitely consider using air as the working gas and a larger regenerator. In return, you can do without the additional equipment for helium handling. The advantage of helium over air diminishes as performance increases. This applies in particular to the targeted power range of the New Stirling engine technology.

The complete Plant

The plant is fed with heat from a central single source.

This could be a central furnace/burner unit that is fed with diesel or LPG or hydrogen or something else combustible. It is also conceivable that waste heat from a industrial process is used in order to improve its efficiency. If the New Stirling System cannot be installed close to the heat source, air recirculation systems can be provided.

In the examples below, the heat is transferred via heat pipes to a heater installed in the high-pressure pipe system and to the outer surfaces of the expansion cylinders.

Layout of complete New stirling plant

Notes on construction and thermodynamics

Since the heater and cooler are installed in the pipelines, there are not four, but six process steps in the pV diagram. The lowest pressure is created behind the cooler (before entering the compression cylinder). The highest pressure does not arise at the exit of the compression cylinder, but after passing through the heater.


These steps have been marked in the pV diagram above. The heat removal in the regenerator ➁ ➂ (shell side) corresponds to the heat supply ➃ ➀ (tube side) in the direction of the working cylinder. The bigger the heat exchange in the regenerator, the smaller the remaining amount of heat to be supplied / removed in the heater / cooler. A large part of the heat is trapped in the system.

This directly results in the potential to increase performance and efficiency: The increase in the amount of heat transferred in the regenerator either reduces the required size of the cooler or increases the temperature difference (and thus the efficiency) between the hot and cold side (shifting the curve ➂ ➃ down).

The performance of the plant stands and falls with the correct design of the regenerator!
The regenerator is the key element to the efficiency of the plant

From over 100 sample calculations with helium as the working gas with a maximum temperature of 800°C, the following results have emerged:

  • By varying the cylinder diameter (0.25m - 0.30m - 0.35m - 0.40m), stroke length (0.30 m - 0.38 m - 0.44 m - 0.56 m), number of cylinders (4-6-8) and speed (300rpm - 400rpm - 500rpm - 600rpm) a series with outputs from 800kW to 15000kW at a pressure ratio of 3 bar: 75bar (compression: 25) can easily be created. The maximum mean piston speed never exceeds 11.5m/s.
  • The quality grade was set at 0.75...0.85 based on a conservative estimate of the loss mechanisms and in comparison to internal combustion engines. This results in effective efficiencies of 46% ... 62%.
  • By increasing these values, for example the speed and / or the compression, even higher performance and better degrees of effectiveness can be achieved.
  • The above-mentioned efficiencies resulted primarily from a reasonable design of the regenerator: As a rule of thumb, it was found that a U-shaped counterflow tube bundle heat exchanger which has approximately the same length as the crankshaft and about twice the diameter of a cylinder (with k = approx. 150 ... 200 W/(m2K) has achieved an efficiency of 85 ... 90%, so that an exit temperature (cold side) of 150°C and an exit temperature (hot side) of approx. 630 ... 680°C can be reached. The bigger the machine, the more favourable these numbers are.

    Even better efficiency of the regenerator is theoretically possible, but would increase its size disproportionately. A reasonable design of the regenerator means that the heater and cooler each have to achieve a temperature difference of only approx. 100 ... 150 ° C.

  • If the cooler is also designed as a tube bundle heat exchanger, its performance and size can be estimated at 15 ... 30% compared to the regenerator in order to achieve the lowest temperatures of 50 ... 100°C.
  • Based on a conservative estimate of the loss mechanisms and in comparison to internal combustion engines, the effective efficiency reaches up up to 65%.

      Combustion engine New Stirling engine
    Frictional losses to 15% up to 5%
    Cooling water losses up to 25% up to 15%
    Exhaust gas losses up to 35% up to 10%
    Radiation, wall heat losses to 15% up to 10%
    Burn losses,
    imperfect thermodynamic process
    up to 40% up to 15%
    Total: approx. 65%
    (Empirical value)
    approx. 35 ... 55%
    residual efficiency (1-total) approx. 35%
    (Empirical value)
    approx. 45 ... 65%

If you add up all the maximum losses for combustion engines that are reported in studies, dissertations and research
  • Bock S., Mau G .: Exploitation of exhaust heat from diesel engines. In: The diesel machine in land and ship operations. Vieweg + Teubner Verlag, Wiesbaden.
  • Simon Hirzel: "Industrial waste heat utilization" (short study)
  • Prof. Dr. Andreas Wimmer: "Thermodynamics of the Combustion Engine", Graz University of Technology
  • Dr. Benedikt Vogel: "Efficient and clean diesel engines"
are published, you get a total of well over 100%. At this point, common sense is challenged to see how the machines really work ...
How do these favorable values ​​for the loss mechanisms of the New Stirling Technology come about? Read about the obvious and indisputable

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