Patent Application
20070044467
This invention is related to a heat engine and particularly to an improved Stirling cycle engine incorporating features to equalize pressure among separated volumes of working gas within the engine.
The basic concept of the Stirling engine dates back to a patent registered by Robert Stirling in 1817. The engine operates by causing a working gas to shuttle between regions of temperature difference accompanied by volume and pressure variations. Stirling engines have a reversible thermodynamic cycle and therefore can be used as a means of delivering mechanical output power from a source of heat, or can act as a heat pump through the application of mechanical input energy. Using various heat sources such as combusted fossil fuels or concentrated solar energy, mechanical power can be delivered by the engine. This energy can be used to generate electricity or can be directly mechanically coupled to a load. Numerous potential applications for Stirling engines have been identified, for example including: as prime moves for motor vehicles, solar energy production, waste heat recovery, and remote location electricity generation.
The Assignee of the present application, STM Power, Inc. (previously named Stirling Thermal Motors, Inc.), has made significant advances in the technology of Stirling machines through a number of years. Examples of such innovations include development of a compact and efficient basic Stirling machine configuration employing a parallel cluster of double-acting cylinders which are coupled mechanically through a rotating swashplate. In many applications, a swashplate actuator is implemented to enable the swashplate angle and therefore the pistons' stroke and swept volume to be changed in accordance with engine operating requirements.
Although the Assignee has achieved significant advances in Stirling machine design, there is a constant need to provide further refinements. In a double-acting, multiple-cylinder Stirling engines, isolated volumes of the working fluid, typically helium or hydrogen gas, are shuttled through the engine. In accordance with the thermodynamic cycle of a Stirling engine, these isolate volumes are cyclically compressed and expanded and shuttled between spaces having temperature differences. Due to dynamic conditions during operation, leakage, and start-up conditions, changes in the mass of gas contained in each of the isolated cycle volumes occurs. These differences in “charge” mass or volume in the isolated cycle volumes lead to imbalances and roughness in operation of the machine. Moreover, such imbalances place undesired mechanical forces on the moving parts of the engine, increase noise and vibration of the engine during operation, negatively affects thermal efficiency, and increase starting torque.
Even with ideal sealing among the working parts of the Stirling engine and uniform charge volumes of the working gas during operation, once the engine is shut down, the cycle volumes will be stopped at various stages of compression. Inevitably, the working gas will leak from high pressure areas to low pressure areas over a period of time. This results in a difference in charge volume between cycles since each defines a separated volume. Thus, upon starting the engine, a significant difference in charge volume exists between working gas cycles. This invention provides a system for equalizing working gas charge volumes between the isolated cycle volumes.
One approach toward providing pressure balancing between isolated cycle volumes in a multiple-cylinder Stirling engine is described in Assignee's U.S. Pat. No. 5,813,229. That patent describes allowing each of the cycle volumes to communicate via a small diameter capillary tube. Although this system will result in pressure balancing over time, it has the disadvantage of creating a constant loss in efficiency due to an exchange of gas between cycles, even where pressure balance conditions do not exist. This occurs since the capillary tube is exposed to out-of-phase pressure variations and consequently there is a constant shuttling flow of gases through the capillary tubes.
In accordance with this invention, a system is provided for allowing minute transfers of working gas between cycle volumes to occur in a manner which enables their minimum pressures and consequently their total charge volumes to be equalized. This has the affect of producing a smoother running engine and addresses the previously mentioned shortcomings of Stirling engines in accordance with the prior art technology.
The Stirling engine innovations of the present invention may be implemented in numerous engine configurations, including the types previously developed by the Assignee, including those described in U.S. Pat. Nos. 4,481,771; 4,579,046; 4,615,261; 4,669,736; 4,836,094; 5,611,201; 5,706,659; 5,722,239; 5,865,091; and 5,938,207, which are hereby incorporated by reference. Basic features of many of the Stirling machines described in the above-referenced patents are also implemented in connection with the present invention.
The system of the present invention utilizes a piston having a hollow connecting rod which passes through a pair of separated rod seals. The interior passage of the connecting rod communicates with an annular piston seal volume between a pair of axially separated piston sealing rings. The connecting rod hollow passage communicates with volumes for each cylinder which are all connected together via passageways to form an equalization volume or plenum. This plenum is maintained at a minimum cycle pressure level through the use of a valves such as one-way check valves which communicate with the individual cycle volumes. In operation, the equalization volume is maintained at the lowest minimum cycle pressure in the system. Working gas is able to move into any cycle volumes which exhibit a minimum pressure which differs from the plenum pressure by leakage of working gas past the piston rings.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates from the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings.
With reference to
A pair of piston rings 20 and 22 provide sealing in the radial space between piston 16 and the inside diameter of cylinder bore 14, and these components defined an annular piston seal volume 58. Sliding rod seal 24 allows reciprocation of connecting rod 18 while providing a fluid seal. Similarly, rod seal 26 also provides a fluid seal for connecting rod 18. Rod seals 24 and 26 are separated to define partial equalization volume 28 for each of the cylinder assemblies 12. Each of pistons 16 act as moving boundaries of pairs of separated working gases cycle volumes, designated as cycle volumes “A”, “B”, “C” and “D” in
One-way check valves 40 are provided which communicates the cycle volumes A, B, C, and D to equalization volume 28. Check valves 40 are oriented such that gas flow only occurs from the equalization volume 28 to the connected cycle volume when a pressure difference occurs between them in the direction designated in
Connecting rod 18 incorporates a central passageway 42 which communicates with the annular piston seal volume 58 between piston rings 22 and 24. A more detailed illustration of this configuration is provided with reference to
For unloading Stirling engine 10, a series of valves 54 are employed which open ducts 36 with the equalization volume 28 and can be opened and closed by remote electrical controls. When valves 54 are opened, for example through energizing a solenoid, the Stirling engine 10 does not operate through a closed thermodynamic cycle. Valves 54 are provided for unloading engine 10 for use in start-up conditions or when complete unloading of the engine is desired during operation. It is noted that valves 54 communicate between the same spaces as one-way check valves 40 (which are pressure actuated). The difference between the valves is a greater flow capacity of valves 54 and their external actuation, as compared with the one-way check valves 40. Two separate sets of valves 40 and 54 are not essential to realize the benefits of the present invention. In another implementation of the invention, valves 40 can be eliminated, with valves 54 functioning for opening during start-up conditions through a remote control signal, and when closed, acting as a one-way pressure actuated valve, thus acting as check valve 40.
Equalization volumes 28 each define in their aggregate, a plenum which is maintained at the minimum cycle pressure experienced by the cycle volumes by means of operation of check valves 40 (or as mentioned above, valve 54). Working gas cannot transfer from one cycle volume to another without going through common plenum 56. In this manner, the gas pressure within plenum 56 is held a minimum cycle pressure in each cycle through the operation of check valves 40. As mentioned previously, when Stirling engine stops, the pistons 16 of each cylinder assembly 12 stop at a different position and each of the isolated cycle volumes A, B, C and D will have a different volume and pressure. After the Stirling engine 10 remains stationary for a period of time, the pressure of these isolated cycle volumes A, B, C, and D will tend to equalize due to leakage, for example across piston rings 20 and 22. This inequality in the charge volume among the cycle volumes for a stationary engine with give rise to high torque required to start the engine and imbalances during operation. To reduce the starting torque, the valves 54 are activated to open which communicates each of the cycle volumes to the common plenum 56. This condition unloads the engine and allows the charges to equalize among the cycle volumes for the first few revolutions of the engine. After a short time after start-up, valves 54 are deactivated and the system reaches steady-state operation.
The connection of the space between piston rings 22 and 24 to the plenum 56 held at a minimum pressure, provides the equalization function for the system of this invention. This connection prevents a net transfer of gas charge between adjacent cycles without involving the plenum 56. In operation, plenum 56 is held at an “average” minimum pressure for each of the cycle volumes A, B, C and D. When variations in the minimum pressure for an individual cycle volume occurs during its cyclical variation in pressure, leakage of working gas to or from that volume occurs past rings 20 and/or 22.
Numerous varieties may be provided within the scope of this invention. For example, in a physical implementation of this invention, the various ducts 29, 36, and 38 may be passageways or communication paths without requiring a separate pipe or coupling.
While the above description constitutes the preferred embodiment of the present invention, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope and fair meaning of the accompanying claims.
