Abstract:
A hydraulic accumulator system for providing pressurized oil to a transmission in a vehicle. The system includes a transmission oil storage chamber to store oil at a predetermined level; and a hydraulic accumulator mounted in the oil storage chamber and including an accumulator piston mounted below the level of oil, separating an accumulator oil area from a remainder of the oil storage area and movable to increase and decrease the oil in the accumulator oil area, and a spring biasing the piston toward the accumulator oil area. The system also has a hydraulic assembly including a first oil passage for removing the oil from the remainder of the oil storage area, a second oil passage for directing oil into and out of the accumulator oil area below the predetermined oil level and a hydraulic circuit directing the oil from the first oil passage into the second oil passage.

Description:
BACKGROUND OF INVENTION 
     The present invention relates to a hydraulic system having a hydraulic accumulator for an automatic transmission and more particularly to a hydraulic accumulator that assists with maintaining a hydraulic pressure to allow for immediate engagement of the transmission upon startup for a vehicle that operates with an engine stop-start system. 
     In order to improve the fuel economy of vehicles that use an internal combustion engine for propulsion, some vehicles employ a stop-start strategy where, when the vehicle is stopped, for example, at a traffic light, the engine is automatically turned off. When the light turns green and the driver releases the brake to apply pressure to the accelerator pedal, the engine is automatically started to allow the vehicle to accelerate. For such vehicles that also have an automatic transmission, a need arises to somehow maintain the hydraulic pressure in the transmission so that the transmission can engage immediately after the engine is automatically restarted, which will allow the vehicle to accelerate without hesitation. 
     In order to overcome this concern, some employ an electric pump for pressurizing the hydraulic fluid rather than the conventional engine driven pump. While this allows for the maintenance of the hydraulic pressure while the engine is shut off at the stop light, this system may be more costly and require more packaging space than is desired. It also may require an additional battery to power the electric motor. This system may also require continuous operation even when a pressure charge is not required due to a delay between pump motor startup and hydraulic pump prime time. 
     Another attempt to overcome this concern uses an external gas accumulator to store extra hydraulic fluid outside of the transmission hydraulic circuit. This also has drawbacks in that it may cost more and take up more package space than is desired. In addition, an external gas accumulator may cause a fluid level change in the oil pan during period accumulation (lower fluid level) and discharge (higher fluid level) that create undesirable conditions, such as oil starvation or spin losses, during engagement of transmission clutches. 
     Another attempted solution is to use a spring-loaded piston accumulator with a mechanical latch. This approach calls for control system redesign to package it internally. It also presents the same issue as a gas accumulator by changing the fluid level in the oil pan during operation. Such a spring loaded accumulator may require one hundred percent capacity fluid charge in order to mechanically latch the accumulator&#39;s piston, which may present an issue when frequent stop-start events occur. 
     SUMMARY OF INVENTION 
     An embodiment contemplates a hydraulic accumulator system for providing pressurized oil to a transmission in a vehicle. The system may include a transmission oil storage chamber to store oil at a predetermined level; and a hydraulic accumulator mounted in the oil storage chamber and including an accumulator piston mounted below the predetermined level of oil, separating an accumulator oil area from a remainder of the oil storage area and movable to increase and decrease the amount of oil in the accumulator oil area, and a spring engaging the accumulator piston and biasing the piston toward the accumulator oil area. The system also may have a hydraulic assembly including a first oil passage for removing the oil from the remainder of the oil storage area, a second oil passage for directing oil into and out of the accumulator oil area below the predetermined oil level and a hydraulic circuit selectively directing the oil from the first oil passage into the second oil passage. 
     An embodiment contemplates a method of operating an automatic transmission for a vehicle comprising the steps of: driving a transmission oil pump with an internal combustion engine; while the engine is operating, pumping oil from an oil storage chamber, the oil storage chamber having an accumulator mounted therein and including a movable accumulator piston defining a top of an accumulator oil area, with the oil being drawn from a remainder of the oil storage chamber outside of the accumulator; selectively directing oil flow from the pump through a hydraulic assembly and to the accumulator oil area below the accumulator piston against a bias of a spring mounted in the oil storage chamber, the accumulator piston being located below a predetermine fluid level in the oil storage chamber; ceasing operation of the engine; and, while the engine has ceased operation, directing oil from the accumulator oil area into the transmission. 
     An advantage of an embodiment is that the hydraulic accumulator system maintains a minimum hydraulic charge pressure while the engine is off (under stop-start operating conditions), thus allowing the transmission to engage immediately when the engine is automatically restarted. The hydraulic accumulator can operate with a short stroke and a large surface area, thus allowing for easier integration with the transmission oil pan (or bottom of the transmission housing as the case may be). In addition, the fluid level in the transmission oil pan, while filling the accumulator, can remain relatively constant. These allow the accumulator to be relatively easily added to an existing transmission assembly, while also avoiding significant increases in packaging space taken up by the assembly. Another advantage is that the energy of the accumulator is stored in springs rather than containing a compressed fluid inside a pressurized housing, allowing for a lighter structure. An additional advantage is that a partially charged accumulator may still be used in brief stop-start situations, thus improving the functionality for a variety of operating conditions. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic, side view of a hydraulic accumulator mounted in a transmission oil pan. 
         FIG. 2  is a schematic, bottom view of the hydraulic accumulator mounted in the transmission oil pan. 
         FIG. 3  is a schematic diagram of a portion of a transmission hydraulic circuit that employs a hydraulic accumulator in the oil pan. 
         FIG. 4  is a schematic diagram of an alternate embodiment of a portion of the transmission hydraulic circuit that employs the hydraulic accumulator in the oil pan. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 and 2  illustrate a hydraulic accumulator system  20  having a hydraulic accumulator  22  mounted in a transmission oil storage chamber  24 . The oil storage chamber  24  is an oil pan or bottom of a transmission housing. The oil pan is typically used to store the transmission oil in vehicles with rear wheel drive transmissions while the bottom of the transmission housing is typically used to store the transmission oil in vehicles with front wheel drive transmissions. Thus, the overall result is the same—a transmission oil storage chamber. The hydraulic accumulator  22  is integral with this oil storage chamber  24 . Thus, the oil storage chamber, as used herein, is not an inflatable or expandable hydraulic storage area (such as a bladder), and is not separate from the typical location where the transmission oil is stored. 
     The oil chamber  24  includes a cylindrical recessed portion  26  in which the hydraulic accumulator  22  is mounted. The side walls  28  of the oil chamber  24  extend above the level of the cylinder portion  26 , with the oil chamber  24  also including a mounting flange  30  for mounting and sealing of the oil chamber  24 . The oil chamber  24  includes a hydraulic passage  32  that connects the oil located in the accumulator  22  to other portions of a hydraulic circuit, discussed below. Cast or molded-in pockets  34  extend downward from the bottom wall  36  of the oil chamber  24 . 
     The hydraulic accumulator  22  includes support rods  38 , each having a support head  40  at one end mounted in a respective one of the pockets  34 , threads  42  at the opposite end and a support flange  44  adjacent to the threads  42 . The support rods  38  are retained in the pockets  34  by threaded inserts  46  that each thread into the top of a respective one of the pockets  34  and have a hole through which a respective rod  38  extends. A spring  48  is mounted between each support head  40  and corresponding threaded insert  46 . The hydraulic accumulator also includes an accumulator piston  50  that is mounted onto the support rods  38  and secured against the support flanges by nuts  52  threaded onto the threads  42 . Seals  54  are mounted around the perimeter of the accumulator piston  50  and seal the area in the cylindrical recessed portion  26  below the accumulator piston  50  (the accumulator oil area  51 ) from the remainder of the chamber above (and possibly also around) the accumulator piston  50 . The fluid level  56  of transmission oil in the oil pan  24  is intentionally configured to be above the height of the accumulator piston  50 . The opening of the hydraulic passage  32  into the recessed portion  26  is below the height of the accumulator piston  50 , allowing the passage  32  to be used to direct oil into and out of the accumulator oil area  51 . 
     While the springs  48  are configured to operate in compression to bias the accumulator piston  50  downward, an alternative configuration where the springs operate in tension to bias the accumulator piston  50  downward can also be employed. For example, the springs can be mounted between and secured at their ends to the threaded inserts  46  and the support flanges  44  (or bottom face of the piston  50 ), operating in tension to bias the accumulator piston  50  downward. 
     The hydraulic accumulator  22  works by pumping oil through the hydraulic passage  32  into the accumulator oil area  51  under the accumulator piston  50 . As the oil is pumped into this accumulator  22 , the accumulator piston  50  is pushed upward against the bias of the springs  48 . The seals  54  keep the oil from escaping past the accumulator piston  50  to the upper portion of the oil pan  24 . Even with the increase in oil in the accumulator  22 , the fluid level  56  in the oil pan  24  can remain essentially constant—it is just a matter of how much oil is above and how much oil is below the accumulator piston  50 . The oil is essentially pumped from above the accumulator piston  50  to below it. The oil chamber  24  does not expand or contract during this operation—the movement of the accumulator piston  50  against the bias of the springs  48  creates the usable hydraulic pressure. The oil storage chamber  24  itself forms part of the hydraulic accumulator  22 , thus the hydraulic accumulator  50  is integral with this oil storage chamber  24 . 
       FIG. 3  schematically illustrates a hydraulic system  60  that may be employed with the hydraulic accumulator system  20  illustrated in  FIGS. 1 and 2  to provide hydraulic pressure during stop-start vehicle events. The hydraulic accumulator  22  is schematically illustrated in  FIG. 3  mounted in the oil storage chamber  24 , with the accumulator piston  50  below the fluid level  56 . A pump  62 , which is driven by the vehicle&#39;s internal combustion engine  61 , draws transmission oil from the oil storage chamber  24 , from outside of the accumulator oil area  51 . 
     The output of the pump  62  directs the oil into a main regulating valve  63  that includes a variable solenoid  64 , which controls through which output of the main valve  63  the oil flows. That is, the oil may be partially or totally directed into a return line  66  that directs oil back to the pump inlet, or the oil may be partially or totally directed into an oil line  68  that leads to a line  70  directing oil to the automatic transmission control system or to a line  72  that directs the oil to a check shuttle valve  74 . The check shuttle valve  74  includes an on/off solenoid  76  that can actuate the valve  74  to selectively allow or block the flow of oil therethrough. The output of the valve  74  directs oil into a line  78  leading to a one-way ball valve  80 . The ball valve  80  is configured to allow flow when the pressure is above a predetermined threshold, and block the flow of oil below this threshold. The ball valve  80  also always blocks flow in the opposite direction. The ball valve  80  and check shuttle valve  74  are part of a hydraulic charge circuit  82  for the hydraulic accumulator  22 . 
     The output of the ball valve  80  directs the oil into an oil charge line  84  that directs the oil into the hydraulic passage (shown in  FIG. 1 ) for charging and discharging the accumulator  22 . The output of the ball valve  80  also directs oil into a line  86  that connects to an input of another shuttle valve  88 , which is controlled by an on-off solenoid  90  that can selectively allow or block the flow of oil therethrough. An oil line  92  leading from the output of the valve  88  directs the oil to areas of the transmission that control one or more clutches in the automatic transmission. This shuttle valve  88  and oil line  92  are part of a discharge circuit  94  for the hydraulic accumulator  22 . 
     The operation of the hydraulic system  60  and hydraulic accumulator system  20  shown in  FIGS. 1-3  will now be discussed. When the vehicle engine is started, the engine driven pump  62  pumps transmission oil from the transmission oil storage chamber  24  to the main regulating valve  63 . The variable solenoid  64  is controlled to regulate the transmission hydraulic pressure by controlling how much oil is directed toward the transmission control system and how much oil is directed through the return line  66 . The hydraulic pressure is employed to operate the automatic transmission. 
     When it is desirable to pressurize the hydraulic accumulator  22 , the on/of solenoid  76  actuates the check shuttle valve  74  to allow flow through it. As the oil pressure differential acting on the ball valve  80  reaches the predetermined threshold (cracking pressure), the ball valve  80  opens, allowing oil flow through the charge line  84  and hydraulic passage  32  into the cylindrical recessed portion  26 . Thus, the hydraulic charge circuit  82  is opened for charging the hydraulic accumulator  22 . The shuttle valve  88  in the hydraulic discharge circuit  94  is of course closed during this time. This shuttle valve  88  may be designed with small clearances to minimize fluid leakage during periods when flow through it is not desired, i.e., when the accumulator is charged and no discharge oil flow is desired. 
     As the oil is pumped into the accumulator  22 , the accumulator piston  50  is lifted upward against the bias of the springs  48 , thus storing oil under pressure within the accumulator oil area  51 . This is accomplished without affecting the fluid level  56  in the oil storage chamber  24  as the oil is, in effect, pumped from above the accumulator piston  50  to below the accumulator piston  50 . The ball valve  80  prevents back flow through the charge circuit  82 . When the accumulator  22  approaches the maximum desired capacity, the solenoid  76  may be actuated to close the shuttle valve  74 . 
     When the vehicle comes to a stop, for example at a traffic light, the engine may be automatically shut off (stop-start operation) in order to save fuel. The accumulator  22  remains filled with the oil under pressure. When the light turns green and the operator lets up on the brake, the engine is automatically restarted. At this time, the solenoid  90  opens the shuttle valve  88 . The oil under pressure in the accumulator  22  will flow out through the discharge circuit  94  to control clutches (not shown) where hydraulic pressure is needed in the transmission in order to immediately set the vehicle in motion when the engine is restarted. Of course, as the engine is started, the pump  62  again begins to pump oil through the hydraulic system  60 . Even with oil flowing out of the accumulator  22 , the fluid level  56  in the oil storage chamber  24  can be maintained with return oil. 
       FIG. 4  schematically illustrates another hydraulic system  100  that may be employed with the hydraulic accumulator system illustrated in  FIGS. 1 and 2  to provide hydraulic pressure during stop-start vehicle events. The hydraulic accumulator  22  is schematically illustrated in  FIG. 4  mounted in the oil storage chamber  24 , with the accumulator piston  50  below the fluid level  56 . A pump  102 , which is driven by the vehicle&#39;s internal combustion engine, draws transmission oil from the oil storage chamber  24  from outside of the hydraulic accumulator  22 . 
     The output of the pump  102  directs the oil into a main regulating valve  103  that includes a solenoid  104  that controls through which output of the main valve  103  the oil flows. That is, the oil may be partially or totally directed into a return line  106  that directs oil back to the pump inlet or partially or totally directed into an oil line  108  that leads to a ball check valve  110 . The ball check valve  110  allows for flow from the main regulating valve  103  when the pressure difference is above a predetermined threshold but blocks flow in the other direction. 
     A line  112  directs the oil from the check valve  110  to an accumulator feed valve  114 . The accumulator feed valve  114  may be actuated by a solenoid  116  or alternatively through regulating the hydraulic flow  117  (both options shown in  FIG. 4 ). The accumulator feed valve  114  has two sets of flow paths for the oil. A first set allows flow from line  112  to another oil line  118  that eventually leads to the accumulator and blocks flow from an oil line  119  that extends from oil line  118  and eventually leads to forward and low reverse clutches  122  in the automatic transmission. A second set allows for oil flow from the accumulator through the valve  114  and through a line  124  that directs the oil to the forward and low reverse clutches  122 . 
     A feed limiting valve  126  is located between the accumulator feed valve  114  and the accumulator  22 . In addition, a ball check valve  128  allows for flow in one direction around the feed limiting valve  126  when the pressure differential at the check valve  128  exceeds a predetermined value. The feed limiting valve  126  is configured to block the flow at a predetermined pressure. 
     The operation of the hydraulic system  100  and hydraulic accumulator system  20  shown in  FIGS. 1 and 4  will now be discussed. When the vehicle engine is started, the engine driven pump  102  pumps transmission oil from the transmission oil storage chamber  24  to the main regulating valve  103 . The solenoid  104  is controlled to regulate the hydraulic pressure of the transmission by controlling how much oil is directed toward the transmission control system and how much oil is directed through the return line  106 . The hydraulic pressure is employed to operate the automatic transmission. 
     When it is desirable to pressurize the hydraulic accumulator  22 , the solenoid  116  actuates the actuator feed valve  114  to allow flow through it into oil line  118 . This allows oil flow through the oil line  118 , the feed limiting valve  126  and hydraulic passage  32  into the cylindrical recessed portion  26 . Thus, the hydraulic charge circuit is opened for charging the hydraulic accumulator  22 . The feed limiting valve  126  may be set to modulate pressure at a predetermined pressure threshold. That is, for example, the feed limiting valve  126  may be set to allow oil flow up to sixty pounds per square inch of pressure in the hydraulic accumulator  22  and then block flow above that pressure. 
     As the oil is pumped into the accumulator  22 , the accumulator piston  50  is lifted upward against the bias of the springs  48 , thus storing oil under pressure within the accumulator oil area  51 . This is accomplished without affecting the fluid level  56  in the oil storage area  24  as the oil is, in effect, pumped from above the accumulator piston  50  to below the accumulator piston  50 . 
     When the vehicle comes to a stop, for example at a traffic light, the engine may be automatically shut off (stop-start operation) in order to save fuel. The accumulator  22  remains filled with the oil under pressure. When the light turns green and the operator lets up on the brake, the engine is automatically restarted. At this time, the solenoid  116  moves the accumulator feed valve  114  to the position that directs oil flowing from oil line  118  into oil line  124  to the forward and low reverse clutches  122 . The oil under pressure in the accumulator  22  will push open the check valve  128 , causing oil to flow from the accumulator  22  to the clutches  122 , where hydraulic pressure is needed in the transmission in order to immediately set the vehicle in motion when the vehicle is restarted. 
     Of course, as the engine is started, the pump  102  again pumps oil through the hydraulic system  100 . Even with oil flowing out of the accumulator  22 , the fluid level  56  in the oil storage chamber  24  can be maintained with return oil. 
     While certain embodiments of the present invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.