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.
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
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.
|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|
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.
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?
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.
Due to these circumstances, the actual advantages of helium over air are partially offset.
Advantages of helium:
Disadvantages of helium:
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.
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.
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.
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).
From over 100 sample calculations with helium as the working gas with a maximum temperature of 800°C, the following results have emerged:
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.
|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%|
imperfect thermodynamic process
|up to 40%||up to 15%|
|approx. 35 ... 55%|
|residual efficiency (1-total)||
|approx. 45 ... 65%|