Patent Application
20150337869
The present application relates to accumulators for transmissions within vehicle powertrains.
Accumulators are a part of a transmission within a vehicle powertrain. They store hydraulic potential energy when the engine is shutdown. If the internal pressure of the accumulator is less than the pressure from a hydraulic pump, then the hydraulic fluid flows into and fills the accumulator. The accumulator stores this volume of fluid, under pressure, to store potential hydraulic energy. Upon an engine restart command, accumulators supply pressurized hydraulic fluid to the shift elements necessary for the transmission to transmit power following engine startup. This is useful for vehicles utilizing engine start/stop systems.
Engine start/stop systems shut down a vehicle engine when no torque is needed, for example when the vehicle is stopped at a traffic light. This helps reduce fuel consumption, but increases the number of times the engine needs to be restarted. It is advantageous, therefore, to more quickly supply the energy necessary for the shift elements upon an engine restart command.
An accumulator for a vehicle includes a cylinder defining a bore having an inner surface, and a piston moveable within the bore. The piston includes a seal and a guide section defined by a truncated sphere. The guide section is configured to orient the piston within the bore such that the seal maintains contact with the inner surface of the bore.
An accumulator includes a cylinder defining a bore and a cylinder axis, and a piston moveable within the bore. The piston has a piston axis. The piston axis and the cylinder axis define a tilt angle. The accumulator further includes a seal disposed on the piston, and a guide section formed on the piston. The guide section has a curvature such that the seal maintains contact with the cylinder at a maximum tilt angle exceeding 2 degrees.
A powertrain for a vehicle includes, an engine, a pump mechanically driven by the engine to pressurize hydraulic fluid when the engine is running, a plurality of shift elements, a hydraulic control system configured to route pressurized fluid from the pump to the plurality of shift elements, and an accumulator configured to store the pressurized fluid and supply the pressurized fluid to the plurality of shift elements when the engine is not running The accumulator includes a cylinder, a piston disposed within the cylinder defining a chamber, and a spring biasing the piston to reduce the volume of the chamber. The piston includes a guide section having a truncated spherical portion.
Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments may take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures may be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations.
Referring to
The larger pressure of the hydraulic pump 16 creates a pressure difference allowing the hydraulic fluid to fill the accumulator 22. When the transmission pump 16 has a pressure less than the pressure within the accumulator 22, the accumulator 22 will not fill. The hydraulic controls, through a valve and check valve, allow the accumulator 22 to store the hydraulic fluid under pressure to maintain a stored hydraulic potential energy while the engine is shutdown to save fuel. The hydraulic controls 18, upon the engine 12 restart command, direct the accumulator 22 to discharge the necessary hydraulic energy to the shift elements 20. Storing more hydraulic energy requires either increasing the packaging space or an accumulator 22 with a higher energy density. An accumulator 22 that stores more hydraulic energy density more quickly energizes the necessary shift elements 20 upon engine 12 restart.
As shown in
As stated above, packaging space within the accumulator 22 may be important. Increasing the packaging space, thereby allowing for a spring 30 with a larger outer diameter 44 to fit within the piston 28, increases the hydraulic energy. Storing more hydraulic energy may result in a higher stored energy density. Removing the need for guide bushings increases packaging space within the bore 26 and increases the hydraulic energy density of the accumulator 22.
In order to increase the outer diameter 44 of the spring 30, the piston 28 is formed with a guide section 34. Through heat treating or coating, and low micro finish, for example polishing, the guide section 34 may be formed having a truncated spherical curvature 36. The guide section may be formed using surface hardened steel with a Rockwell hardness of at least 50 RC. This allows the guide section 34 to prevent the wear typically absorbed by the guide bushings. In addition, the truncated spherical curvature 36 of the guide section 34 reduces contact between the piston 28 and the bore 26. The truncated spherical curvature 36 of the guide section 34 reduces the piston surface area 38 moving against the bore 26. This reduces drag imposed by friction and improves accumulator 22 discharge response time.
The lack of piston surface area 38 contact with the cylinder 24, resulting in the reduction in drag of the piston 28 on the cylinder 24, coupled with the increase of hydraulic energy density further allows the accumulator 22 to more quickly supply energy to the shift elements 20 required for engine restart. The accumulator 22 response time may be reduced to approximately 250 milliseconds. This allows a vehicle powertrain 10 to restart the engine 12 before the hydraulic pump 16 is capable of supplying energy to the vehicle transmissions 14. Supplying the hydraulic energy necessary for an engine 12 restart as well as the improved response time of the accumulator 22 improves the overall fuel economy of the vehicle.
Referring to
The diameter 42 of the guide section 34 may be substantially equal to the outer diameter 44 of the spring 30. This provides greater balance of the piston 28 on the spring 30 to further reduce drag between the piston 28 and the inner surface 40 of the bore 26. Further, the diameter 42 of the guide section 34 may also substantially equal the inner diameter 32 of the bore 26. Therefore, the outer diameter 44 of the spring 30 may be substantially equal to the inner diameter 32 of the bore 26. The increased bore packaging space allows the spring 30 to have a larger outer diameter 44. With a larger outer diameter 44, the spring 30 is able to further support the piston 28 under a higher pressure. This allows for an increase in pressure in the cylinder 24 and as such an increase in the hydraulic energy density of the accumulator 22.
A spring 30 with a larger outer diameter 44 is able to support a greater volume of hydraulic fluid which may increase the pressure within the cylinder 24. The increase in volume and the resulting increase of pressure results in an increase in the hydraulic energy density of the accumulator 22. Increasing the hydraulic energy density of the accumulator 22 improves the response time of the accumulator 22. Storing a greater volume of hydraulic fluid under a greater pressure, through the use of a valve and check valve, permits the accumulator 22 to more quickly energize the shift elements.
Further, the increase in the spring diameter 44 allows the accumulator 22 to have a longer piston stroke volume. A spring 30 with a larger outer diameter 44 is able to compress further, allowing the piston 28 to have a longer stroke. Increasing the stroke volume of the piston 28 allows the accumulator 22 to have a higher hydraulic energy density. As stated above, a high hydraulic energy density allows the accumulator 22 to respond faster when supplying hydraulic energy to the transmission 14. Therefore, increasing the diameter 44 of the spring 30 and forming the guide section 34 with a diameter 42 substantially equal to the outer diameter 44 of the spring 30 allows for a significant reduction in response time of the accumulator 22.
Referring to
For example, the spring 30 may be misaligned by approximately 2°. This small degree misalignment α may result in a tilt of the piston 28 against the inner surface 40 of the bore 26. When the piston 28 is tilted on the spring 30, the curved surface 36 of the guide section 34 may contact the inner surface 40 of the bore 26. The guide section 34 acts as an adjustment mechanism compensating for the misalignment α of the spring 30.
Being tilted reduces a clearance γ between the guide section 34 and the bore 26. By reducing the clearance γ between the guide section 34 and the bore 26, the distance between a base edge 48 of the piston 28 and an inner surface 40 of the bore 26 is increased. This is due to the angular misalignment α of the spring 30. The increase in clearance γ may require the piston 28 to maintain a seal 50, at a greater distance, with the inner surface 40 of the bore 26. The guide section 34, having a diameter 42 substantially equal to the inner diameter 32 of the bore 26, accounts for this increase in clearance γ and allows the piston 28 to maintain a seal 50 with the inner surface 40 of the bore 26.
The guide section 34 accomplishes this through a ratio between the length 52 and diameter 42 of the curved surface 36. The ratio of the length 52 to diameter 42 of the guide section 34 is greater than a tangent of the misalignment α of the spring 30. This allows the guide section 34 to compensate for the misalignment α of the spring 30. The ratio of the length 52 and diameter 42 may be such that the guide section 34 compensates for greater than 5° of a tilt angle β between a piston axis 56 and a cylinder axis 58.
Since the guide section 34 compensates for a tilt angle β greater than 5° and the misalignment α of the spring 30 may be approximately 2 to 3°, the guide section 34 is further configured to account for and orient the piston 28 within the bore 26. The truncated spherical curvature 36 of the guide section 34 orients the piston 28 within the bore 26. A self-orienting guide section 34 allows the piston 28 to float on the spring 30 without the use of guide bushings. The guide section 34, therefore as part of the piston 28, allows the piston 28 to self-orient within the bore 26 despite floating on a misaligned spring 30. This allows the cylinder 24 to utilize a spring 30 having larger outer diameter 44, despite the potential small degree misalignment α of the spring 30. The self-orienting guide section 34 may increase the hydraulic energy density of the accumulator 22 by approximately 20%.
The truncated spherical curvature 36 of the guide section 34 further prevents binding between the piston 28 and the bore 26. As explained above, the curved surface 36 of the guide section 34 minimizes contact between the piston 28 and the inner surface 40 of the bore 26. Due to the spherical nature of the curved surface 36, the piston 28 may only contact the inner surface 40 of the bore 26 at a single point. Therefore, even despite a misalignment a of the spring 30, the guide section 34 of the piston 28 further aids in reducing wear on the piston 28. Minimizing the contact between the piston 28 and the inner surface 40 of the bore 26 allows the accumulator 22 to last longer. This may save time, cost, and manufacturing expenses.
Reducing the binding between the inner surface 40 of the bore 26 and the guide section 34 further reduces the friction drag force between the piston 28 and the cylinder 24. Reducing the drag force not only reduces damage to the guide section 34 of the piston 28 due to friction, but also improves the response time of the accumulator 22. Further, reducing the friction drag force allows the accumulator 22 to use the hydraulic energy to energize the shift elements 20, rather than using the hydraulic energy to overcome the friction drag force. Therefore the guide section 34 allows the accumulator 22 to store more potential hydraulic energy, have a higher energy density, and an improved response time.
While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments may be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics may be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. These attributes may include, but are not limited to cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. As such, embodiments described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and may be desirable for particular applications.
